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How habitable is the Earth?

(Pay attention at the back: this is a trick question.)

We H. Sapiens Sapiens appear to be an infestation on this planet. After the slow-burning evolution of hominins in Africa, our ancestral populations erupted out into Eurasia in a geological eye-blink, spread into the Americas by way of the Bering land bridge (sea levels being somewhat lower during the ice ages) and finally reaching even the remotest islands of oceania around twelve thousand years ago. Today we're ubiquitous. Even our pre-industrial ancestral cultures, from those resembling the inuit to the antecedents of the tuareg, occupied a slew of geographical environments that put cockroaches to shame.

So you'd think that, to a first approximation, the Earth is inhabitable by human beings. And this tends to colour our approach the prospects of finding extrasolar planets that might be hospitable to human life (if we could ever get there from here).

Actually, I think this is not quite the case. In fact, to a first approximation, from the perspective of prospective interstellar colonists, the Earth is uninhabitable. That we could imagine otherwise bespeaks a profound cognitive bias on our part (and a degree of relativism: because when all's said and done, the Earth is a lot less hostile than, say, the surface of Venus or the cloud base of Jupiter).

Why is the Earth uninhabitable?

Let's play a thought-experiment ...

I want you to imagine that, instead of being a perplexed mostly-hairless primate reading a blog, you're the guiding intelligence of an interstellar robot probe. You've been entrusted with the vital mission of determining whether a target planet is inhabitable by members of your creator species, who bear an eerie resemblance to H. Sapiens Sapiens. To gauge the suitability of the target world you've been given an incubator that can generate decorticated human clones — breathing meat-machines with nobody home up top. When you get to the destination you're going to transfer them to the surface and see how long they survive. If it can make it through 24 hours (or one diurnal period), congratulations! — you've found a potential colony world; one so hospitable that a naked and clueless human doesn't die on their first day out.

Your first destination planet is the cloud-whorled third planet out from an undistinguished G2 star, orbited by an airless, tidally-locked moon with roughly 1.3% of the planet's own mass. (Sound familiar? It should be.) You start sending down meat-machines to probe the surface at random. What conclusions do you draw about the inhabitability of Earth?

Let's start with Earth in its current configuration.

78% of the surface area is seawater. Drop a naked meat puppet there and it's going to go glug glug glub ... tritely, this is Not A Good Start.

Of the remaining 22%, about one third is either mountain ranges, deserts, or ice caps. It's reasonable to say that, in the absence of protective equipment, the meat probes are going to die of exposure in less than one diurnal period — possibly in as little as an hour if they're unlucky enough to land in the middle of the Antarctic winter.

We're down to about 15% of the planetary surface — 15% that isn't lethal without life support equipment such as boats, tents, and clothing. Our meat probes can breathe the air without their lungs freezing or dessicating. They aren't going to drown rapidly. And they aren't going to roll off a cliff. They might get a tad sunburned or hypothermic depending on the weather, and they might be eaten by a mountain lion or bitten by a rattlesnake, but they stand a reasonable chance of making it through 24 hours on the surface without dying.

Triumph! We have confirmed that a small part of this planet is inhabitable. Except ... I cheated. I pulled a fast one on you. Because I picked Earth in its current configuration — as it is today.

As a species, H. Sapiens has only been around for somewhere in the range 70,000-200,000 years. We are fine-tuned for survival on the Earth of this time frame. However, the Earth is a lot more than 200 kiloyears (Ky) old; the surface formed roughly 4.6 Gy ago (gigayears — 1Gy = 1,000,000,000 years). And we can expect the Earth to persist for about another 3-5 Gy, until the sun leaves the main sequence of the Hertzsprung-Russell diagram and becomes a red giant, presumably swallowing the Earth (or at any rate rendering it too crispy for comfort). So if we're being honest (and not cherry-picking our candidate stellar colony mission targets) we've got a 8-10Gy span to probe.

Back up far enough in the Earth's time-line (before about T minus 4.6 Gy) and we run into the formation of the solar system — and the proto-Earth, before its postulated oblique impact with Theia, a proto-planet roughly the size of Mars — debris from which collision condensed into our moon, Luna. Earth definitely wasn't a candidate for human inhabitation in those days — a largely airless blob of molten rock, under continuous heavy bombardment by planetary embryos and small planetesimals thrown out of the asteroid belt by Jupiter, as the two large gas giants (Jupiter and Saturn) churned the protoplanetary disk. Indeed, just about anywhere in the inner solar system at any time prior to the end of the Late Heavy Bombardment (which ended at T minus 3.8 Gy) is a poor candidate for a space colony — it was the LHB that resurfaced and cratered the moon, and presumably did a similar number on the Earth's then-fragile surface. On the other hand, this period left us geological evidence in the shape of minerals dated to the Hadean aeon. We know relatively little about the Hadean, although there's some recent evidence to support active plate tectonics and a surface temperature compatible with liquid water.

But don't get the idea that late Hadean Earth was a fun place to be. For one thing: that great big moon of ours didn't condense from a debris cloud at its current orbital distance. Tidal dragging is widening the lunar orbit by about 3.8 metres per century; it's now orbits roughly twice as far out as it did when it formed. Which in turn means that the young Earth spun on its axis far faster than it does today, and the tides the newborn molten-faced moon raised during the Hadean aeon would have been something to behold (preferably from a very high altitude). In fact, early Earth was a very alien world indeed. To hit on wikipedia (because I'm feeling too lazy to type):

Recent evidence suggests the oceans may have begun forming by 4.2 Ga.[22] At the start of the Archaean eon, the Earth was already covered with oceans. The new atmosphere probably contained ammonia, methane, water vapor, carbon dioxide, and nitrogen, as well as smaller amounts of other gases. Any free oxygen would have been bound by hydrogen or minerals on the surface. Volcanic activity was intense and, without an ozone layer to hinder its entry, ultraviolet radiation flooded the surface.
We don't know when life got started on Earth. However, we do know that the early history of life relied on anaerobic processes for a surprisingly long time. It wasn't until roughly 580 My ago (at the end of the Proterozoic era) that free oxygen came to dominate our planet's atmospheric chemistry. Back to wikipedia:
The most widely accepted chronology of the Great Oxygenation Event suggests that oxygen began to be produced by photosynthesis by organisms (prokaryotic, then eukaryotic) that emitted oxygen as a waste product. These organisms lived long before the GOE, perhaps as early as 3,500 million years ago. The oxygen they produced would have almost instantly been removed from the atmosphere by the weathering of reduced minerals, most notably iron. This 'mass rusting' led to the deposition of banded iron formations. Oxygen only began to persist in the atmosphere in small quantities shortly (~50 million years) before the start of the GOE. Without a draw-down, oxygen can accumulate very rapidly: at today's rates of photosynthesis (which are admittedly much greater than those in the plant-free Precambrian), modern atmospheric O2 levels could be produced in around 2,000 years.
There are elaborate and fascinating competing theories to explain why it took so long for oxygenation to take off: even the Moon gets blamed (the huge 50 metre tides that churned the anoxic early oceans and would have sucked premature photoautotrophs to their doom deep below: oxygenation had to wait for the moon to migrate out far enough for the tides to die down, permitting photosynthetic organisms to thrive near the surface). The point to take away from this is that, between T minus 4.6 Gy and T minus 0.56 Gy, the Earth's atmosphere was largely free of oxygen. Meat probe says: "aaargh, choke".

Even after the oxygen catastrophe, our space probe isn't going to find a terribly hospitable planet. The sun's luminosity is increasing by around 6% every billion years. The Proterozoic earth was a cold place by our standards; there were ice ages in which glaciation closed in on the equator. And there was continental drift. Continental drift leaves faint traces: we know that 250 My ago, our current continents were united in a single land mass (called Pangaea). There's some evidence that between 800 My and 550 My another single supercontinent (Pannotia or Vendia), and of another mass 1000-850 My ago (Rodina). And there seems to be a correlation between the occurence of "snowball Earth" events (those equator-reaching ice caps) and the unitary supercontinents. Back to wikipedia:

Most paleoclimatologists think the cold episodes had something to do with the formation of the supercontinent Rodinia. Because Rodinia was centered around the equator, rates of chemical weathering increased and carbon dioxide (CO2) was taken from the atmosphere. Because CO2 is an important greenhouse gas climates cooled globally.
Supercontinents too close to the equator: snowball earth. (Meat probe turns blue and shivers.)

So here's the upshot: of the 4.6 Gy of Earth's known history, there's only been enough oxygen in the atmosphere for us to survive for about 0.5 Gy. For roughly 90% of the Earth's history we couldn't even breathe the air. And about 10-25% of the time, there have been ice ages so savagely fierce that the glaciers reached the tropics: odds are good that any meat probe landing on solid ground during these periods would rapidly die of exposure. So historically, Earth has only been inhabitable about 8% of the time — assuming you are lucky enough to find some solid ground. Once you factor in the random surface distribution, we're down to about 2% survivability.

Now let's look at the future of the solar system.

We know the sun is steadily brightening by about 6% per Gy. It's postulated that within a couple of Gy, solar output is going to have some unpleasant effects. Ultraviolet radiation can split the covalent bonds that hold water molecules together, high in the atmosphere: and hydrogen ions (or, more likely, hydrogen molecules) can be blasted right out of the ionosphere by the same mechanism. The slow, steady loss of Earth's water is a one-way process, but exacerbated by warming (more water vapour in the upper atmosphere means more hydrogen is lost). As hydrogen loss proceeds, we end up with a carbon-dioxide dominated atmosphere and a runaway greenhouse effect like that of Venus.

There are other mechanisms that might render the Earth uninhabitable by our kind of life. Over geological time, the partial pressure of oxygen in the atmosphere has risen. With more solar energy inputs, it may be that oxygen levels continue to soar. Above about 28%, even waterlogged biomass will burn handily: and there are indications that atmospheric oxygen (currently down around 16%) has been well over 20% in the past. If oceanic photoautotrophs pump out too much of the stuff, the continents may well be burned back to bedrock by the resulting lightning-triggered fires.

All of this leaves aside the prospects for either an anthropogenic catastrophe, or the evolution (or creation) of new types of chemoautotrophs that have a drastic effect on the Earth's atmospheric chemistry. Or something else. Phase of the moon, perhaps. (What happens when the moon's tidal drag diminishes further, reducing long-term deep-ocean mixing? Anyone got a clue?)

The upshot is, we may well be most of the way through the Earth's inhabitable epoch. In which case, of the 4 Gy remaining, we may have 0.3 — 1 Gy to go with an oxygen-dominated atmosphere and water close to its triple point — the minimum necessary critera for human survival on a planetary surface.

So, back to the gedankenexperiment. Currently, a random meat probe dropped on the Earth's surface has something like a 15% chance of finding it survivable. But a random sampling over the historical epoch would return a survivability probability of around 1%. And over the future epoch, it's likely similar, unless we're erring massively on the side of pessimism about the prospects for our atmospheric composition remaining stable.

Ergo: to a space probe searching for somewhere that our kind of life can thrive, a truly random sampling of the Earth's surface (distributed over both time and area) would probably result in the conclusion that the planet is uninhabitable. See also: Fermi Paradox

Of course, a random sampling of Mars or Pluto would give an even lower probability of finding the planet to be inhabitable. But that's not the point. The point is this: we are finely tuned survival machines that have evolved to survive in a niche on one particular planet in one particular epoch. Even our own planet is unimaginably hostile to our kind of life for most of its history. And while survival outside that niche is possible with the assistance of a horrendously complex toolkit we call "civilization", we've yet to try it somewhere where we can't count on the basics (free oxygen and triple point water).

208 Comments

1:

Wow - I've heard the "Earth is uninhabitable" line before - but never spelled out quite that way.

I am thinking it all boils down to the idea that there are no simple questions - or simple answers.

2:

Charlie, good point about the lethality of most of Earths planetary surface, namely the ocean. Seafaring means trusting oneselfs life to a fragile artificial environment. Even a mere mile from shore the naked human will die with high probability (as he can't even see the theoretically rechable safe shore). Man's achievement in settling environments hostile to his life or at least in using them to generate nourishment (fishing) is remarkable.

3:

As a comment to this and the metapost about the blog (metametacomment?), this is one of the reasons I read your writings.

Interesting stuff, that is. I have never really thought the habitability of Earth in that way.

Somehow I like to think that the 0.3 - 1 My should be enough to think of some way to let the culture continue somehow. Of course, if we can survive in the short term.

Personally, I expect not to see the year 2100. I'd like to live for a long time if it's possible in a non-debilitating way, but can't count on it. If I can be sure that some legacy remains for at least a couple of generations, it'd be swell.

Also having the humanity survive in some way - or a descendant thingie as our meat probes are somewhat fragile as you pointed out - would be a nice idea.

Hm, not sure if I can express my thoughts well. I'm trying to say that I place some value on the culture of humans surviving. Not as it is now, but somehow as a changing thing.

Of course it can and probably will change into something unrecognizable to me in the coming hundreds or thousands or billions of years, even if there is a continuation.

4:

Excellent post, makes the prospects of interstellar biodomes (lozenges, tubes or dodecahedra) all the more appealing.

It really throws into stark relief how vital our (limited) intelligence is to our survivability, especially when you look at the range that most creatures inhabit. Outside of humanity few land based creatures have anywhere close to our range.

An interesting by-product of evolution: it clearly isn't intelligent as it tends towards specialisation which is inherently a bad trait in all but the short term.

5:

Wonderful post Charlie, even by your high standards.

I noticed, however, that you left us wondering about your own Fermi Paradox thoughts...

6:

Interesting thought experiment, but I'm not convinced our incubator/spaceship is as smart as it should be, which I guess is what your conclusion was as well. Why is it wasting resources on meat probes rather than using more economical methods to figure out what planets to investigate further? For instance, if I were designing a thing that sent meat probes to planets, pre-Neoproterozoic Earth (and even that's a stretch) is one I wouldn't have bothered with in the first place given the lack of atmospheric oxygen. So with a pre-survey, I'd rule out Earth for a while until things had some time to settle down. Put it on the backburner for a few billion years or invest in some terraforming to at least generate oxygen if you're too impatient to wait that long. In theory, there should be a bunch of candidate planets out there (although these guys disagree: http://en.wikipedia.org/wiki/Rare_Earth_hypothesis) so that the incubator doesn't have to waste time and resources randomly sampling a planet for habitability over a wide range of time/space. *shrug*

7:

I heard them meat probes grill up really good.

Only question is dry rub or that sweet sauce.

8:

From the perspective of 10 parsecs away, looking towards Earth, we'd be able to detect life for maybe the last 4 Gyr, looking at atmospheric spectra. We'd be able to detect intelligent life, by radio emission, only over the last 100 years. SETI, then, is a bit of a long shot.

One of the main theories for the creation of life is hydrothermal vents, at the bottom of the ocean. Life apparently developed in the water, without an oxygen atmosphere. So, if alien life was like this, what would its view of the solar system be ?

Most of the oceans we know of are _not_ on Earth: instead they are under the ice at Europa, Callisto, Titan, maybe, Enceladus .. and other possibilities. So an alien probe would look there first. Would it bother with Earth ? Only if it had specially adapted spacecraft that could handle our atmosphere, and gravity. They might have drills, fusion drives, etc. but unless they've previous experience on high-atmosphere planets, they may never have flown in air. Checking if theres life in our oceans might not be worth the effort ...

9:

Side note: If you haven't noticed yet, this diary entry is already linked in io9.

10:

John: I did the Fermi Paradox (and the Singularity!) in "Accelerando". (Or did you miss that bit? Middle section ...)

Katran: try to be less literal-minded -- this isn't a thought experiment about a plausible design for an interstellar colony probe, it's making the point that we are products of a very peculiar environment indeed ... and that this has implications for the Fermi paradox, among other things.

Alastair: the atmospheric signature of life isn't 4Gy old, it's much more recent (go hunting for "oxygen catastrophe"). And there's some question over whether any of our radio signals would be identifiable as technological artefacts at 100 ly -- even the big early warning radars they ran in the 50's-80's. You're right about the ice-moon ocean thing, though. It may turn out that part of the reason for the Fermi paradox is that most places where intelligent life evolves, the world comes with a Ceiling (and if you break it, you die).

11:

A very interesting post Charlie!

The question of habitability is so heavily influenced by who is asking, that it becomes a moot point. The problem with this thought-experiment is that the meat-probe is based on a human. Given that we are specialized to live in this current environment (our evolution is very fast compared to geological change) then of course we are going to look for THIS kind of environment. If the alien probe was from a sea-faring race, they would see the vast oceans as a good thing. If the alien probe was from a race that finds oxygen toxic, they might have found the pre-GOE environments on Earth more pleasant.

Furthermore, averaging out inhabitability over both entire surface area and all the historical epochs is not something an actual probe would ever do. It treats inhabitability too objectively. For example 15% survivable surface might be really good compared to some planets, and the vast oceans provide so many life supporting/climate supporting effects that without them or fewer of them the increased land masses might not be habitable! Also the probe would likely calculate how long it would take is creator-species to reach the Earth and how long they would like to colonize it (and for what purposes, what if they want to strip-mine the Earth clean?) to determine which epoch to take into account (not all of them).

As I mentioned earlier, biological evolution happens a lot faster than geological change. You can mention something like the covalent bonds of water molecules being torn apart after millions of years, but what will life on this planet look like then? It may not have the same habitability requirements that life today has (and thanks to science we may have changed even more than non-technological life would have!). Life could very well continue to adapt to Earth's changing environment until it truly is completely fried or incinerated by the Sun as a red giant.

So it's a cool thought experiment, but not a fully fledged idea of what habitability means.

12:

I tend to feel the same way -- when playing with the Drake Equation I've put in some pretty pessimistic figures about eh habitability of worlds. I think I worked out something like 1 intelligent species per billion years.

Though I reality, I wonder how much we really know about how suitable different planet configurations are. Perhaps the ratio of land/water/etc we have on Earth are the best it gets. Or maybe we're especially unlucky and most other worlds in the same position would be paradise.

I think there's a good chance though that if we ever do get to the interstellar colony stage, that we'll end up engineering the colonists to fit the environment. Either that or we'll just stop using planets and build habitats around stars that we fancy.

13:

Andrew G: Your SF masterwork of the day is "Surface Tension" by James Blish. Hmm, coming up to time for a re-read ...

14:

There's a variant on this thinking used as a better way to model the Drake Equation's underlying question. You look at the purely physical processes involved in setting the stage for Life As We Know It (LAWKI) - and work out how long in a planet's evolution those take, and how long the window is for that aspect of the process, given the solar system's evolution, etc.

The Drake Equation's approach was quasistatic - but looking at it from an evolving time window point of view, not just the odds of X happening but how long the conditions prerequisite for X exist - is much more accurate.

We were bouncing it around 20 years ago at various SETI informal discussions and over the last 5-10 years people started doing papers on it. The problem is, that figuring out the steps to model, and then figuring out reasonable first order models for all the steps, is not easy. A simplified look back at Earth's history like Charlie did here indicates the complexity, much less figuring out what factors might apply elsewhere in the universe...

15:

I'm with Katran, and it's not due to literal-mindedness about interstellar probes; the thing I find compelling about human beings versus the habitability of our landscapes is that we are 1) we are deeply into exploiting our environments and 2) we have vast cultural resources to adapt ourselves to hostile environments. We have never interacted with the earth as your meat probe would (or rather failed to interact), and "we" in this case encompasses even the most primitive of life-forms. We respond to the environment, retract, expand, hunt for food, lure food to us, migrate, run away, etc... Factor in the ability to learn, and we (just humans, this time) can't be too focused on perfect environments. I see terraforming in our future. We terraform our current planet all the time, as it is.

16:

Charlie, I've gotta agree with number 6. Let's assume a minimally intelligent probe (after all, the civilization which built it must be substantially more advanced than we are, and capable of designing a probe that's capable of simple bounds-checking. The probe would (minimally) do the following:

1.) Check the temperature extremes of the "randomly chosen" place before dropping the meat probe. The intelligent creatures your probe represents aren't so dumb that they'll go to an artic/desert area without protection.

2.) Make sure that the area where the meat probe will be dropped is not covered with water, lava, acid, etc. The probe's inventors are perfectly capable of avoiding obvious hazards, and the probe "knows" this, or at least is programmed by someone who knows this.

3.) Protect the meat probe from predators. After all, an advanced civilization can deal with the local sabre-tooths, (or whatever.) The meat probes tests for more subtle dangers - diseases, noxious gases, molds, fungi, nano-tech, indigenous intelligent life, etc. After some basic habitability has been proven, then the protection might be removed - half the problem of dealing with the local fauna is knowing what they do - and the method by which the meat probe dies will certainly be educational...

4.) Disect the meat-probes and take samples, then do blood tests, etc., to determine how they died. This does not require much more than Viking-level technology.

Your other points, however, are well-taken.

17:

This is dumb. I think that any species being ingenious enough to traverse the vast distances of space would be able to master what conditions warrant the wearing of a coat. And who says they'd have to come to Earth? there's a vast variety of environments available in the Solar System. They could pick any number of what might be to them a nice vacation spot.

18:

Free oxygen first appeared in the atmosphere more than 2 GYa. Yes, it only reached modern levels much later (after the formation of the banded iron deposits, for a start), but remote observation could certainly have picked up the signature then. I don't know whether there's any biogenic aspect of atmospheric absorption spectra prior to the arrival of free oxygen - I'm not sure that anyone knows that yet - but I wouldn't be surprised if there is.

19:

Well I'll be damned if that doesn't make Olaf Stapledon seem careless and implausible. And Arthur C. Clarke's "Against the Fall of Night" and "City & The Stars" totally implausible, too.

You'd think Stapledon--who took the (extended) time to mention each species of winged human that flew above the ruined arcologies of Venus--would've mentioned the solar engineering necessary to render the human species capable of surviving on Earth BEFORE the sun began to expand, which he does account for. Can't have one-eyed psychic minotaurs on Pluto at the end of history if their ancestors were burnt to a crisp from oxygen fires. Olaf, I am disappointed in you.

And Clarke gets away by implying that some sort of massive post-singularity intergalactic battle sapped the energy from the stars. Seems like a cop out, really.

Excellent post.

20:

Further thought brings one other issue to mind. "Habitability" is a rather slippery concept. If you're from a primitive civilization, meat-probes may be useful. On the other hand a really advanced civilization might look at a planet and say, "Oh, it's in the liquid water zone. That's good enough. Let's drop off some nano-tech and we'll come back when it's done." Then the nano-tech would prepare then planet with whatever improvements needed to be made, including the provisioning of a moon if necessary. The nano could even build colonists, if necessary.

This could explain the Fermi paradox. Your civiliation broadcasts for a few years, your transmission gets picked up by the neighbors, and they reply with a couple ounces of very intelligent powder sent by solar sail...

21:

Sorry Charlie, but you screwed the pooch on this one. This is nothing more than the classic fallacy of looking at probabilities after the outcomes they are based upon. Let's let Alexandrov have his say:

“Invent bloody argument, like this. Golfer hits ball. Ball lands on tuft of grass — so. Probability ball landed on tuft very small, very very small. Million other tufts for ball to land on. Probability very small, very very very small. So golfer did not hit ball, ball deliberately guided on tuft. Is bloody argument. Yes? Like Weichart's argument.”

So beings living in a reducing atmosphere will make this argument, beings in an environment of liquid ammonia will this argument, The Black Cloud living far out in space will make this etc. Thus, while one particular patch of territory is something of a tough go for most life in the universe, the probability it will be a nice spot for some living organism could be rather high.

22:

Yes, I'm aware of the oxygen catastrophe. The point of the 'maybe' in my post was that life was present before the oxygen catastrophe; the first disputed traces of life are at 3.5 - 3.8 Gyr, and perhaps before. We have generated spectra of what an Archean-like atmosphere would look like, and are busy speculating whether we could detect life under such an atmosphere.

The detection of life in non-Oxygen atmospheres is obliquely what my current PhD research is leading to. We need to be a bit more broad-minded about what the 'habitable zone' is. Even if we assume liquid water, most of it is under ice, or under too much atmosphere. If you look at Uranus, for example, you could say its an ocean planet that is currently _too_hot_ for life: the weight of its H/He atmosphere keeps the water vapour underneath above th 647 K critical temperature; it may blossom later when an inflated red giant sun blows away most of its atmosphere.

23:

@8: Alastair, I thought you were going to take the words right out of my mind, but nope.

Problem with not having oxygen is the good ol' redox potential. I've got the chart my soil microbiology teacher gave me around here...somewhere. Anyway, life as we know it (ignoring the Black Cloud) runs on redox potential. It needs an electron donor, and electron receptor, and a membrane or billion to set up gradients to extract energy from the pairings. Oxygen is a really handy electron receiver, and it makes for the biggest gradient. You can power life through all sorts of other redox reactions, but all of them generate less energy. To my knowledge, multicellular life uses oxygen exclusively, and while I don't think that rules out multicellular anaerobic life, you've got to get around the lower energy available in the absence of oxygen when designing the thing. While I expect people will find microbial life on ice moons, unless there's free oxygen available (through some unknown process), I wouldn't expect multicellular intelligent aliens. It would be nice to be wrong about this.

As for the probe, what kind of self-respecting, cost-conscious bureaucrat would aim an interstellar probe at anything other than a really nice, clean "free O2!" spectrum attached to a planet or moon of earthlike conditions? Talk about a waste of resources! This is a variant of ScentOfViolets argument, in that whoever's sending the probe will aim very carefully. The randomness you're talking is where my suspension of disbelief comes crashing down.

I'd also point out that the last 450 mya haven't been all that habitable either. Want to go for a run on an Ordovician beach? How about during the Triassic? A lot less oxygen back then. Granted, the Carboniferous had a lot more oxygen, and I'm not sure how good that was, either.

24:

Nice post! I'm sure you've read this already, but if not: Bill Bryson's "A Short History of Nearly Everything" spends a couple of hundred pages talking about this stuff in a really enjoyable manner.

25:

Great post until you consider everything it ignores.


I'll give you points for a really interesting way to discuss the formation/destruction of the planet. Kept me reading the whole way through. But it isn't so useful from a habitability standpoint.

26:

Some food for thought here

re.: Drake Equation : My problem with that one i that it relies on a large amount of assumptions, all of which could be off by orders of magnitude. Granted that we know more now than when Drake created it, but still ...

@21 Scent : I thought I recognised the quote when I read it, and sure enough, The Black Cloud makes for an interesting thought experiment in itself.

27:

Regarding speculations about some non-oxygen breathing life that might have found pre GOE earth tempting--it probably wouldn't have been multicellular.

As far as I know, even the most chemically capable life (bacteria/archaea, on this world) needs two things--a carbon source (which might be modified to something-else-source if you're thinking non-carbon based life) to replenish its substance ("flesh" if you will) and an electron acceptor, for energy. Prokaryotes (bacteria and archaea) can use a lot of different things as electron acceptors. Multicellular life uses oxygen. Oxygen is a great choice because hanging electrons on it gets you lots of energy, and because there's lots of it (now) in the atmosphere. The Pre-GOE atmospere was what they call "reducing' which is a way of saying there wasn't anything that, like oxygen, gives you lots of energy when you hand it electrons. I haven't seen it stated in so many words, but I suspect that one of the reasons multicellular life doesn't show up in the fossil record until fairly late is that there had to be a fair amount of oxygen available for energy before multicellular life could even happen.

So:
580-500 mya (million years ago) photosynthetic bacteria's oxygen production finally overwhelms the ability of dissolved iron to precipitate it out as rust in the banded iron formations. This is also when the first complex multicellular life appears--and the latter part of it coincides with the Cambrian Explosion.

And, not incidentally, this is about when life starts scuttling and growing out onto land.

So if there was multicellular (and intelligent at that) life running around in a reducing atmosphere I'd sure like to know what they're using for energy.

28:

Scent of Violets: But part of what I read in the post is that habitability is a moving target, we and the and The Black Cloud are spectacularly well adapted for our particular niche. And the inverse of the post is that the Earth has been habitable for 3.5Gyr. Just not for us.

It's a similar point that Leonard Susskind makes with the Anthropic Principle in his final post in Edge: most of the megaverse is inhospitable to us.

All of which reminds me somewhat of Stephen Baxter's Manifold trilogy which I takes a look at Fermi's Paradox and how dangerous a place the universe is.

29:

Mandamus @17: wooden spoon award for missing the point. (As in: you mistook the arrow for a spoon.)

ScentOfViolets: life seems to require (a) a redox gradient (for energy), (b) a carbon source (until we discover something weird and silicon-based -- sigh), (c) replication-capable molecules, and (d) some kind of catalyst (RNA seems to have filled in for this in the pre-DNA, pre-enzyme world: not as good at the job as poly-peptides, but workable). In turn, maintaining a redox gradient seems to require some sort of partitioning system (read: semi-permeable membranes) and diffusion gradient. Diffusion gradients ... well, that looks to be where multi-cellular life comes from: the size of individual cells is limited by the surface area to volume ratio across which diffusion of nutrients can occur.

Now, let's get around to your probabilities point. We have an existence proof for multicellular redox gradient based complex tool-using life forms (i.e. us). We don't have an existence proof for Fred Hoyle's black cloud, or for hypothetical ammonia breathers (whether or not we're missing a plausible energy cycle for them to run on). I can't disprove their existence ... but we're getting into Invisible Pink Unicorn territory here (or, if you're feeling combative, into comparative theology -- "my god's no less provably real than yours").

Evan: I'll let Stapledon off the hook because he was writing before anyone understood how the stellar fusion cycle worked. (Indeed, "Last and First Men" predates the discovery of the neutron.)

30:

There are some interesting analogues to the life-in-an-uninhabitable-region problem: perhaps its deliberate ?

Many species, such as Salmon, etc. go to great lengths to lay their eggs in desolate, barren rivers, seashores, etc. far away from other life. Its a survival strategy: leave the eggs (with sufficient food) in a place where there is otherwise never enough to supply life, so that there are no predators. Perhaps life started on Earth because this is a backwater, with no competition ?

Even if you avoid panspermia, you can apply the same idea to highly-energetic areas of space. Perhaps this is a low-energy-gradient region, and all mature species move to take advantage of high-energy/entropy gradients around black holes, too energetic for life to evolve around ?

Heteromeles, Cat @23, @27: Agreed on the advantages of oxygen. Methanogenic and other life has an order of magnitude less energy to play with, and typically gets swamped by oxygen-comsuming species. But if you've all the time in the world (and around smaller M stars, you've lots more than around our G2, with only a billion or so years to go), then things might evolve.

Then you can look at the redox equation from the other side. Look at the work of Mark Kuchner on Carbon planets: if you're in a region where the C/O ratio > 1, you end up with Carbon and Carbide planets, rather than Silicate ones like Earth. There, you have a reducing chemistry rather than an oxidizing one.

31:
As far as I know, even the most chemically capable life (bacteria/archaea, on this world) needs two things--a carbon source (which might be modified to something-else-source if you're thinking non-carbon based life) to replenish its substance ("flesh" if you will) and an electron acceptor, for energy. Prokaryotes (bacteria and archaea) can use a lot of different things as electron acceptors. Multicellular life uses oxygen. Oxygen is a great choice because hanging electrons on it gets you lots of energy, and because there's lots of it (now) in the atmosphere. The Pre-GOE atmospere was what they call "reducing' which is a way of saying there wasn't anything that, like oxygen, gives you lots of energy when you hand it electrons. I haven't seen it stated in so many words, but I suspect that one of the reasons multicellular life doesn't show up in the fossil record until fairly late is that there had to be a fair amount of oxygen available for energy before multicellular life could even happen.
Could not the hypothetical lifeforms run in the opposite direction, so to speak? Donating instead of gathering electrons? It strikes me as being possible that that could work out, though I'd love to hear from someone who actually knows something about chemistry.

The argument strikes me as somewhat...incomplete. Sure, you have a low individual chance of finding something exactly fitted to you, but if you look at the entire population of stars around you, you're bound to find a few good candidates. And if you can colonize space or other 'uninhabitable' environments, why then so much the better and you can go anywhere. Though as you have pointed out before, this is...doubtful at best.

Hm, another idea farm post...?;)

32:

Also noteworthy, now it occurs to me: orbit too close to your star, you end up with hydrogen stripping (and hydrogen comes in useful if you're running hydrocarbon chemistry :). Orbit too far out and the energy available to run your biosphere is too low: either everything runs v-e-r-y s-l-o-w-l-y or it doesn't get started at all (if there's a potential barrier to overcome). Tidal heating (e.g. in the oceans of Europa) are an alternative input to sunlight, but still ... not brilliant.

And then there's the temperature regime. It's a long time since I studied enzyme kinetics but IIRC there's something like a 10% reaction rate improvement per degree celsius (until the environment gets so hot that the hydrogen bonds start breaking and your intricately structured peptides turn into cross-linked mush).

I find it plausible that there might be something creeping towards life on Titan ... but it's not going to get there any time in the next couple of Gy, simply because it's trying to cook up reactions in the deep freeze.

33:

Charlie@29:

life seems to require (a) a redox gradient (for energy), (b) a carbon source (until we discover something weird and silicon-based -- sigh), (c) replication-capable molecules, and (d) some kind of catalyst (RNA seems to have filled in for this in the pre-DNA, pre-enzyme world: not as good at the job as poly-peptides, but workable).

It does? Could you explain why?

I can't disprove their existence ... but we're getting into Invisible Pink Unicorn territory here (or, if you're feeling combative, into comparative theology -- "my god's no less provably real than yours").

I'm sorry, but that's just not the way it works - see your first quote. You're perfectly free to claim that these sorts of probabilities apply . . . provided that you can demonstrate that only life of the sort you imply in your first quote is possible. Otherwise, it's just another "I don't see how the eye could have evolved, therefore God Did It" sort of argument.

34:

Testing by unsupported meat puppet seems a little pointless. Humans can't exist without a supporting ecosystem. To my knowledge we can't even manufacture food without other organisms. Even in the most benign environment, and with magically provided oxygen, a human isn't going to survive indefinitely.

35:

Hmmm... in some sense, this is the inverse of the automated probes in Larry Niven's Known Space series. Those probes were programmed to find a habitable spot, with no specification as to how large that spot had to be. The result was colonies like Mt. Lookitthat (sp?) and Jinx (only inhabitable in a very narrow band). Niven's test was a bit tongue in cheek; presumably so is the one you use here. As an example, a world that is habitable over most of its surface would fail the test if it happened to have carnivores that would eat up those brainless meatballs.

Equally misleading false positives from the test:

A world with breathable oxygen and a nice climate but +/- 100 degrees temperature variation over a year. Land in spring or fall, and it passes the test. One season later, 100% mortality rate.

A world where heavy metals are pretty pervasive. The meatballs live for a few months, so it passes. Unfortunately the 1-year morality rate is 100%.

In short, it's not a very good testing mechanism, and the implications or derivations from it are at most interesting, not realistic. Nice discussion about the habitability of the earth, tho.

36:

Thanks Charlie for the fascinating post.

Also, thanks to many of you who made comments which were entertaining, interesting and quite informative.

You are some of the most articulate commenters on any of the blogs I frequent.

37:

Charlie@OP: (What happens when the moon's tidal drag diminishes further, reducing long-term deep-ocean mixing? Anyone got a clue?) I does has a clue, as it happens :) On the current Earth, ocean mixing is driven by the thermohaline cycle rather than tides per se (ice forming at the poles leaves cold, salty, oxygenated water that sinks, runs across the seafloor right past the equator, and eventually rises back up; that same ice melting in spring drives cold, less salty water out across the polar seas towards the equator). If one were to shut down this system (in a greenhouse Earth, for instance) you'd end up with an anoxic deep sea. This has happened in geological history, and the consequences are believed to include mass extinctions and various dire effects on atmospheric chemistry.

Charlie @32: What you're talking about is called Q10. It's specific to individual enzymes; most of ours work best at 37 degrees for reasons that should be obvious, but there are some that work fine at nearly 100 or right down to zero. The former come from hydrothermal vent organisms, the latter from things that live in very cold water.

til@31: in a word, no. You only get energy in one direction.

all@multicellular-life-and-free-oxygen. Multicellular organisms don't process their own oxygen (or their own sunlight): they hold prokaryotes prisoner and make them do it. You call them mitochondria (and chloroplasts). If we're doing evolutionary arm-waving, there are other prokaryotes that can use completely different electron-transport chains that could hypothetically be held prisoner in the same way. Evolutionarily speaking. Finding a suitable energy gradient for them to exploit is up to you (sulfur compounds continue to be popular in many neighbourhoods).

38:

The Martians put you up to this one didn't they? They'll never get over the whole Terraforming plan the Bush administration had.

Oh, dear. I said too much about your masters!
:)

39:

It's a nice way of looking at the habitability of the Earth, but including the future epoch is not good, since the universe is (probably) going to expand forever, and so to be truly time agnostic, you need to include the total future of the universe as well.

Hence, if you picked any random point in the Universe's history to look at the Solar system, chances are you'd be looking at a single large cold carbon crystal the size of the Earth, with a mass of about 1e30 kg.

Probability of meat survivability = 0!

(With apologies to Douglas Adams)

40:

Chris L@37:
Tides are still important, as Charlie was pointing at, for mixing. Even though the Thermohaline cycle is the driver of the deep circulation, without (tidal) mixing, the water doesn't go down. Without mixing, the deep water is cold and saline, and sinking water from the poles doesn't go all the way down. This is what happens at the Northern Pacific, for example: while there is sufficient mixing (for topographic reasons) at the Northern Atlantic, at the Northern Pacific the sinking water only reaches the Intermediate layers.

Also, are you referring to Peter Wards euxinic ocean work
(as in "Under a Green Sky")? While various parts of the ocean have been euxinic at certain times geologically (that is, anoxic, with sulphur-driven biochemistry instead), they were not stagnant : there was a vigorous circulation: see Meyer and Kump, 2008. It was dire for oxygen-breathers, but life continued. Also, it was incomplete, which is why we're still here: oxygenic life continued in parts of the ocean. (There may be a genetic remnant of those times in our ability to survive low levels of HS gas without harm).

41:

What about the possible nasty effects of micro-organisms on the alien would-be colonists? One of the things that has always bugged me (no pun intended) about planetary colonisation scenarios presented in science fiction, after handwaving away the fearsome problems of interstellar travel, is that humans manage to walk out onto the surface of planets with flourishing biospheres, but are barely affected by the local microbes. Admittedly I don't know much at all about how the immune system works, but it is something that is often not considered in such scenarios. Any biologists around these parts?

42:

Trick question indeed: 'How habitable is the Earth?' is present tense, and strongly implies 'How habitable is the Earth *right now*?'

Using it to mean 'How habitable is the Earth *averaged over geologic time*?' is a bit of a stretch.

The argument that shirtsleeve environments might not be common in the universe can be made without such legerdemain.

43:

@37: from the multicellular and oxygen crack. Yes, you're right. Absolutely. I'll even agree that we eukaryotes are way too proud of being the alleged masters of a system that still is arguably run by the bacteria and archaea. However, I'm pretty sure that we multicellular types wouldn't be around if there was enough oxygen. We're energetically expensive, and the oxygen-carbohydrate gradient is about as good as it gets for supplying energy. Don't think sulphur's good enough for that.

I suppose it would be possible to have intelligent life under a reducing atmosphere, but it would have to look something like those intelligent microbial communities in Cosmonaut Keep. If they're around, I think I'd call them/it Gaia now. Hmmm. Maybe that's what magic mushrooms are about...They were designed by Gaia to allow communication. Duuuuuude....

@41. Andy, I think Heart of the Comet was one of the few that dealt seriously with the disease aspects of alien biospheres, at least that I remember. My personal pet peeve is: why does it always have to be an alien virus? It's not as if the same genetic code is going to exist planet to planet (the code is arbitrary, so far as I know), and without equivalent codes, viruses can hijack your genome. I'm far more worried about the equivalents of fungi, and to a lesser extent, bacteria, simply because they can eat you as a resource base, rather than hacking your genome to make copies.

My other pet peeve is, as a sufferer of allergies, why is it that characters in B-Grade Sci-fi books always get hyper-efficient artificial immune systems to deal with alien bacteria? Sounds like a good recipe for autoimmune hell on steroids to me. It's like having too many hyper-vigilant, heavily armed police around a neighborhood. You don't want too many cops, because then they get bored, and start causing trouble to justify their existence.

44:

Alastair@40: I stand somewhat corrected. Although if you'd asked me, I would have said that the North Pacific was probably the least important contributor to the deep-ocean circulation. Doesn't most of the deep water come up from Antarctica and run all the way to the north temperate zone?

Andy@41: By and large, things that can get past the mammalian immune system have been practicing that trick for a very long time. Not that some alien super-bug couldn't maybe take up residence in (say) our lungs. But a large collection of things that would like to take over our bodies already live right here on earth. Lots of things bug me about most colonisation stories, but not so much that one (a few stories have done it, anyway; it's a plot device in "The War for Eternity", and there's a short story I've forgotten the name of where the colony flourishes by all their babies are killed off by the aforementioned lung-tenant).

45:

Sure, most of the Earth is uninhabitable by meat puppets. So? Humans aren't meat puppets. We needed some fairly constrained parameters on our environment in order to evolve, but once we got to tool-making, we started moving away from those constraints. Humans survived the last Ice Age, some of them living on the ice their entire lives¹ as you say, Charlie, "Meat probe turns blue and shivers."

And now, we could, if we were willing to pay the price, colonize the sea bed, at least in the shallower regions. Meat probe gets all wrinkley and stops breathing. So the real question is "Habitable for who?". Insects have done a pretty good job of it in all kinds of habitats over the last 400 million years or so; termites, bees, and ants have even used some of the environment-modifying tricks we've developed. I think we stand a chance of doing that well ourselves if we ever figure out how to live with ourselves and our effluents.

1. One paper I've seen says that humans migrated across the Bering Strait into North America at a rate of about 8 miles per year. With an average life expectancy of perhaps 30 years, the ones born in Siberia certainly wouldn't have made it to the end of the ice in western Washington by the end of their lives.

46:

Missing me, then?

47:

Apropos 46: Only someone living in Scotland would describe the triple point of water as ideal for life.

48:

Isn't the meat argument itself fallacious as it only exists form our survivability perspective right now? Without a global catastrophe then a thousand years from now we are likely to be aliens to our current selves and much better able to survive in a much wider range of conditions. Unless we are talking Space Opera where technology _doesn't_ change humans.

And that is assuming that we haven't gone non-biological by then and that we are still Earth's dominant species. Though AI might still be "only 20 years away" ;-)

49:

Charlie @ 32:

Orbit too far out and the energy available to run your biosphere is too low: either everything runs v-e-r-y s-l-o-w-l-y or it doesn't get started at all
No, not really. Jupiter has lots of hydrogen, a core that has been releasing heat since the planet first condensed, and tens of thousands of kilometers of temperature-stratified atmosphere / slush of varying composition that ought to provide one or two paradisiacal environments for some sort of life, probably carbon-based. It just won't breath oxygen. I'm still not convinced that multicellular life can't evolve without that; granted it's got the required high energy density, but at high pressures and temperatures I would think that quantity would make up for lack of quality in some other reaction cycle. And there might easily be some other simple trick the equivalent of eukaryotes could turn by enslaving a prokaryote with the right chemistry.

As for silicon instead of carbon, I sort of doubt it. There's a lot more carbon than silicon in the universe (carbon's lighter, and is a preferred resultant of some stellar fusion reactions), and carbon is more versatile in terms of chemical bonds. I've been wondering if sulfur might work, but don't know enough chemistry to figure it out.

50:

@ Chris L and @ Alasdair

Regarding the possiblity of multicellular life in a reducing atmosphere...

Prokaryotes show up 3.8 billion years ago. Oxygen doesn't build up in the atmosphere until about .58 billion years ago. That's 3.2 billion years that life was around in a reducing atmosphere, free to evolve into multicellular life if the energy conditions would support it, whether by taking appropriate organelle-precursors hostage or by other methods. It doesn't seem to have happened as best we can tell by an admittedly spotty fossil record.

0.58 billion years ago oxygen starts to build up in the atmosphere. Multicellular life shows up a fairly short time (by geological standards, granted) later.

Long time in reducing atmosphere--no multicellular life. Short time in oxygen bearing atmosphere--multicellular life. It is admittedly not as good as running a thousand such experiments, but still...

@41 andy: microbes are usually co-adapted with the living things they infect--have proteins shaped to trigger receptors (for other things) on the surface of cells to get themselves inside, etc. Some of our nastiest plagues (including plague) originally infected other animals, granted, but animals related to us by common descent, which life forms on other planets wouldn't be. A truly "martian" bacterium would likely be less of a threat than a terrestrial bacteriophage (virus that infects bacteria)--both are specialized to parasitize organisms very different from us.

@43-- I think you mean "without equivalent code viruses *can't* hack your genome" in which case I entirely agree, and you bring up a good point. The genetic code is completely arbitrary--that it is the same in all terrestrial life is due to common descent, not chemical necessity. You wouldn't expect that to match in non-terrestrial life (unless we're looking at a germs-on-an-asteroid-smashed-off-Earth's-surface scenario) which means serious problems for anything that (like a virus) has to hijack your cellular machinery to reproduce and make the new protein coats for its offspring.

I mentioned why I wouldn't be all that worried about bacteria--fungi would potentially be another matter--though again the enzymes they secrete to break down bio-matter might be too different to affect our molecules. Heck, they might not even be protein.

Regarding the original post, are we interested because we want new digs? Or because we want neighbors? Because life arose under conditions humans can't live in (and there's reason to think it *can't* arise under conditions humans can live in.)

Oh, and Charlie--are you aware that there is a school of thought that says that the first pre-life organic molecules may owe more to freezing than to the classic "warm pond?" Of course, there is another school of thought that fingers upwellings of near-superheated seawater as the culprits, so I dunno.

51:

Alastair@40:

Could you fill out that Meyer and Cump (2008) reference? I wasn't aware that there was great deal of circulation in a Canfield Ocean.

Ward's Under a Green Sky certainly gives the impression that worldwide oceanic eutrophication is induced, or at least initiated, by a halt in thermohaline circulation brought on by greenhouse heating. At least it gave me that impression.

While on the topic of earlier stages of the Phanerozoic - even once the oxygen levels are up, disposition of the continents is going to limit habitable area. The interior of Pangaea, for instance, is unlikely to have been very clement. And nobody ever seems to remember that an entire continent has had almost all of its terrestrial fauna and flora extirpated by continental drift carrying it down over the South Pole, and in the recent past (as such things go on the plate tectonics scale). Antarctica used to be a nice place to live. Well, a place where you could live, at least. If you were quick on your feet and didn't go near the water.

52:

sooo, the earth is uninhabitable...yet we ALL live here...??? enough said..plz ppl, dont bother with this man's article. its like me saying, "everyone on the planet is the same color". we KNOW ITS NOT TRUE. and neither is this. i read about 2 lines and skipped right down to the comments. this man is a damn fool, and he's trying to hard to make a point that isn't even possible to prove. WE HAVE ALL LIVED HERE OUR ENTIRE LIVES!!!! how in the EFF is this guy gonna tell us earth isn't habitable. what a joke

53:

I think I know how those meat probes feel, most days.

54:

Charlie @10: You're right about the ice-moon ocean thing, though. It may turn out that part of the reason for the Fermi paradox is that most places where intelligent life evolves, the world comes with a Ceiling (and if you break it, you die).

Unless this intelligent life is positively buoyant. In which case their world comes with a Floor.

Well, a Ceiling too, but it is the Floor which is breakable.

55:

Maybe we ought to see if Planets for Man can be reprinted. It's about time for another edition, anyway, with updates for the last fifteen or twenty years of research and discovery.

(ISTR it has considerable discussion on what the boundaries actually are for human habitability. My copy is in a box.)

56:

Cat@50:

Prokaryotes show up 3.8 billion years ago. Oxygen doesn't build up in the atmosphere until about .58 billion years ago. That's 3.2 billion years that life was around in a reducing atmosphere, free to evolve into multicellular life if the energy conditions would support it, whether by taking appropriate organelle-precursors hostage or by other methods. It doesn't seem to have happened as best we can tell by an admittedly spotty fossil record.

Post Hoc, ergo Propter Hoc. This isn't a good argument and yet it seems to get used a lot in the biological sciences, particularly when the topic is evolution, and most especially when it's a just-so story you're talking about. The urge to explain why a particular trait confers survival points seems to be overwhelming, and contra the other direction, to dismiss the lack of a trait as the result of being detrimental to survival.

57:

Sounds like a good rationale for understanding just how rare a commodity life is, should one wish to make it such...Imagine all the commotion meat-probe-right's groups would make!

58:

Cat@50, SoV@56: If I've ever heard a firm date for the rise of eukaryotes (cf multicellular life), I can't recall it. I assume Cat's talking about the Ediacaran fossils with the ~500 million years ago, but they're a bit... enigmatic. (BTW, SoV: the reason us biological arm-wavers like to assume a survival advantage for stuffs is that she's a cruel old world, nature red in tooth and claw etc etc, and most organisms don't survive.)

I think the argument is, as heteromeles suggested, whether an appropriate energy gradient might not exist without oxygen under some circumstances (increasing pressure does some interesting things to reaction rates, fer instance, and we're back to ice moons and gas giant slush).

59:

PJ@55: looks like it already got a reprint.

60:

Hey Charlie -- doesn't the triple point of water imply extraordinarily low pressure, e.g. about that of the atmosphere of Mars?

Perhaps s/triple/freezing/g?

61:

An animal is the sum of all its adaptions and think that humanity can't be defined unless you include all our exo-somatic adaptations. Remember, the only reason we don't have fur any longer is due to the exo-somatic adaptation of clothing (or at least wearing animal furs).

Humans without these adaptions are like an unprotected foetus and are not designed to live anywhere...If the cold doesn't get you, then the sunburn will.

But with the extras we can live almost anywhere and are getting better at it all the time.

62:

the human race, with all its fault right now doesn't deserve to polute the univers'al possibilities...
i can't envision us going anywhere near the break of our gravitywell or reach 3My on our timescale.

first we have to get rid of the baggage and then we might leave earth to have a chance at something resembling the future.

if we survice the transition to he-man, we might become "masters of the universe" as far as we can reach; but otherwise, we will be stuck here and die a historically humiliating way.

atm, humanity won't stand a chance on a universal scale.

63:

@50 Cat: Show some respect, substrate! Fungi are underappreciated. There's this whole field of mycoremediation, where practitioners use fungi to break down things like, oh, rocket fuel, nerve gas, and many VOCs (see Stamets, Mycelium Running). Working mycologists generally have the opinion that some fungus, somewhere, can break down just about anything organic. Not as well as HF, certainly, but they may be the best option for cleaning up soils contaminated with industrial organics.

There are a large number of fungal species (mostly Onygenales) which eat people, and I suspect that something similar probably exists elsehwere.

Of course, there are bacteria that can live on anything, so I'd expect them to live on us too.

64:

ChrisL@58:

BTW, SoV: the reason us biological arm-wavers like to assume a survival advantage for stuffs is that she's a cruel old world, nature red in tooth and claw etc etc, and most organisms don't survive.

The problem with that line of reasoning is that it gets applied indiscriminately to every trait, or lack thereof. Why do you see Fibonacci spirals in in sea shells? Obviously, it must confer some sort of survival advantage. Why are the cells in a honeycomb hexagonal? Survival advantage again. Why aren't there any mammals with green fur? Or six legs? Obviously, green fur is disadvantageous for survival(though we may not know precisely why), ditto for hexapodia.

In fact, of course, Fibonacci spirals and hexagonal structures prevail for reasons having nothing to do with survival advantages, but rather basic physical processes and geometry. Similarly, there's nothing contra-survival about having green fur or six limbs, but due to reasons of historical contingency, those features simply aren't found among the modern class of mammals.

Is having a multicellular structure impossible without the energy budgets oxygen affords? I don't know. But I do think it a fallacy to argue that because this is what we see in the fossil record, this must necessarily be the case. Incidentally, this is one of the best reasons I can think of to do the interplanetary/interstellar exploration thing. These sorts of questions seem to be hard to answer without some sort of comparative research. They're also the biggest reason why I would want to live a long time :-)

65:

RE: Fermi Paradox in Accelerando. (responding to Charlie gently teasing me about not remember the embedded essay)

I couldn't remember how you snuck that in to Accelerando so I googled the question and found my OWN damned 2006 Amazon review of Accelerando ...

"The embedded essay I most enjoyed reading, however, is on one of my all-time favorite topics -- the Fermi Paradox. This is one of those conumdrums that bothers a very few people a great deal and is irrelevant to most of humanity.

In short, we ought by all rights, to be overrun by little green beings. The puzzle is that we appear to have much of the galaxy to ourselves. To the Fermi fan-boys this is the biggest question around, to which matters of theology or epistemology are merely academic.

The answer to the Fermi Paradox is most often expressed in the terms of the Drake Equation. The best bet is that something utterly inevitable ends all technological civilizations like our own in well under a thousand years. The most popular candidate for an "inevitable fate" over the past 23 years has been the Singularity (Greg Bear's 1982 short story 'Blood Music' is the earliest version of the Singularity theory I know of, Vernor Vinge developed the ideas extensively in the early 1990s.) Stross takes these ideas and pushes the boundaries. Why might a post-singular entity find travel unappealing? Why would it be hard for entities like us to live near such a beast -- even if it didn't spend any time thinking about us? "

Problem is, I was trying not to be a spoiler, so I didn't fill in the details and, you know, it's been 3 years and a lot of reading since then.

I had to go back to Google to get the last memory jogger. In the post-singular society low bandwidth living is intolerable. Very funny. :-)

66:

Of course an alien probe would have trouble finding a usable planet if they looked at the Earth over the last few billion years!

Now, if they were looking for a nice warm planet with plenty of resources, there's a very nice little place two slots further in towards the Sun - a little Ferroforming, and you'd have a great new home with plenty of alloys and energy to spawn new mechanoforms.

For the cool-inclined types, there are plenty of free feeding zones between the fourth and fifth planets. Admittedly, things are a bit less energy-intensive there, but with lower gravity to deal with, you don't need to hurry so much. Good retirement habitats.

"That's ridiculous. How can meat make a machine? You're asking me to believe in sentient meat." - Terry Bisson

67:

After reading this post, I was left with the strong sense that one of the main purposes of civilization is to terraform regions to be more comfortable for humans, and that we've been slowly getting better at it as we try something new and figure out what goes wrong.

Once we built cities, we had to learn about microorganisms and sanitation.

More recently, pressure for increased energy efficiency, pushed us toward sealed insulated buildings, which then created "sick building syndrome". However some groups have figured out mixtures of plants to grow in these sealed office buildings to filter out VOCs, and provide enough oxygen for the humans inside.

Though we can't build a large closed ecosystem now, we are slowly learning pieces of what we need to know through a slow process of incremental development.

68:

We find it hard to live on senescent planets like Earth. Our preferred habitat is someplace that has very recently condensed from supernova debris and has lots of free energy from radicals and unsaturated chemicals after radiolysis of stable compounds.
This planet has such a low rate of radioactivity that it barely even has plate tectonics!
Thank god for nuclear reactors and waste cooling ponds.

69:

If you're breathing 16% oxygen in Scotland at the moment, you may wish to consider evacuation - here in NZ we've had ~21% since the start of human infestation, at least ;-)

** Atmospheric composition by volume available at http://en.wikipedia.org/wiki/Earth%27s_atmosphere

70:

>>try to be less literal-minded

Haha, ok, point taken. I got the main picture re: Fermi paradox and implications that any variable for habitability should include some time-dependent probability, I just thought that the thought experiment framing could use some work. I work in a place where the very name defines its uninhabitability (Death Valley) and admittedly, I only try to do my work in the winter and carry a lot of water around with me in the truck. However, the area has had human habitation for thousands of years; not to mention it supports a wide range of animal life including mammal species which are definitively less smart than humans, yet still manage to make it, so you don't even have to have a very smart meat puppet/spaceprobe to expand the range of potentially habitable areas/time periods on the planet.

71:

"They're made out of meat."

"Meat?"

"Meat. They're made out of meat."

"Meat?"

"There's no doubt about it. We picked several from different parts of the planet, took them aboard our recon vessels, probed them all the way through. They're completely meat."

"That's impossible. What about the radio signals? The messages to the stars." ...

(On preview, I see 65 nipped me at the post. It was the first thing I thought of. Author's page.)

72:

SoV@63: Fibonacci spirals and hexagonal cells both make maximum use of available materials, and there IS a survival advantage (or rather, a fitness advantage, the two not quite being synonymous) in that. I'm an unabashed adaptationist, but I'm entirely prepared to agree with you on contingency and the role of chance. As long as we remember that evolution isn't a random process.

Leaving aside the shambling zombie of Stephen J Gould, it's easiest to assume that most features of organisms are the result of selection and go from there; the selection might be historical rather than current, of course. Modern organisms are the result of billions of cycles of overproduction and brutal culling, and it really doesn't pay to ignore that.

73:

A couple of things: for a start, surely any culture that could try this experiment, or even close, wouldn't? It seems a daft way to replicate the Soviet VENERA/VEGA program, especially as probing the Earth would be significantly easier than Venus. We don't have a permanent cloud layer that's opaque to every damn thing but radar, and our atmosphere doesn't dissolve radio antennas, and the Earth's surface conditions are not such as to require something built as heavily as a submarine but light enough to launch on a rocket and aerobrake down to a survivable deceleration on landing.

As James Nicoll says, anyone who planned to spend time on another planet would start by doing a full orbital survey and probably installing a gaggle of weather, surveillance ("Bug-eyed monster detected in position XXXXX.YYYYY, course 220 degrees, speed 10 mph, estimated weight 230 tons"), navigation, and maybe space solar power satellites. (After all, by definition you're dragging a whole lot of kit out of your gravity well, so you might as well fit the place out with all those aids to survival that our lower orbits unaccountably don't provide without a lot of help.)

Of course, as someone pointed out, the survivability of the Earth for those meat probes is even worse, because the period in which it's reasonably survivable also involves a wide range of other meatware, including social technological primates. Hmm, shall we cook it or study it?

By the way, if the majority of intelligent life exists in a world with a Ceiling, I suppose that means they worship the Ceiling Cat, or rather the Ceiling Xkhjg?

74:

@35 - a daft point i know, but i love the idea of a 100% morality rate after a year on a heavy metal planet.

(In the beginning was the word, and the word was Kerrang!)

75:

Bruce @45, about migration rates between Asia and the Americas.

There's some evidence for communication--styles of stone tools and such--between Europe and North America. It could, I suppose, be parallel development, but there are ways the journey could have been made.

Point is, you look at the modern Arctic fringe, and people do travel along the boundary regions, ice and sea. Whether the Atlantic or the Pacific, we have good evidence of how it might be done.

And 8 miles per year seems a very low figure. What's it based on? If it's based on distance and the dates of archaeological sites. I'd want to look closely at the error bars on the dating.

Anyway, if the people didn't know how to live on the oceanic edge of an ice sheet, it's a bigger into-the-unknown than almost any fictional space voyage.

And if the ice-sheet were to recede a bit, expose some coastal land, and then return, any evidence of intermediate settlements will vanish. Lower sea levels too.

76:

Wooden spoon with crossed eyes to "me" @53.

Robert @61: that's a good point. But it requires us to contemplate how much of our extended phenotype we can pack down for travel in a (hypothetical) interstellar tin can. At this point the number of variables in the thought experiment explodes, so I'm leaving that question for another time.

77:

Alex, the point of the thought experiment is not "let's design a probe for determining whether an exoplanet is inhabitable" but "to what extent are we optimized for survival in a restricted environment?"

For example, I picked the survival filter for the meat probes as being 24 hours or one diurnal period (we know damn well that the diurnal period fn other planets may vary by orders of magnitude from our own baseline) because human beings tend to die of exposure in -- if we can survive unprotected for more than 24 hours, the environment is probably friendly (barring subtle threats -- incompatible biology, heavy metal poisoning, radiation, and so on).

If I'd set the filter time at 5 seconds, the thought-experiment would tell us a lot less; by that yardstick, the Moon or Mars are "survivable". Without a space suit.

If I'd set the filter time at two centuries, nowhere is survivable ... because unmodified humans don't live that long.

You've got to pay attention to how the parameters of the experiment are rigged. (And what they tell you about the experimenter's interests and expectations.)

78:

Some thoughts about the alien bacterium/fungus thing.

What you need is something (probably a bacterium or fungus analogue) which excretes some kind of generic protein chomping enzyme to digest complex proteins and turn them into simpler stuff that the organism can use to get energy and to reproduce. If the reaction conditions for this enzyme are fast enough then your hypothetical meat probe turns into a pool of goo crawling with new copies of organism in a few minutes.

If this organism was (say) only around on a particular continent then the colonists would be in great shape as long as they colonized the other one(s).

79:

FrancisT: "generic protein chomping enzymes" are a bit of a tall order, for several reasons.

(1. What's the energy source it uses to cleave those peptide bonds? (Yes, you specified a rapid reaction. Really slow is entirely do-able via hydrolysis without any external energy input (which is how proteases generally work). Fast is something else.) 2. More to the point, how does the organism that excretes it avoid being chomped to death? 3. How does the enzyme handle self-recognition (so that individual molecules don't chomp each other up first, before they get to the target)? 4. How did the universal chomper evolve ...)

Also: bacteria and fungi have an annoying habit of spreading far and wide on the wind and the tide.

As it is, we have plenty of very limited protein chomping enzymes (go google on the function and purpose of lysosomes if you want to know more), but what you're asking for is the protease equivalent of a magic swiss army chainsaw with no "off" switch.

80:

Lars @40:

The reference is "Oceanic Euxinia in Earth History: Causes and Consequences", Annual reviews 2009, doi:10.1146/annurev.earth.36.031207.124256
Basically, people confuse anoxic, methanic, swamp-like conditiions with the euxinic Canfield oceans. Oceanic euxinia (sulphide, rather than oxygen-, dominated ocean) is caused by nutrient buildup and trapping: that can be due to lack of oxygen reaching the deep ocean due to a greenhouse, but the ocean circulation is still vigorous.

Back to Charlies point, should lower tidal mixing mean no deep circulation, it doesn't necessarily mean a Canfield Ocean under Green skies: that depends on how much nutrients (especially phosphorus) reaches the deep oceanm due to weathering, etc. I would still think its bad news: we'd be waiting geological timescales for nutrients to be subducted into the mantle and brought back to the surface as rocks.


Chris L @44: Yes, the N Pacific is fairly irrelevant for the THC; the question is why; after all, it is the largest ocean. There has been a body of work on what alternate circulation patterns can take place, including deep circulation patterns that go via the Pacific instead. Changes in ocean circulation can help explain the glaciation events such as Dansgaard-Oescheger events, but they normally involve sinking at the Antarctic instead of Greenland driving the circulation.

My interest is what all this means for Ocean Planets, where there are no continents ...

81:

1) I highly recommend Ward and Brownlee's Rare Earth. (http://books.google.ca/books?id=SZVV26vCSi8C&dq=rare+earth&source=gbs_navlinks_s) It's mentioned by comment #6 but it needs another advert. It basically sets up the series of events that had to happen for metazoan life to exist on this planet. Whether all these events would have to happen on an alien world (e.g. is a moon necessary for advanced life) are at current merely thought experiments but still it's a great book for realistically trying to give feedback on how many extrasolar planets out there have life that we can see without a microscope.

2) _H. Sapiens Sapiens_ is bad taxonomy for 2 reasons. 1/2, you shouldn't abbreviate a genus without first using the whole genus once in your publication. I'm dockin you half a point here because everyone reading this is actually a member of this particular species and thus figured it out.
1, don't capitalize anything smaller than genus level.
the other 1/2, and I'm sure there's anthropologists out there who would disagree with me here (and wikipedia does too, but, eff them), but, until someone does some genetic testing to prove that there's a fossil hominin out there who is also a subspecies of _H. sapiens_, I think _H. sapiens sapiens_ is a redundant trinomial. Just go with the basic binomial until the need for the trinomial is categorically shown.

...anyway, everyone here is already rapping you for other science bits, so I'm not going to. The point of your thought experiment is laid out in the last paragraph but everyone seems to be hung up on the "but yeah what if the meat probes had spacesuits" issue. (okay, maybe that wasn't everyone, maybe that was just me at first.)

The truly horrifying result of your thought experiment is anyone who gets to the end of it, realizes the point, and then looks at people who are fighting against unchecked greenhouse gas emissions as if they are loons. Humans and the organisms we use for food have spent our entire existence living within a biosphere based on a particular range of atmospheric CO2 levels. That we humans are going above that range ...and realizing we are going above that range ...and not really caring ...boggles my brain. The people who run the world are basically intentionally poisoning our own well, while arguing that we can't stop poisoning the well because it's too expensive.

Anyway, good thought experiment. Thanks for giving me a good distraction from work for a short while.

PS to #23: I would love to take a walk on a Triassic beach. Atmospheric oxygen levels were high enough for mammals and dinosaurs to exist, which leads me to think that it also would have been good enough for humans. A bit low probably but survivable.

82:

Charlie @80:


(1. What's the energy source it uses to cleave those peptide bonds? (Yes, you specified a rapid reaction. Really slow is entirely do-able via hydrolysis without any external energy input (which is how proteases generally work). Fast is something else.) 2. More to the point, how does the organism that excretes it avoid being chomped to death? 3. How does the enzyme handle self-recognition (so that individual molecules don't chomp each other up first, before they get to the target)? 4. How did the universal chomper evolve ...)

1) I didn't specifiy a fast reaction. I just said it would be worse with a fast one but even if slow it could be nasty. We flat out don't know enough about enzymes to state that no enzyme can do a basic hydolysis reaction fast.
2) I haven't thought this through enough to say for sure but one possible approach would be if the proteins in the organism's skin were of an opposite chirality around critical points.
3) Chirality works here too. If the enyzme and the organism are chirality X in the critical point and the "prey" (and humans) are chirality ~X then the enzyme only works on chirality ~X bonds
4) How it evolved. Assume our new planet is by default the opposite chirality to earth (or of mixed chirality). I'm willing to defer to a biologist but my recollection is that there seems to be no reason why earth's life is based around one particular chirality. NOw this enzyme that turns out to smack all earth chirality proteins (or most...) might turn out to also catalyse a particular opposite chirality reaction that is less serious in terms of how it affects life. It evolved the way other enzymes do (i.e. trial and error / god / magic) in order to facilitate this other reaction.

83:

@82: you hinted at it.

Quote:
Although D amino acids can be found in nature, only the L isomers are used to form proteins. The D isomers are most often found attached to the cell walls of bacteria and in antibiotics that attack bacteria. The presence of these D isomers protects the bacteria from enzymes the host organism uses to protect itself from bacterial infection by hydrolyzing the proteins in the bacterial cell wall. (http://chemed.chem.purdue.edu/genchem/topicreview/bp/1biochem/amino2.html)

My previous comment got eaten, so I've kept this short.

84:

Too short alas: I meant to say that incorporating amino acids of opposing chirality into proteins would twist them out of shape and render them non-functional. D-amino acids are toxic to most lifeforms and we have special enzymes to deal with them.

85:

artiofab @81:
"Humans and the organisms we use for food have spent our entire existence living within a biosphere based on a particular range of atmospheric CO2 levels. That we humans are going above that range ...and realizing we are going above that range ...and not really caring ...boggles my brain."

If you want to really freak yourself out, try to realize that the 380 ppm CO2 level is the LOW level most humans would ever have seen, and that we daily put ourselves in indoor environments with drastically higher levels of the stuff. The marker for "adequate" ventilation is 1000 ppm. The OSHA limit is 5000 (yes, five THOUSAND) ppm for extended periods, and 35,000 ppm for short periods.

Most organisms on the planet aren't that sensitive to noticeably higher levels of CO2 (except plants - they love it). Over that short 580 million year history of "livable times" on Earth, the CO2 levels were usually well over 1000 ppm. The current "high" 380 ppm is near the low end of what's been seen on the planet for most of that time.

86:

"Why aren't there any mammals with green fur? ... there's nothing contra-survival about having green fur... but due to reasons of historical contingency, those features simply aren't found among the modern class of mammals."

Sure they are. Some polar bears have green fur from algae growing in the hollows of the hair roots, and it even happened to a kitten in Denmark the other year. Not to mention certain bipedal mammals you often see in urban environments...

87:

Chris@:72

Fibonacci spirals and hexagonal cells both make maximum use of available materials, and there IS a survival advantage (or rather, a fitness advantage, the two not quite being synonymous) in that. I'm an unabashed adaptationist, but I'm entirely prepared to agree with you on contingency and the role of chance. As long as we remember that evolution isn't a random process.

Care to tell me how Fibonacci spirals make maximum use of materials? But whether they do or not is really irrelevant; what matters is whether or not selection pressure was ever applied. Take bunch of straws and squeeze them. Their cross-sections become hexagons. And you didn't need to do any figuring to get them either. Now, while it may be true that hexagons solve a particular min/max problem, it's not necessarily true that the organisms which incorporate them experimented with other forms before as a matter of expediency settling down to the 'best' one. And there's a lot of that sort of thing going on with Earthly life. Note that there's a lot of information necessary for making another living thing which simply isn't incorporated in it's parents genes(contrary to what was thought several decades ago.) So there's no way you can get selection pressure to operate directly(that's freighted with all sorts of caveats, I'm afraid) on those particular characteristics.

Your remark about evolution not being 'random chance' is interesting. What do you mean by this? It was my impression that this is very much the case, for various values of 'random'.

88:

Actually, there are a couple mammals reported with green fur. One I recall is the green monkey Chlorocebus sabaeus and there's a species of galago anomalure--at least as reported by Ian Sanderson, who was trapping them. They all seem to be confined to arboreal habitats in tropical Africa. If you stretch it, sloths normally have green hair through algae and as noted in 86, polar bears get green fur too.

Probably there's a biochemical/evolutionary reason why mammals don't normally produce green hair, and it tends to show up in groups that also produce blue skin (i.e. primates). Thing to remember is that when you look at the vertebrate tree of life, the split between mammals and birds/reptiles/crocs/turtles is a big, deep, dichotomous branching. When we're looking at green vertebrates above the level of amphibians (i.e. herps and birds), they all share a common ancestor that doesn't include us. So if they're green and we're not, a good answer is that early on, our clade lost the ability for some reason, and only re-evolved it a few times in specialized cases.

This is analogous to the question "why does Australia have so many poisonous snakes?" The answer isn't something weird about the Australian climate. It's simply that one of the first snakes to get to Australia happened to be a poisonous elapid, and it's descendants have undergone an adaptive radiation to take up niches that, on other continents, are filled by less poisonous or non-poisonous colubrids.

89:

My bad for not reading carefully, but Charlie: 12 THOUSAND years to settle remotest Oceania? Try 1,200 years or less, unless you believe in the Lost Continent of Mu.

90:

"You've got to pay attention to how the parameters of the experiment are rigged. (And what they tell you about the experimenter's interests and expectations.)"

Well, apparently Charlie is interested in bombarding the Earth with decorticated meat puppets. Kinda squicky. Is it a fetish thing?

That said, I guess we need to thank the Israelites for stopping the Manna colonization.

91:

It is a bit weird - especially if all those meat sensors were clones of Charlie.

You have to wonder what the $Stross_Habitable_Planet_Probe_v0 is up to when it's dropping hundreds and thousands of naked clones of it's creator all over random planets around the galaxy

Methinks there's an ulterior motive afoot!

92:

This is a somewhat interesting commentary, but it is not remotely plausible. It serves only to point out to people who must be surprisingly ignorant that our concept "Earth" is mostly a result of our preconceived notions.

No probe is going to drop "meat puppets" on a planet to determine viability. That's a wholly absurd concept that is mostly only worthy of a chuckle. I'd call this satire except satire is usually humorous.

The proper target for this sort of analysis is not whether earth is "habitable" but should instead be pointed at Global Warming alarmists to demonstrate how absurd their notion of "Preservation" of our current "climate" truly is.

93:

@87: Your remark about evolution not being 'random chance' is interesting. What do you mean by this? It was my impression that this is very much the case, for various values of 'random'.

Sigh, common pitfall. Selection pressure is directional and decisively non-random.

94:

Then again, perhaps the Earth was carefully designed and positioned exactly where it needs to be for life to flourish.

Perhaps it began as a perfect garden, protected from harsh solar energy by a water filter atmosphere creating a plush greenhouse environment the world over.

And perhaps something happened to destroy that initial garden and leave us with the world we have today.

Or you could, with confident erudite surety and Whitey Herzog-Bill Russel diagrams, simply speculate what it was like billions of years ago and what it will be billions of years in the future.

More importantly, can you tell me with the same surety what the weather will be next month?

95:

Koblog, CosmicConservative: whoever you are, you get the jester's crown award for unintentional (probably) humour.

Have I been slashdotted again, or something? (I know about the io9 news item.)

96:

Robin@91: Maybe, but I would really worry if it were mobile phones that were dropping from the sky.

97:

"We H. Sapiens Sapiens appear to be an infestation on this planet"

After this risible attempt at some kind of thought, I have to wonder why Instapundit linked you. I have no idea if this goes anywhere, but you should think about passing on cliches as lead in.

98:

@ 56 ScentofViolets

(shrug) We have a good reason to think oxygen is necessary for multicellular life in energy requirements and redox potential. We have another good reason in that we have a long unicellular period in a reducing atmosphere followed by a quick flowering into multicellularity in an oxidizing atmosphere.

The desire to understand why a trait "confers survival points" (not exactly an issue here anyway as unicellular and multicellular life are both life, and unicellular is actually arguably more "successful" than multicellular) is hardly a bad thing. You can tell a good theory by how much observed fact it explains.

But okay, you don't buy it. (shrug) Good luck finding that multicellular life that uses something other than oxygen as an electron acceptor. If you find it, it will be fascinating. Write and let us know how it went!

@ 58 Chris L.

All multicellular life is eukaryotic. Not all eukaryotes are multicellular. Think yeast, for example. Eukaryotic just means it has a well-separated nucleus; most eukaryotes have organelles as well, including mitochondria (hijacked bacteria that use oxygen to produce energy) and in the case of plants, chloroplasts (highjacked bacteria that use energy to produce oxygen).

Eukaryotes show up about 2 billion years ago, so they were around for about 1.4 billion years in a reducing atmosphere before life went multicellular in the fossil record.

I have to admit that when I think about bacteria and unicellular eukaryotes I think about terrestrial conditions. I had not considered the possiblity of energy gradients in situations that differ wildly from those of earth. I can't eliminate the possibliity that something other than oxygen might work in Jupiter's slush--it might be possible to eliminate that possibility but you'd better consult a real chemist :-)

@ 63 heteromeles

I do respect the chemical abilities of fungi. I happen to respect the chemical abilities of bacteria even more; I've never heard of a fungus that secretes sulfuric acid, for example.

However it's also the case that terrestrial digestive enzymes are "targeted" to particular short sequences (usually only 2 amino acids long). For example trypsin cuts proteins between lysine and proline and between arginine and proline. Pepsin is a bit less choosy; it cleaves after phenylalanine, tryptophan or tyrosine. But the point is that terrestrial digestive enzymes don't just cut anything, anywhere--they cut particular sequences found in terrestrial proteins. One might argue that non-terrestrial digestive enzymes would be different--but the producer of the enzyme has to continue living with the enzyme around, so there is going to be some kind of limitation on the enzyme or you get into the "what container do you keep the universal solvent in" problem.

Also, it's a bit of a jump (only a bit of a jump, though) to assume that a non-terrestrial fungus-like organism would even use proteins--let alone that their proteins would use the same set of amino acids, with the same chirality (handedness) as we do. Ditto enzymes to dismantle polysaccharides (do they even use poly saccharides? The same ones as we do?) or degrade fatty acids (see above for questions).

I agree that fungi can chemically affect all sorts of weird, in some cases non-biological, substrates. But they are substrates the fungus has had around for a while. If our molecules don't closely resemble something already in the environment of the non-terrestrial "fungus" I wouldn't expect them to be able to digest us.

Plus, if you're only looking for 24 hour survival of the meat probe, it's a rare fungus that will kill in a day.

99:

As a side-effect of H. sapiens and the ecosphere co-evolving, our "human nature" has changed rather quickly at least twice before, and (as you show in fiction) is changing even faster right now.

Marc Hauser, the author of Moral Minds and other books, Professor of Psychology, Human Evolutionary Biology, and Organismic and Evolutionary Biology at Harvard University, in “The Origin of the Mind”, Scientific American, September 2009, pp.44-51.

Hauser suggests that Evolution by Natural Selection of the human brain, made a sharp change somewhere between 800,000 and 45,000 years ago...

During a critical phase of “the stone age” -- the late Lower Paleolithic period (between 400,000 and 200,000 years ago), people hunted and shared meat differently than they did in later times. Instead of a prey's carcass being prepared by just one or two persons resulting in clear and repeated cutting marks — the forefathers of the modern butcher — cut marks on ancient animal bones suggest something else. It was the chaotic introduction of different rules of the game.

The cut marks that multiverse archaologists are finding are both more abundant and more randomly oriented than those observed in later times. What this could mean is that either one person from the clan butchered the group's meat in a few episodes over time, or multiple persons hacked away at it in tandem. This finding provides clues as to social organization and structures in these early groups of hunters and gatherers.

Among human hunters in the past 200,000 years, from southern Africa to sub-arctic Canada, there are distinctive patterns of how people hunt, who owns the products of the hunt, how carcasses are butchered and shared. The rules of sharing are one of the basic organizing principles of hunter-gatherer cultures. From 200,000 years ago to the present day, the patterns of meat-sharing and butchering run in a long clear line. But in the Qesem Cave, something different was happening. There was a distinct shift about 200,000 years ago, and archaeologists and anthropologists may have to reinterpret hunting and meat-sharing rituals.

Meat-sharing practices can tell present-day archaeologists about who was in a camp, how people dealt with danger and how societies were organized. The basic logic of butchering large animals has not changed for a long time. Everyone knows how to deal with the cuts of meat, and we see cut marks on bones that are very distinctive and similar, matching even those of modern butchers. It's the more random slash marks on the bones in Qesem that suggests something new. Qesem, which means "magic" in Hebrew, was discovered seven miles east of Tel Aviv. [Stiner, 2009].

Mary C. Stiner, Ran Barkai, Avi Gopher. Cooperative hunting and meat sharing 400-200 kya at Qesem Cave, Israel. Proceedings of the National Academy of Sciences, 2009; 106 (32): 13207 DOI: 10.1073/pnas.0900564106; http://www.sciencedaily.com/releases/2009/10/091014111547.htm

100:

@98: Cat, true on that. I've looked more at the non-protein degradation from fungi. To use one example, the ability of oyster mushrooms to break down things like nerve gas and other contaminants appears to derive, not from their co-evolving with nerve agents, but from the fact that these chemicals are similar chemically to wood components which the fungi did evolve to break down.

I'm a pretty conservative type, and my first bet is that if Charlie's meat puppet could survive 24 hours in an environment, then the organisms around it would probably be built with amino acids fairly close to ours. This is just good old parsimony in chemistry clothing. That said, I would expect the local protein scavengers to attempt to break down the meat puppet. It's possible, probable even, that it will take longer than 24 hours, but in the longer term (i.e. colonization), these microbes will probably be a bigger problem than, say, alien viruses.

101:

Disappointed in somewhere-or-other @97: paper runcible spoon award for poor reading comprehension, aggravated by distraction.

NB: I refer you to the Blog Policy FAQ and the moderation policy before you post again.

102:

Interesting piece.

"Most paleoclimatologists think the cold episodes had something to do with the formation of the supercontinent Rodinia. Because Rodinia was centered around the equator, rates of chemical weathering increased and carbon dioxide (CO2) was taken from the atmosphere. Because CO2 is an important greenhouse gas climates cooled globally."

Most paleoclimatalogists draw their paychecks from the notion CO2 is VERY VERY important. Ologists of genus geo and other fields that are apolitical and old enough to drink tend to roll their eyes at the notion varying ppms of CO2 have much discernible effect on Earth's temperature -- and the ERBE data suggests the GCMs are full of smelly brown stuff on this point.

It's much more likely Rodinia glaciated the same way Antarctica did -- by moving over a pole.

In the Cryogenian period the Earth experienced large glaciations, and temperatures were at least as cool as today. Substantial areas of Rodinia may have been covered by glaciers or the southern polar ice cap.

http://en.wikipedia.org/wiki/Rodinia

103:

"Ergo: to a space probe searching for somewhere that our kind of life can thrive, a truly random sampling of the Earth's surface (distributed over both time and area) would probably result in the conclusion that the planet is uninhabitable."

One implication is that any colonising species won't look for somewhere their kind could thrive. Instead, they'd build it. Anybody who would choose to live on the surface of a random planet would be looked on as as quaint and strange as we'd look on someone who chose to live homeless in the woods.

104:

@89

Not 12,000 years to settle Oceania; farthest Oceania settled by 12,000 years ago.

Regards
Luke

105:

As others have noted, the kind of technical background, infrastructure, and economy required for interstellar exploration implies the ability to make appropriate habitat from very raw materials. Also, interstellar transport is rather expensive. So explorer probes are probably not looking for real estate.

However, a viable alien biosphere would be a scientific bonanza, and littering it with brainless Strossbodies would be horrific vandalism against the science of biology.

From this we can conclude that our fine host is some form of alien creationist, trying to pollute our pristine Earth DNA with his own replicators in order to discredit biologists on his home world.

We're on to you Mr. Stross, if that is your real name. If I see one of your decerebrated clones in my yard, it's going straight into the incinerator, and I will additionall apply liberal quantities of HF to the area your biological pollution has contaminated.

106:

Martin: what, you're going to stuff my clone up your ass?

I think you'd better lay off the Bolivian marching powder.

107:

How did you know where I keep my incinerator!?

Also: LOL

108:

Now, this would never happen naturally, but what if a planet was engineered to have large quantities of FLOURINE instead of oxygen? Even more oxidizing, so you can get more energy out of it, right? Of course, the whole tearing apart organic molecules thing is a downer, but oxygen does the same thing, IIRC.

(All this talk of redox potentials...)

109:

I'm alive, healthy and breeding successfully. It must be an illusion.

110:

@103 So Luke, when do you think Hawaii and Aotearoa were reached? Charlie's "around twelve thousand years ago" is a slip for "around twelve hundred years ago" or "around one thousand years ago".

111:

Thanks for the reference, Alastair. Should be in our library.

112:

Charlie and @ 81 surely it should be Homo sapiens sapientes africanus ???
[ Man who thinks he thinks, who came from Africa ]

@ 98 - a fungus that will kill in a day?
Try Amanita phalloides or A. virosa , for that matter!

@ 102 like "The Culture" in Bank's novels then? Making orbitals etc....

@ 103 maybe Aus was settled 12 000 years ago - but what about the Land of the Long White Cloud (Aotearoa)?
It was not settled - by seagoing canoes, from the EAST and NORTH until about 1000AD

113:

Very interesting post. Having grownup on Isaac Asimov I'm pretty familiar with the uniqueness of thalassogens and the chemical propinquity of oxygen, and all the rest, but our history underscores that in fact the planet is habitable for humans only in some senses and in some places and even then the human animal needs to be far more resiliant and durable to really thrive, or why would it have taken so long to reach the kind of numbers we have. Without a significant amount of technological knowhow we wouldn't have gotten far and the real question is why stop now?
Here we are with our population exploding and resources in quesion. It'd be a pity if we went backwards when we are so close to breaking out of this convenient nursery planet with all its opportunities and limitations. Now is the time to use the technological power we have harnessed to create for our species the opportunity to travel and thrive in space where resources are abundant beyond our earthly experience and the human craving to expand can be expressed not just until the next big asteroid visits, or our arm of the Milky Way enters a dusty patch and freezes the oceans, but for a time frame that is consistent with the time frame which includes our sun and its neighbors. Abandonning carbon, ignoring nuclear fission, shortchanging fusion and hobbling our efforts to develope space with cold war era balistic missiles as the means of transport which we know is too darn expensive to ever open the door wide enough for practical purposes when we know that even at its worst we can continue to power our civilizations current energy intensive ways for another few hundred years, by which time, unless we fail to pull our heads out of the sand, we'll be using solar from space, fusion from the abundance of hydroge,mining the asteroids for elements that are now so precious or dirty we can barely work with them here but in asteroids being processed in the vacuum and zero-g of space they will be plentiful enough to be used everywhere. Diamonds, platinum, rare-earth elements as well as frozen hydrogen di-oxide and diesel fuel if we still have a use for it.
Let's hope those who wish to see us in a less energy intensive civilization are reconginzed as being unrealistic and irrelevant and are ignored, and that those who see that the path forward and upward is dependent on our using powerful and dense energies are encouraged, supported and successful. We won't be going out to into space powered by windmills and by switching to CFLs, and while we're here on this barely habitable planet we are subject to the vagaries of weather, climate, resource shortages and the inevitable visitation by a genuinely civilization-ending bollide that already has our address stamped on it, even if we can't see it, provided we don't kill ourselves off before all that over the appearance of shortages. Now that is the kind of faith that needs spreading if humans truly want to join the community of intelligences that make it beyond the sad imperitive of our biological destiny.
When the kind of scaling that has happend with computers happens to our energy industry, as it will when fusion and/or space based solar is developed, a radical change will occur in our civilization, and the result will be, for those who would love to see Earth as a paradise, a period of time when we will wield the kinds of energies and capacities to make it so.

114:

@93:

Sigh, common pitfall. Selection pressure is directional and decisively non-random.

What does this mean in this context? The word 'random' is taking a beating with lead pipes here it seems. Are you saying that if we do a rewind and reset back to 50 million years ago and then let evolutionary processes play out again that everything evolves into the species we see today? I don't think I would agree with that, even though the 'selection pressures' would be the same the second time around. This, incidentally, was a problem Darwin wrestled with all his life, finally distinguishing between random environmental exigencies and sexual selection pressures(iirc.)

115:

FYI, current O2 content of the atmosphere is 20%, not 16%.

I think your criteria are unreasonable, too. For example, while it's true we ourselves would die if dropped into the ocean, it's also true that something very much like ourselves could have evolved to live pretty comfortably in the ocean, or in very close proximity to it. Desmond Morris thinks we actually did evolve as littoral waders, for what it's worth.

And the oceans are of course far more moderate places to live than the land masses, relatively unaffected by such things as glaciation. There's not even any obvious need for oxidative respiration. It's certainly convenient, and high energy, but, again, as long as there's liquid water, it's not obvious something like us couldn't evolve in a reducing atmosphere. The only thing oxidative respiration gets you is a much more efficient combustion of your energy sources. So reducing-atmosphere beings have to eat more, and more often, is all.

I don't think anyone looking for intelligent life would restrict himself to the parts and times of our own planet when we ourselves, one particular species, are comfortable. Any planet with liquid water will probably do.

Furthermore, even by the worst statistics you've got (e.g. a 1% habitability probability for our planet), the Fermi Paradox is unresolved, because of the *enormous* number of stars in the Galaxy (at least 100 billion). Plug even your worst numbers into the Drake equation, or something like it, and you should *still* have, if life evolves as readily as we imagine it does, and civilizations last as long as we hope they do, several hundred, if not several thousand, of radio-emitting civilizations in the Galaxy, which means we should have heard of at least one by now.

116:

@113 Are you saying that if we do a rewind and reset back to 50 million years ago and then let evolutionary processes play out again that everything evolves into the species we see today?

Nonsense. But there would be broadly similar species in broadly similar habitats, depending on which way the climate swings. Thinking back further, who knows if mammals would have taken the dominance they have after the demise of the dinosaurs? If the impact hadn't happened, for all I know we would have intelligent troodontids today. Or no intelligent (technological) species at all. But the beasties would still be broadly recognisable.

117:

@111: Nope, Amanitas will kill you in about 4-5 days, presuming you are human (other animals like cats die much faster). I think the one you're looking for is Gyromitra, as Gyromitrin hydrolyzes into monomethylhydrazine, which will kill you in 24 hours if you ingest enough.

There's always the possibility of being killed by something you ate, but the more interesting question is what can eat you if you don't share the same suite of amino acids.

118:

What would a meat puppet think of the Berkeley Pit in Montana? It's an Acid Lake but there is life (of a "simple" sort) in it. It seems given half a chance life can adapt.

119:

Cat@98:

(shrug) We have a good reason to think oxygen is necessary for multicellular life in energy requirements and redox potential. We have another good reason in that we have a long unicellular period in a reducing atmosphere followed by a quick flowering into multicellularity in an oxidizing atmosphere.

I'll see your shrug and raise you two (sighs). Your first 'good reason' hasn't been elaborated on here; your second, which prompted my original comment is nothing more than post hoc reasoning. If you've got something, by all means share it.

But okay, you don't buy it. (shrug) Good luck finding that multicellular life that uses something other than oxygen as an electron acceptor. If you find it, it will be fascinating. Write and let us know how it went!

Sigh. I didn't say I didn't buy it. This is exactly the sort of snarky and dishonest 'reasoning' that is the base of most people's gut loathing of libertarians and creationists. And this is not some sort of nasty political blog where you can pretend that any skepticism towards what you say is in itself a statement to the contrary and equally assertive. In reality, since you're the one making the claim, you're the one that has to put up. I don't have to prove one blessed thing. Since you think this is such a neat way to go, the proper formulation is for you to find another planet with multicellular life that uses oxygen. As you just said, why don't you go out and attend to that, and let us know what you find ;-)

Alternatively, you could make some sort of implicit admission that skepticism is in no way a contrary claim(Does God exist? I don't know, and I'm skeptical of any claim that one does, but that in no way is the same as asserting the nonexistence of God), and try to make an honest case for your beliefs. If one of my students tried the stunt you just tried to pull, he wouldn't just receive an 'F', he'd be marked down for a zero.

120:

truth@108

Now, this would never happen naturally, but what if a planet was engineered to have large quantities of FLOURINE instead of oxygen? Even more oxidizing, so you can get more energy out of it, right? Of course, the whole tearing apart organic molecules thing is a downer, but oxygen does the same thing, IIRC.

As I've said once already, finding out these sort of things is one of the best reasons for interplanetary and interstellar exploration. Campbell had beings who breathed hydrogen, positing that it doesn't matter which way you go. Smith had whole galaxies infested with evil chlorine breathers. Are such things likely? Possible? I don't know, and there's no real substitute for going out there and looking. It could turn out that only carbon-based life is possible, and that it only happens once every ten million Earth-like planets. Or life, complex, intelligent, tool-using life, is quite common, but it only seems to occur in humanoid form with alien women being strangely attractive to Tellurian men except for those odd bumps on their foreheads(even if they run on top of a silicon-based chemistry.) Or . . . well, take your pick. I liked the Wang's Carpets idea Egan had, as one nontraditional possibility.

121:

ScentOfViolets: issues with fluorine are (a) it's so electronegative that it's hard to free it (and remember, the free oxygen in our respiration cycle is produced for us by a barkingly complex electron transport chain in chloroplasts: what kind of energy inputs is fluoride-cleaving photosynthesis going to run on?), and (b) while I'm not 100% clear on this, some brief web poking suggests that its relative abundancy in the galaxy at large is around a tenth or less that of oxygen -- it appears to be formed by fusion reactions in rather large stars, and seeded via Type II supernovae.

Wearing my SF writer hat I'm going to take a wild-ass guess that stable fluorine ecosystems don't really get off the ground because they would need really high energy photons as input for photosynthesis, which tend to imply bright (and hence very short-lived) stars. Bit of a bummer if you've just gotten around to inventing the equivalent of chloroplasts when the sun goes supernova, what?

122:

I'd just point out that the evidence right now seems to be flowing against sudden oxygenation (http://www.sciencenews.org/view/feature/id/44350/title/The_iron_record_of_Earth%E2%80%99s_oxygen). The first oxygen-producing critters seem to have shown up around 2.316 billion years ago, and (apparently multicellular) eukaryotes may have started showing up around one billion years ago, although the fossils start showing up around 500 mya. Apparently, the molecular boffins now think that plants, fungi, and animals independently evolved multicellularity, which makes some sense based on their life cycles.

Another good point is that some prokaryotes (notably the myxobacteria) do form multicellular structures, and archaea form biofilms which aren't exactly multicellular...but aren't exactly unicellular either. I'm still looking for an example of obligately anaerobic multicellular critters in any domain. In this case, lack of evidence is proof of a sort. If there aren't any, why not? Why are the multicellular prokaryotes (to my limited knowledge) aerobic? That suggests that oxygen matters.

So what does this have to do with finding human-compatible biospheres? Possibly quite a bit. Adding a billion years or so of oxygenated atmosphere to a planet's history does skew the probabilities a bit, although eating cyanobacteria for a living isn't high on my list of great ideas (don't bother with the chloroplast argument, either, I'm talking about the wild, free-living cyanobacteria that are so good at poisoning things).

On the flip side, we've got that interesting question of why the prokaryotes didn't go into complicated multicellular structures, other than through eukaryogenesis.

123:

*59
Cool. IIRC, Habitable Planets is the heavy-duty version; the other is the somewhat-less-technical commercial version. (Still think it should be updated to add more recent developments.)

It gets into things like temperature, O2 partial pressure, and other habitability factors, even in the commercial version.

124:

One more piece of evidence about where humans originated comes from experiments wherein people are completely isolated from the outside world. When they arrange their days as they see fit, they always end up living days of a bit more than 25 hours.

125:

I can't believe so many people took Charlie's idea of dropping meat probes so literally, as if this is something alien civilizations should consider in the exact way it's described. Charlie obviously used the meat probe device for dramatic effect and to inject some dark humor into the piece.

126:

For what it's worth, the following weren't settled until the last few centuries even though they are less remote than Oceania, although some were known earlier:-

- Mauritius, Reunion and Diego Rodriguez (the Mascarene Islands in the Indian Ocean);

- Madeira, the Azores, Sao Tome y Principe and similar Portuguese discoveries or rediscoveries (in the Atlantic Ocean);

- the Cocos-Keeling Islands and one of the places called Christmas Island (in the Indian Ocean);

- various islands roughly between Australia and New Zealand, e.g. Lord Howe Island and Norfolk Island (maybe these count as Oceania, though).

Madagascar was only settled about two thousand years ago. The sub-antarctic islands are only arguably more remote than Oceania, but while some like the Falkland Islands have been settled for a mere couple of centuries, others like Kerguelen (more habitable than Iceland, let alone Greenland) haven't strictly speaking been settled even now as the populations have always been transient. And there's always Rockall to consider...

127:

@P. M. Lawerence, 126: Well, not only are the sub-antarctic islands about as remote as the Oceanian ones, they are also less habitable. Iceland and Greenland are quite large as well, and...well, quantity has a quality all its own, you know?

@Charlie, 121: Well, yeah fluorine is less common than oxygen (O is the 3rd most common element in the universe, after all). That's why I suggested that unidentified people made a world with lots more flourine than oxygen. Bit of a bummer to hear about the difficulty of reaction, though I really should have remembered that.

Unfortunately, there don't seem to be many good substitutes for oxygen in the electronegative (redox potential) sense. Nitrogen is too tightly bound, sulfur too solid, chlorine too rare, and most other elements all three!

128:

Didn't Hal Clement once propose a stage BEYOND an Oxygenating atmosphere - a Nitrating one??

Or was that just a McGuffin for the story?

Other off-the wall possibilities:
Turn it round, and use the other "side" of the table - except Beryllium is very rare, being a product of spallation IIRC.
Or one row down - which sustitutes Sulphur for Oxygen.
I do also wonder about Chlorine or Bromine as the "active" elements, since they are not so violent as Flourine, but under what conditions would you get such an atmosphere?
Since we know from our presently available samples, that other atmospheres are likely to be Ammonia / Methane type mixes.
Um.

129:

@13 charlie:
I was going to ignore your scouting party scenario, but then we encounter it again in the Blish story synopsis you referenced.

Why would a civilization capable of producing intelligent space-probes bother inhabiting planets? It looks like hard-sf, but sneeze and it turns into pixie-dust.

130:

Jeremy, you just got hooked on the metaphor.

The scouting scenario is a thought experiment intended to give us a framework for querying what makes a planet inhabitable ... and for looking at the Fermi paradox.

It is no more an actual proposal for planetary exploration than Erwin Schroedinger's contraption was a serious design proposal for a feline euthanasia machine.

131:

bleepless@#124, the 25-hour day thing is an obvious hack to allow for the various biological clocks (and there are a *lot* of them, in every cell: much gene expression shows a 24-hour periodicity) to reset themselves daily as hours of available sunlight vary over the course of a year. Bear in mind that the Earth's day has never been longer than it is now. I don't think we originated 100 million years in the future...

132:

@130 charlie: yeah, i can accept it just sets up a thought experiment here. i was reacting more to "surface tension" and the sense that i've encountered this conceit once too often elsewhere.

having said that, your response @76 seems to make the terms of the thought experiment beholden to this probe scenario, at the expense of a fairer consideration of our planets habitability.

133:

I have to wonder why Instapundit linked you.

I believe the reason why the vacuous, self-promoting, Internet-waffling-but-comments-off, torture-propagandising hack linked here must be some sort of regrettable accident.

More to the point, this is really the killer argument against the anthropic principle, not that it has ever impressed me as a philosophical argument. Over the long term, the universe has not been particularly suited to us, and in fact it's only just about OK now.

(Regarding fungi; yes, A. phalloides will do you a power of no good, but you've got to eat it first. Colonising creatures like us, and then killing them, which is the ecological role you'd worry about, doesn't seem to be a great niche for fungi on earth.)

134:

Charlie@121,

Massive, single stars need not be the only source of hard
radiation for a Florine ecosystem. Take two stars, neither more massive than a solar-type star and put them so they orbit about each other with a period of days to a few weeks and you can get one or both stars to emit far more UV and X-ray radiation than the the Sun. These binary star systems are the RS CVn type (http://en.wikipedia.org/wiki/RS_Canum_Venaticorum_variable) and they are not that rare. Since neither component is no more massive than a solar-type star, they are also long-lived. Perhaps sufficiently long-lived to get your Florine ecosystem going.

135:

Charlie@121: I see this as breaking down into two problem sets. The first is whether or not it's possible for a planet to form with the 'right' amount of flourine - which takes us into Anderson/Clement world-building territory. Life powered by a flourine-based oxidizing agent(it doesn't have to be elemental flourine, after all; Automobile batteries use PbO2/Pb for reduction/oxidation iirc) might be very implausible simply because no such panetary systems are possible. Or, if not impossible, don't last long enough, as you point out.

The other, more significant, problem is whether or not such life will ever arise, even given optimal conditions lasting billions of years. This to my mind is the by far more interesting of the two. I don't know either way of course, but it might well be that after examining hundreds or thousands of life-bearing locations, we see only a carbon/water chemistry, and multicellularity only energized by oxygen. No silicon beasties, no flourine or chlorine or hydrogen breathers, no chilly oceans of ammonia teaming with alien fish. No crystaline life, or gaseous life, nothing exotic at all unless explicitly manufactured to be so.

Otoh, given the optimal prebiotic environment, maybe silicon-based, flourine-breathing life is commonplace. At this other extreme - call them the pro-lifers :-) - you see people (again, we're all wearing our science-fiction hats, right) like Steven Baxter who posit a universe teaming with life in the Planck era prior to inflation, then life arising again in the quark soup immediately after inflation(each existing for aeons on it's own time scale), then again, and again, during each successive era - the Hadron era, the Lepton era, and so on and so forth down to the present day and presumably beyond, way past 10^100 years after the Big Bang.

Does life arise everywhere there's an energy gradient, low entropy, and a complex substrate? Is there life inside the Sun or in the atmospheres of gas giants, or in Black Clouds? I dunno. But I think it's worth the expense of a trillion or so, or a thousand trillion or so dollars to find out. Suitably amortized, of course :-)

136:

David:

With binary systems like that, don't you run into massive problems getting any planets to have a steady orbit for long, due to the orbital dynamics? It won't do the fluorine-metabolizers any good if their star system is stable but each of the planets gets tossed into one of the primaries or out of the system after a few million or hundred million years. IIRC correctly the only known stable positions would be the Lagrange points, which I think would be a mite close.

137:

Alex @ 133
They colonize us pretty well, and don't have to kill us to be successful.

138:

People, people. Almost everybody has flourine in their kitchens. The dangerous stuff is fluorine.

139:

Interesting article, thank-you. I just want to pick up on the anaerobic/aerobic respiration side of things.

As an energy source many bacteria use a wide range of energy sources, including elemental sulphur, ammonia, hydrogen, iron, and carbon monoxide. (They're all still carbon-based life-forms though). The thing about aerobic respiration is that its so much faster and more efficient that it makes big organisms much easier to evolve.

Initially oxygen was released as a waste product by organisms about 2.5 billion years ago, and gradually the quantities began to be significant- but only in the seas, at first locking up the freely dissolved iron. Eventually the ocean ran out of iron, and levels in the seas began to rise- at this point oxygen becomes a real problem that must be dealt with. Colin Tudge, in "The Variety of Life" suggests that the first eukaryotes might have originated when an anaerobic archaean kidnapped an anaerobic bacterium in order to deal with the oxygen that was being pumped out by all the photosynthesisers. Eventually (at 580-odd mya) the oxygen levels rise enough to give serious quantities in the atmosphere- but it takes a good hundred million or so years after that before there's enough oxygen to sustain a meat-puppet in any comfort.

I'm not aware of any multicellular obligate anaerobes, but there are lots of anaerobic bacteria and archaeans, and aerobic respiration (or at least techniques to deal with free oxygen) have evolved many times independently.

140:

"By the time I got through reading that manual, I didn't see how anybody could stay alive anywhere at all, let alone in a spacesuit."

;)

141:

Dave Bell @ 75:

8 miles per year seems slow to me because I know damn well I'd walk faster than that to get someplace warm. OTOH, the figures that I recall for mass migrations in the last few thousand years in western Asia and Europe (e.g., the original Celtic and Germanic tribes) were on the order of 100 miles per century.


Clifton @ 136:

Systems of binary stars of approximately the same mass don't have Lagrange points. The Lagrange solution to the 3 body problem is only stable if the masses of the 2 larger bodies have a ratio of 1:25 or greater.

142:

Well, considering this scenario as a tought experiment to decide what makes a planet inhabitable, I would say that the answer, more than the planet, is the intelligence and activity of the probe itself: take those same probes, give them average human intelligence, and the chance to choose about 10 Kg of at least stone-age level equipment, and most of them (not all) will manage somehow to survive the full 24 hours... if you give them more intelligence/skills and access to more high-level equipment, the parameters could grow quite a bit.
By the way, as a side consideration, all this talking about the fragility of our current biologic configuration in the face of such a variable environment, is making me wish that we'll become able to make ourselves a bit more resistant as soon as possible (I remember an old Isaac Asimov novel about some robot probes sent on Jupiter to establish contact with an hostile civilization...)

143:

Charlie @ 121:
[fluorine's] relative abundancy in the galaxy at large is around a tenth or less that of oxygen -- it appears to be formed by fusion reactions in rather large stars, and seeded via Type II supernovae.

There are three proposed sites of fluorine formation -- Type II supernovae, Wolf-Rayet stars (very massive), and asymptotic giant branch (AGB) stars (not very massive), with stellar winds carrying away the fluorine in the latter two cases. It isn't clear which of these is most important.

In any case, the abundance of fluorine is closer to one ten-thousandth that of oxygen.


Wearing my SF writer hat I'm going to take a wild-ass guess that stable fluorine ecosystems don't really get off the ground because they would need really high energy photons as input for photosynthesis, which tend to imply bright (and hence very short-lived) stars. Bit of a bummer if you've just gotten around to inventing the equivalent of chloroplasts when the sun goes supernova, what?

Actually, you can get high-energy photons from the active chromospheres of low-mass stars (M dwarfs); these put out proportionally more extreme UV and X-rays than does a star like the Sun. I think the low abundance of fluorine is really going to be more of a problem.

144:

Truth is life, please spell my name right.

I already pointed out that the sub-antarctic islands were arguably more remote than Oceania. But it's only arguably, and certainly some of them aren't. And the idea that size affects this is irrelevant, both because that wasn't one of the original criteria and because many of them are large. The ones I mentioned, the Falkland Islands and Kerguelen, have areas comparable to Wales.

145:

Clifton @ 136:

Planets orbiting around the binary will be stable, as long as they're at a distance of roughly twice the binary separation or larger (precise limit depends on the mass ratio of the binary and how eccentric it is). In the specific case of an RS CVn binary, the binary separation is much smaller than Mercury's orbit, so it's perfectly possible to have a planet in a stable orbit around the binary and still get lots of light.


Bruce Cohen @ 140:
Systems of binary stars of approximately the same mass don't have Lagrange points.

Minor nitpick: "Lagrange points" are the 5 locations where a test particle would remain stationary relative to the two massive bodies (in the limit of circular binary orbits). Whether they're stable or not doesn't affect the nomenclature. (You're correct about the condition for the L4 and L5 points to be stable.)

146:

#85... Okay, yeah, sure, I'll bite on your enviroskeptical commentary

1) "If you want to really freak yourself out, try to realize that the 380 ppm CO2 level is the LOW level most humans would ever have seen"

You are either mistaken OR have access to data that the world's climate scientists don't. Do you have a citation for your statement that 380 ppm is low for the last 150/200ky?

2) ", and that we daily put ourselves in indoor environments with drastically higher levels of the stuff. The marker for "adequate" ventilation is 1000 ppm. The OSHA limit is 5000 (yes, five THOUSAND) ppm for extended periods, and 35,000 ppm for short periods."

This is irrelevant to my point. I was talking about long-term ecosystem-wide atmospheric concentrations, not what is ok for humans for 8 hours a day, 5 days a week.

3) Most organisms on the planet aren't that sensitive to noticeably higher levels of CO2 (except plants - they love it).

Citation(s) needed. Exactly whether or not plants will "love" a higher CO2 world is still under discussion. Even if plants do love it, will they be the plants we _want_ to love it, e.g., will agricultural staples be negatively affected?

4) Over that short 580 million year history of "livable times" on Earth, the CO2 levels were usually well over 1000 ppm. The current "high" 380 ppm is near the low end of what's been seen on the planet for most of that time.

This is all, once again, irrelevant to what I said. The CO2 values during the Cretaceous (if known) have nothing to do with the more important question "Should we do something to keep CO2 values in the 'safe range' that has been around during the existence of agriculture?"

...which, of course, you initially said you think that that safe range is totally different than what climate science says, which I guess brings us full circle.

147:

Greg @112: Actually it should be Pongo sapiens, no subspecies, no special genus of our own. We're so closely related to the other great apes that the genus Homo was never anything other than the desire for us to be called "something special man".

...

SoV@114: Selection pressures wouldn't necessarily be the same the second time around, because so much of the selective pressure an organism experiences comes from other organisms. If you haven't encountered it already, the literature on the Red Queen hypothesis should give you a running start on the ideas. The reason Darwin came up with sexual selection was to explain weird stuff that clearly has a negative impact on an organism's chances of survival, like tails so long they slow you down and antlers so huge you can't walk between trees. The critical difference between sexual selection and natural selection is that fitness requires both survival and reproduction.

Developmental plasticity and epigenetic effects (your non-genetic influences on survival) can actually short-cut the evolutionary process. This was my PhD topic and I could bore you to tears with it. Google "genetic assimilation" and "Baldwin effect".

...

I have to share Charlie's bemusement at the reading-comprehension skillz evident in some of the comments here...

148:

@ 100 Heteromeles I do understand that fungi can break down compounds that are only similar to compounds they are adapted to break down. However I'm not as convinced as you are that 1) probe survival requires strong similarity in biochemistry--probes need clean air, reasonable temperature and humidity, and non-lethal terrain, but not digestible food: they're only there for 24 hrs. If Charlie meant that digestible food that can be harvested by a creature with no brain, I'm sure he'll feel free to correct me :-)
2) even if probe survival requires strong similarity in biochemistry--proteins can be quite different from each other. That's part of why the immune system works (mostly) in a targeted way instead of just going off at random, for example. While I agree that possibly by chance probe surface proteins would resemble something in the environment, I wouldn't expect that chance to be very high.
3) I would expect rapid, efficient breakdown of probe proteins to require that fungi be well adapted to breaking them down, not just kind of working on something that's kind of similar to something they know. Perhaps this is unreasonably stringent on my part, but that's where I stand at the moment.

@ 112 Greg You've gotten confused--the original threat was not fungus that probes might eat, it was fungus that might eat probes. In view of the fact that allowing the probes to stuff random material in their mouths would make them even less survival oriented, I think it's fair to assume that Charlie meant non-browsing probes unless he says otherwise.

@ 119 ScentofViolets

Wow. Boy did I get up your nose. I didn't intend to, and I'm not sure what went wrong.

It's not that I didn't use weasel words--at first I thought that might be the problem but I went back and looked at my first post, and it's full of "probably" and "if there was I'd sure like to know what it was using for energy" and "as far as I know" and "I suspect"...

I wrote this whole thing out using more weasel words before I checked back that far, but really, there's no point. If the weasel words in the first post won't satisfy you, then weasel words weren't the problem, so never mind.

I did include a few new details; I'll pick them out.

Oyxgen is a good electron acceptor because it's very electronegative (the most electronegative element but I'm not sure there aren't ions that are more so), and because both it and the product of hanging electrons on it (H2O) move easily though cell walls of terrestrial life--important for getting it in and out of the tissues where energy is generated.

Any extraterrestrial life would be fascinating for obvious reasons, and whether it resembled us or not. Multicellular extraterrestrial life that liked a reducing atmosphere would be doubly fascinating in no small part because I sure would like to know what it was using for energy.

If your problem is with how I have stated my ideas, please provide an example of how I could have stated them better. If your problem is with the ideas themselves, please provide an example of what you think the ideas should have been.

One last quick tip. I haven't compared you to anything nasty. Please don't compare me to anything nasty. Sound ideas can be supported without name-calling.

For that reason, if nothing else, I am glad that I am not one of your students. I got along well with most of my professors but they were all capable of setting me straight clearly, concisely, and in a professional manner.

@122 heteromeles
thanks for the link! This looks like an interesting article and I look forward to reading it (but later--it's after midnight here) :-p

You mention some of the ways that prokaryotes in nature begin to arguably approach multicellularity--bacterial slimes can be quite complex and populated with (as I understand it) many different species of bacteria (possibly with ecological relationships even?) Last I heard it was poorly understood but very interesting.

I don't know if anaerobes produce slimes... that might be one way to approach "multicellularity" that didn't involve oxygen if so. Not that it happened, as far as I know, on this planet, but if a sf writer was looking for a plausible way perhaps this might work. I wonder what kind of weird majority-rules type creature you would get? :-) Probably a small, slow one, I would guess, because of that energy issue, but it's interesting to speculate.

@ 131 Nix writes "the 25-hour day thing is an obvious hack to allow for the various biological clocks" and "I don't think we originated 100 million years in the future..."

Yeah. And I *sure* don't think we evolved on a different planet with a longer day. The chemical similarities between us and other earth life, the anatomical similarities between us and other earth life, the fossil record, they all make it pretty damn clear where we come from.

Though it is cute to speculate :-)

149:

Cat: re slimes, etc. Look up papers by a guy called Phillip Bell. He argues (with good evidence) that eukaryotes could have arisen by a symbiosis of multiple prokaryotes inside the fringes of a big membrane-bound virus. It's a very neat idea that explains some of the peculiarities of eukaryotic cells, and doesn't require the existence of something that's never actually been seen, to wit the anaerobic organelle-free eukaryotic predator-cell that's normal invoked for the endosymbiotic hypothesis.

I suspect the sentence above will read like complete gibberish to a non-biologist... sorry, guys.

150:

Speaking of life on a planet orbiting an M dwarf (Peter@143),
there is a very nice, not too technical review paper on the topic by Jill Tarter et al. at:
http://arxiv.org/abs/astro-ph/0609799v1

151:

artiofab: we had a drive-by link from Instapundit. The Instapundit audience then piled in. There's a high proportion of republibots there who hold some rather quaint theories about reality (probably because they get their science education from Fox News). Feel free to ignore them -- they've mostly gone, now.

Chris L: biology spoken here (at least by me, for rusy values of "biology spoken here").

152:

Clifton @136: there are a fair few planets known in binary systems, including a few where the system is sufficiently old that one of the stars has become a white dwarf. In fact, both of the two known systems where the planets orbit the common centre of mass of the system (P-type orbit) have likely experienced mass transfer between the stars at some point in the past, without destabilising the planets. This provides experimental verification of the long-term stability of such systems.

153:

149: That's...cool. I'd sort of intuitively imagined that symbiosis would be the easiest way to get from prokaryotes to eukaryotes, but inside a big virus? Sensawunda is called for.

137: Indeed they do, but a new kind of athlete's foot isn't what most of us think of as a risk to survivability on an alien world. One that EATS YOUR BRAIN, like the one that parasitises ants and makes them run up trees, well, that's different.

154:

heteromeles@122:

Another good point is that some prokaryotes (notably the myxobacteria) do form multicellular structures, and archaea form biofilms which aren't exactly multicellular...but aren't exactly unicellular either. I'm still looking for an example of obligately anaerobic multicellular critters in any domain. In this case, lack of evidence is proof of a sort. If there aren't any, why not? Why are the multicellular prokaryotes (to my limited knowledge) aerobic? That suggests that oxygen matters.

'Absence of evidence is not evidence of absence'. A while back there was a discussion about why there weren't any higher land animals lacking a rigid skeleton, say, supported by a system of tubes under pneumatic pressure. The answer - aside from historical contingency - was, how would you know they had ever once existed? The fossil record is far from complete, some organisms fossilize better than others, and our search is far from exhaustive. For that matter, we may have such evidence already but have misinterpreted it. ISTR that the forelimbs of anomalocaris were once thought to be separate animals in and of themselves, for example.

On the flip side, we've got that interesting question of why the prokaryotes didn't go into complicated multicellular structures, other than through eukaryogenesis.

If you're going with linkages between coincidental events, how about making the connection between complete and incomplete assimilation? Maybe the same traits and happenstances that led mitochondritic percussors to be incorporated without damage inside a cell are the same ones that enable cells to stick to together without reacting negatively to one another. I don't know. But that's one off the top of my head hypothesis.

155:

Bruce@141:

8 miles per year seems slow to me because I know damn well I'd walk faster than that to get someplace warm. OTOH, the figures that I recall for mass migrations in the last few thousand years in western Asia and Europe (e.g., the original Celtic and Germanic tribes) were on the order of 100 miles per century.

I'm guessing that this is an average and that the variance is considerably higher, say 100 miles/year. Individual groups might break off from the main one to establish new homes, but the direction of their migration could have been almost entirely random. They're almost as likely to travel in reverse as forward - you know, back in the direction of the good old days of my father's father, when he lived in the land of milk and honey, and when kids didn't sass their parents the way they do nowadays.

156:

Marilee@128:

People, people. Almost everybody has flourine in their kitchens. The dangerous stuff is fluorine.

Ouch! You're right, of course. So noted.

157:

#153
It doesn't actually have to eat your brain, or clog your lungs (a much more likely scenario for fatality). It can create enough raw spots to allow opportunistic infections instead ....

158:

ChrisL@147:

Selection pressures wouldn't necessarily be the same the second time around, because so much of the selective pressure an organism experiences comes from other organisms.

You mean that the selection pressures are - wait for it - random?

If you haven't encountered it already, the literature on the Red Queen hypothesis should give you a running start on the ideas. The reason Darwin came up with sexual selection was to explain weird stuff that clearly has a negative impact on an organism's chances of survival, like tails so long they slow you down and antlers so huge you can't walk between trees. The critical difference between sexual selection and natural selection is that fitness requires both survival and reproduction.

Darwin was concerned that environmental selection pressures were random and so there wasn't any apparent direction to evolution, which didn't seem to fit the observational facts. He finally came up with a kludge wherein some selection effects were essentially nonrandom - sexual selection in mates for example. I'm assuming you already know this, despite what you have said above(or maybe you didn't mean to make it sound like a contradiction.) You may assume in turn(he says drily) that I'm fairly well informed on the subject until shown to be otherwise - a hobby as it were.

159:

ScentOfViolets: The problem with going back to the land of your father's father is that your cousins are still living there, and probably wouldn't appreciate the additional mouths to feed. There's a *reason* your father left that land, after all.

160:

Cat@148:

One last quick tip. I haven't compared you to anything nasty. Please don't compare me to anything nasty. Sound ideas can be supported without name-calling.

For that reason, if nothing else, I am glad that I am not one of your students. I got along well with most of my professors but they were all capable of setting me straight clearly, concisely, and in a professional manner.

Looking back at what I wrote, I was quite clear. You (a) equated skepticism with holding a contrary position, (b) attributed to me a position I did not hold, (c) tried to shift the burden of proof on to me, and (d) insisted on an unreasonably high standard of proof. One that you object to yourself, I see. On the whole, I was both concise and clear as to what I objected to. Just like a good teacher should be.

And as a good teacher, I also use repetition: you need to support your ideas, not demand that other people prove you wrong. I see for example that you still have not elaborated on your 'good reasons to think oxygen is necessary for multicellular life in energy requirements and redox potential.' What are these good reasons to think that oxygen is necessary for multicellular life?

161:

Jay@159:

The problem with going back to the land of your father's father is that your cousins are still living there, and probably wouldn't appreciate the additional mouths to feed. There's a *reason* your father left that land, after all.

Exactly so. And that's why there is an overall migration away from this direction. I think to think of these situations like electron drift in a conductor connected across a source of electromotive force. IIRC the overall electron drift is on the order of centimeters per hour, even though individual electrons have speeds of many meters per second. So, per Bruce, it's quite plausible that any specific group of young disaffecteds might have travelled a hundred or more miles in a year before settling down while at the same time having the overall speed of colonization be no more than 8 mi/yr.

162:

ScentOfViolets @ 161:

Yes, those figures I gave were averages, AFAIK, although the second figure I gave was 100 miles per century, i.e., about 1 mile per year, not 100. I don't think the electron drift simile quite works, because the repulsive force between tribes is larger than that between electrons, the numbers are much lower, and the distances are considerably less (that is, they all have different scale factors from electrons to tribes). I think that puts them in a different operating regime, so there's not as much backward drift. Of course we don't know the details of the tribal movements in the colonization of North America, but the tribal migrations in Eurasia worked somewhat like a chain of dominoes, with one tribe's migration pushing the tribe in front of it.

I think there's a misconnect in this thread over the use of the word "random". In the context of evolution it doesn't mean "anything can happen here with equal probability". It doesn't even mean "anything can happen here with a Gaussian probability distribution". What it means is that "there is a probability of a deviation from the current genotype which is a very complex function of the current genotype and the current environment." In particular, some things cannot happen because there's no way to get there from the current state of the genotype; this is part of the contingency aspect of evolution. And because these are changes to the genotype, the changes (if any) that will occur in the resulting phenotype are a complicated function of the genotype and the environmen, a function determined by the details of development of the particular organism and the environment in which it develops.

163:

SoV@158: perhaps we have different standards of "random". You'll no doubt be aware that that creationists and other intellectual irritants are fond of using that term in a misleading fashion. As a consequence many evolutionary biologists twitch when they see it written.

I'm saying that aspects of the environment are indeed random, or the results of chance (probably not quite the same thing: would "random within bounds" satisfy you?). That doesn't mean selection is random. The accumulation of variation is random: genetic drift, mutation, epigenetic oddness. But those are only the raw material of evolution, not the process itself. Selection is never ever random, it operates very strongly in the direction of surviving and passing on your genes to the next generation.

Those are two slightly different processes, and despite what you say I don't think you fully understand the difference between natural selection and sexual selection. Natural selection is mostly about survival, and it pretty well assumes that the successful will reproduce. Sexual selection operates in species where not everyone gets to reproduce: think about male mammals that control a harem, or birds that lek. Traits that have no survival value are selected for in those cases instance because they enhance reproductive success. A really sexy male in a lekking species can father almost all of a local population: that's a huge selective force.

164:

Bruce: well said.

165:

Bruce@162:

In the context of evolution it doesn't mean "anything can happen here with equal probability". It doesn't even mean "anything can happen here with a Gaussian probability distribution". What it means is that "there is a probability of a deviation from the current genotype which is a very complex function of the current genotype and the current environment."

Right. This is the way I usually see it being used. So yes, evolution really is 'random'.

166:

@ 158 ScentofViolets writes

"ChrisL@147:

Selection pressures wouldn't necessarily be the same the second time around, because so much of the selective pressure an organism experiences comes from other organisms.

You mean that the selection pressures are - wait for it - random?"

No, I'm pretty sure ChrisL means they change, based in part on what other organisms are present. Non-random is not the same as unchanging.

It's hard to imagine a random selection pressure. If gravity turned off half the time, maybe? Conditions can vary fast enough that evolution can't keep pace but in that case what you have is a selection pressure for greater adaptability, and a phenotype in the center of the "best adapted at any given second" range.

You continue "Darwin was concerned that environmental selection pressures were random and so there wasn't any apparent direction to evolution, which didn't seem to fit the observational facts."

What? I have just about finished _On The Origin Of Species_ and I can't find *anything* like that in there. Where did he say this?

@ 160 ScentofViolets

You write: "Looking back at what I wrote, I was quite clear. You (a) equated skepticism with holding a contrary position,"

(b is simply a restatement of a and is cut for brevity)

I agree I did you the honor of assuming 1) that you had ideas on the subject of potential electron acceptors and multicellular life that you were willing to risk expressing publicly, and 2) that your comments logically tied in with those ideas. I acknowledge these assumptions have proven to be unwarranted; I admit my mistake and withdraw the honor.

"(c) tried to shift the burden of proof on to me, and (d) insisted on an unreasonably high standard of proof."

I expressed a willingness to be convinced by evidence to the contrary--evidence I grant I knew perfectly well you did not have, and could not get, because it has never been discovered, and probably never will be. In the face of the inevitable long term absence of evidence from non-terrestrial life I see nothing wrong with reasoning by induction from the only examples of life presently known.

"On the whole, I was both concise and clear as to what I objected to. "

No. You weren't.

Specifically you have not made clear what part of relative electronegativity, redox potentials or energy requirements of unicellular and/or multicellular life you believe is incorrect or unclear.

If you desire a thorough introduction to these subjects I recommend college courses in biology and chemistry. The first year course for majors in each subject should do it. If you are a professor there is a good chance your university will allow you to audit them for free. You won't even need the whole thing, just a few weeks in the middle. But it is more material than anyone can convey in a post or a comment.

In the meantime I notice a pattern in your posts. They consist largely of criticisms of other people's ideas or information, and your criticisms are often subtly off, in a way that suggests you don't fully understand the issues.

For example in comment 93 Denni pointed out that Selection pressure is directional and decisively non-random.

You responded (comment 114) by saying random "was taking a beating with lead pipes" and "Are you saying that if we do a rewind and reset back to 50 million years ago and then let evolutionary processes play out again that everything evolves into the species we see today?"

Interesting. First, it looks like you're attempting to attribute to Denni a position zie may not hold for the sake of arguing with that position. I'm pretty sure someone got reprimanded for that not long ago. Second, evolution has parts that are random chance: genetic drift, founder effect, and the heritable variation that natural selection acts on, for instance, and parts that are not random--specifically natural selection, just as Denni said. A person with a good understanding of this, a professor in this field, for example, would have brought up the random parts, but would have had to concede the non-random parts. Such a person would understand that the existence of the random parts means no, of course evolution won't replay the last 50 million years exactly.

For another example "Post hoc ergo propter hoc (comment 56)" is an argument that applies to things that might reasonably be coincidences, and things where the order matters to the argument. "I washed my car, then it rained, therefore washing the car makes it rain" falls into this category because if the rain had come before the carwashing the argument couldn't be made, and because there's no reason other than order to think the two events related. It doesn't work for the issue of non-oxygen-requiring multicellular life failing to evolve in a reducing atmosphere and oxygen-requiring multicellular life evolving in an oxygen atmosphere because 1) an oxygen atmosphere is required for the evolution of life that needs oxygen, and therefore there is good reason to think the two are related, and 2) the argument works no matter whether the oyxgen atmosphere / life came before, after, or during the same time as, the reducing atmosphere / life. For that matter, since life is present in anoxic conditions to this day, and could theoretically evolve multicellularity at any time, the experiment, so to speak, *is* running concurrently. It's as if you knew the phrase "post hoc ergo propter hoc" but didn't really understand what it applies to.

However, you did express an idea last night, cool; good on you. It was:

@ 154 "Maybe the same traits and happenstances that led mitochondritic percussors to be incorporated without damage inside a cell are the same ones that enable cells to stick to together without reacting negatively to one another."

Since the insides and the outsides of cells are pretty different (very different selections of proteins and polysaccharides bonded to the membrane surfaces just for example), I wouldn't expect living stuck to the outside of a cell, and living inside a cell, to be related. I'd tell you I'm prepared to be convinced by evidence, but I wouldn't want to demand an unreasonable standard of proof.

And what on earth are "mitochondritic percussors"? That sounds ... rhythmic. I'd assume you mean "mitochondrial precursors" but I wouldn't want to attribute to you a view that you do not hold.

Anyway, this, like the subtly-off nature of many of your criticisms, gives the impression of someone who would like to be respected as an expert but labors under the disadvantage of not actually knowing much about the field.

167:

@154: The evolution of multicellularity is something that I think is riddled as much with "just so" stories as hard facts. I was taught from the eukaryotic perspective and I learned in the context of algae.

Basically, multicellularity is failed mitosis. Typical unicells divide in a polar way, and on yeast, you even get bud rings that let you count how many times that cell has undergone mitosis. Anyway, you get multicellularity when the two cells don't separate after dividing. This produces threads, and for many fungi, that's as far as they get. Fungi typically go in for branching threads, and aside from odd structures like fruiting bodies and ectomycorrhizal sheaths, they don't even venture into truly two or three-dimensional division.

The next trick is to screw up is to produce sheets (i.e. divide in two directions and fail to flal apart). This seems to be trickier, and relatively few algae have figured out this trick. From there, the next trick was to figure out how to divide in three dimensions, which forms a blob of cells, followed by various tricks for differentiating multi-cellular, three dimensional structures, aka organs.

As I said, this has the whiff of a just-so story, although I have no doubt that someone is working on the genetics of multiplanar mitosis as we speak. Or it could have been disproven years ago, and I was taught by someone who didn't know better.

In any event, there's no particular reason why prokaryotes could go through similar failures of cell division followed by specialization. Yet to my limited knowledge, the most complex three-dimensional structures that prokaryotes produce are the fruiting structures of myxobacteria (which are akin to those produced by cellular slime molds), although cyanobacteria produce sheets and hollow balls. AFAIK, the count of three different cell types for cyanobacteria is as complex as they get: normal photosynthesizing cells, resistant spores, and nitrogen-fixing heterocysts.

Given that the little buggers have been around for eons, it's weird to me that prokaryotes didn't get into multicellularity in a big way. My off-the-cuff guess is that they were so good at swapping genes that they couldn't accumulate the types of genetic screwups needed for multicellularity to evolve. And yes, "off-the-cuff guess" is another term for "just so story."

I just keep hoping that some microbiologist will set us straight at some point.

168:

ChrisL@163:

perhaps we have different standards of "random". You'll no doubt be aware that that creationists and other intellectual irritants are fond of using that term in a misleading fashion. As a consequence many evolutionary biologists twitch when they see it written.

Well, just don't assume any of those things about me, okay? Just like I assume that you're not an evolutionary biologist until explicitly shown otherwise.

I'm saying that aspects of the environment are indeed random, or the results of chance (probably not quite the same thing: would "random within bounds" satisfy you?). That doesn't mean selection is random. The accumulation of variation is random: genetic drift, mutation, epigenetic oddness. But those are only the raw material of evolution, not the process itself. Selection is never ever random, it operates very strongly in the direction of surviving and passing on your genes to the next generation.

I'm sorry, but that's like saying that a random number generator that samples line noise isn't really random because the logic that translates the signal into readable output is deterministic.

Those are two slightly different processes, and despite what you say I don't think you fully understand the difference between natural selection and sexual selection. Natural selection is mostly about survival, and it pretty well assumes that the successful will reproduce. Sexual selection operates in species where not everyone gets to reproduce: think about male mammals that control a harem, or birds that lek. Traits that have no survival value are selected for in those cases instance because they enhance reproductive success. A really sexy male in a lekking species can father almost all of a local population: that's a huge selective force.

Sigh. Whereas I get the impression that you don't really understand what's going on, as per the bolded sentence and the sentence that immediately precedes it. Rather a contradiction, in fact. Um, "Natural selection can only produce adaptation to immediately surrounding(and changing) environments. No feature of such local adaptation should yield any expectation of general progress(however such a vague term be defined)." That's bog standard evolutionary theory. Let me quote the original Darwin for you:

I should premise that I use the term Struggle for Existence in a large and metaphorical sense.... Two canine animals in a time of dearth, may be truly said to struggle with each other which shall get food and live. But a plant on the edge of a desert is said to struggle for life against the drought(1859, page 62).

And it was this biotic competition, as opposed to abiototic competition that led to evolutionary progress. Again from "Origin of the Species":

The inhabitants of each successive period in the world's history have beaten their predecessors in the race for life, and are, in so far, higher in the scale of nature; and this may account for that vague yet ill-defined sentiment, felt by many paleontologists, that organization on the whole has progressed(1859, page 345).

So it's not between 'natural selection' and 'sexual selection', it's between biotic and abiotic competition, according to Darwin, with sexual selection a subset of the former. As I've already said. If you've got a source as to what Darwin was thinking that counters this, by all means, post it.

169:

Cat@166: I see a bit of projection here:

I expressed a willingness to be convinced by evidence to the contrary--evidence I grant I knew perfectly well you did not have, and could not get, because it has never been discovered, and probably never will be. In the face of the inevitable long term absence of evidence from non-terrestrial life I see nothing wrong with reasoning by induction from the only examples of life presently known.

NO. You advanced a thesis. You defend it. It's that simple. I don't have to do a blessed thing other than be honestly skeptical. That you don't know this basic fact - and yes, it is a very basic fact, indicates to me that you really don't know how any of this sort of thing works. Since you seem determined to continue on in this vein, I'll stop after this one bit despite the various degrees of wrongheadedness and contrarianism present throughout your post:

For another example "Post hoc ergo propter hoc (comment 56)" is an argument that applies to things that might reasonably be coincidences, and things where the order matters to the argument. "I washed my car, then it rained, therefore washing the car makes it rain" falls into this category because if the rain had come before the carwashing the argument couldn't be made, and because there's no reason other than order to think the two events related. It doesn't work for the issue of non-oxygen-requiring multicellular life failing to evolve in a reducing atmosphere and oxygen-requiring multicellular life evolving in an oxygen atmosphere because 1) an oxygen atmosphere is required for the evolution of life that needs oxygen, and therefore there is good reason to think the two are related, and 2) the argument works no matter whether the oyxgen atmosphere / life came before, after, or during the same time as, the reducing atmosphere / life. For that matter, since life is present in anoxic conditions to this day, and could theoretically evolve multicellularity at any time, the experiment, so to speak, *is* running concurrently. It's as if you knew the phrase "post hoc ergo propter hoc" but didn't really understand what it applies to.

Reasons 1) and 2) don't make any sense. Period. And your original statement is nothing more than Post Hoc, ergo Propter Hoc; I'll even quote it again:

Prokaryotes show up 3.8 billion years ago. Oxygen doesn't build up in the atmosphere until about .58 billion years ago. That's 3.2 billion years that life was around in a reducing atmosphere, free to evolve into multicellular life if the energy conditions would support it, whether by taking appropriate organelle-precursors hostage or by other methods. It doesn't seem to have happened as best we can tell by an admittedly spotty fossil record.

So. There was no multicellular life prior to the invention of oxygen respiration, but afterwords there was, therefore multicellularity is impossible without oxygen. Pray tell, how is this not post hoc ergo propter hoc reasoning? Now, you've been alluding to some mysterious other evidence about the necessity of oxygen for multicellular life:

(shrug) We have a good reason to think oxygen is necessary for multicellular life in energy requirements and redox potential. We have another good reason in that we have a long unicellular period in a reducing atmosphere followed by a quick flowering into multicellularity in an oxidizing atmosphere.

But so far, despite my repeated promptings(which you say don't exist, natch), you haven't produced it. And since this is where we're stuck, and since you seem to have something of an attitude, that's all I think I need to say to you at this point.

170:

SoV: I use the same identity wherever I post on the internet. Follow the link to my blog. Read my CV linked off that page. Search for the peer-reviewed articles, by me, on ecology and evolution. Look up the people I've worked with. One of us is using a pseudonym here, and it ain't me. I'm an evolutionary biologist, you're a smartarse.

There's no quote about sexual selection in Origin because Darwin came up with idea years after Origin was published. His terminology was also subtly different from what we now use, because he was making it up as he went along; it's not uncommon for his writing to be misinterpreted for that reason.

Now kindly pull your horns in.

171:

ChrisL: I have no idea who you are, or what you have written; I go by what you are posting here. And - to be blunt - pull your horns in yourself, and stop trying to talk down to people. Do I really need to post the places where you've done this? And come off remarkably poorly? Bottom line, don't attempt to patronize people without making some sort of minimal attempt to gauge what they know, and they won't be so sharp with you. I hope that makes sense to you.

Nor did I say that Darwin had anything to say in Origins about sexual selection; I merely quoted him to show where you went off the rails (and yes, he did indeed struggle for a long time trying to reconcile 'directed' evolution with random processes.)

Now, do you want to stop this? Or are you going to keep saying things like "Those are two slightly different processes, and despite what you say I don't think you fully understand the difference between natural selection and sexual selection"? And then get huffy when I pull out cites and sources that disagree with you? I can go either way(though I'd prefer to be noncombative), but really, which way to go is up to you.

172:

heteromeles@167:

Given that the little buggers have been around for eons, it's weird to me that prokaryotes didn't get into multicellularity in a big way. My off-the-cuff guess is that they were so good at swapping genes that they couldn't accumulate the types of genetic screwups needed for multicellularity to evolve. And yes, "off-the-cuff guess" is another term for "just so story."

How would we identify an early anerobe as multicellular? Or an early multicellular organism as being positively anerobic? And where would we find them?

173:

SoV, a lot of very knowledgeable people comment here, and I'm sure I've been proven drastically wrong on any number of occasions in the past. It's all part of the fun, and if it worried me I wouldn't keep coming back.

174:

@172, SoV: It is possible to find prokaryotic fossils, although it's tedious business. It's also possible to get strong hints about the atmosphere around them by the chemistry of the rock encasing the fossil, even if it has been metamorphosed. There's also substantial difference between the size of (most) prokaryotic and eukaryotic cells, and most prokaryotes are smaller.

Basically, if I were a deep-time paleontologist who was working on bacterial fossils in the >1.5 Byr range and suddenly ran across multicellular forms, I would:
a. Work VERY, VERY carefully.
b. Do as much confirmatory work as possible on the chemistry of the fossils and the rock surrounding them,
c. Get said rock dated by as many different methods and independent labs as possible, and
d. If everything checks out, publish it in Science, Nature PNAS (or similar), since it's the kind of once-in-a-lifetime result that makes someone's career.

Note that I was a peripheral participant in a similar find: undergraduate finding weird spores-like fossils in an Ordovician-age road cut, and I hooked her up with a specialist who identify the spores. They published in Science. She got a scholarship to go to Harvard for her PhD, and he got a tenure-track position. People have an incentive to find this kind of stuff, because the rewards are high. It hasn't been found yet, and I think the betting money is that it doesn't exist. Be nice to be wrong, though.

175:

heteromeles@174: Ah, I was thinking about something like this:

In 1999, geologists in northwest Australia discovered traces of oil in rock that formed 2,700 mya. The oil contained sterols, which are fatty acids that hold cell membranes together. Because sterols are only produced by eukaryotes -- organisms whose cells contain a nucleus -- researchers conclude that these sterols were made by primitive eukaryotes.

IIRC, this provoked a bit of controversy at the time. I was wondering if there were more sensitive signatures, with sturdier cross-checks since developed since 1999. It is not at all obvious to me for example, why sterols couldn't have been produced by some ancient and now extinct prokaryote.

176:

@175: Yep, I was thinking of those too (and apologies for not bothering to proof my last post). Extraordinary claims require extraordinary proof, and odd sterols in old rocks don't quite tip the extraordinary scale. There are a bunch of really, really weird fossils from c. 500-600 mya, and they are discussed in the McMenamin's Hypersea. Regardless, the weird old things are eukaryotes to my knowledge, even if no-one knows what they were.

177:

To me the simplest explanation for the Fermi Paradox is that intelligent life is, by definition, smart enough not to talk to us.:)

178:

Charlie at 77: The problem is not that we are is serious disagreement. The problem is that your thought experiment is flawed. A good thought experiment still requires good experimental design. The problems with your probe design aren't incidental - they're fundamental to the problem, because you use them to generate your math.

The way you deprive your meat puppets of intelligence is also poor experimental design. Intelligence is what humans use in lieu of sharp teeth/claws, blinding speed, good camouflage, etc., in order to make our environment habitable. Your meat puppet can't successfully swim, build a fire, descend to a lower elevation before freezing to death, defend itself from even a small predator... all stuff that our ancestors mastered 100,000 years ago.

Thirdly, you assume that every society which might be exploring space is at our level of advancement. Assume good nanotech, and anything in the liquid water zone is probably terraformable. (I assume you've read David Gerrold's Chtorr stuff?)

The sad thing is, you don't require this thought experiment to justify your point of view. Most of our Solar System is uninhabitable to anything we would recognize as life. Even if one assumes oxygen-based life genetically manipulated to survive in extremes of pressure and temperature, the options in our solar neighborhood are probably limited to Earth, Mars, and maybe (though not likely) a couple of the Jovian moons. It's likely, though not experimentally proven, that most of the other solar systems out there - an astoundingly small percentage of the universe - have equally small habitable zones.

You could certainly write a believable, evidence-based science fiction book in which the amount of habitable space in the universe is minimal and I'd take the premise very seriously, but NOT on the basis of the thought experiment you propose.

179:

@ 169 ScentofViolets

"I see a bit of projection here"

Funny. That's exactly what I was going to say to you except I thought it would be rude.

"So. There was no multicellular life prior to the invention of oxygen respiration, but afterwords (sic) there was, therefore multicellularity is impossible without oxygen. Pray tell, how is this not post hoc ergo propter hoc reasoning?"

Consider 2 experiments.

Experiment A: Life in reducing environment gives rise to no multicellular life at all, despite access to every environment available on earth and plenty of time. Experiment ran all over the earth for 3,200 million years and continues to run in a smaller set of environments to this day (total 3,800 million years or so).

Experiment B: Life in oxygen containing environment gives rise to multicellular life, all of which requires oxygen. Multicellularity shows up around 50-100 million years from the start of the experiment.

Whether experiment A or experiment B is run first makes no difference whatsoever as long as both experiments are run.

That's why it's not "post hoc ergo propter hoc."

And I didn't say it was impossible without oxygen--I said "So if there was multicellular (and intelligent at that) life running around in a reducing atmosphere I'd sure like to know what they're using for energy."

You are attributing to me a position I do not hold. I'm sure this was accidental, both times (Danni too), as you dislike it so much when it happens to you that you wouldn't do it to anyone else. But now you have been reminded.

"But so far, despite my repeated promptings(which you say don't exist, natch)"

Sigh. I repeat--what part of "life requires energy" (see Wikipedia "Anabolism") "which all known examples of life get by hanging electrons on an electron acceptor" (see Wikipedia "electron acceptor" and Wikipedia "electron transport chain") "which in all known examples of multicellular life is oxygen" (see Wikipedia "cellular respiration") "which is a good electron acceptor because it is so electronegative" (see Wikipedia "electronegativity") *do* *you* *not* *understand*?

Note that I didn't learn all this stuff from Wikipedia--but it's got good thumbnail sketches of all this stuff as of now, it's free, and anyone with web access can find it, so you have no excuse for ignoring it.

And what a shame you don't like my attitude; after you went to all the trouble of installing it yourself at considerable effort, too!

180:

Well Done, Charlie!
I started thinking along these lines a few weeks ago, directly inspired by your "Atrocity Archives" and "Jennifer Morgue" - namely, the depiction of the division between the undersea "ancients" and the smattering of humans that are permitted to live on Earth's comparatively-arid surface. You really demonstrated the limited habitability, or at least non-anthropocentric nature of this planet. You've been gestating this idea for a while, I take it?

Heh, maybe the next portal our favourite desk-encumbered spy will encounter leads into a primordial Earth!

Regarding climate change and survivability, does anyone remember "A Pail of Air?"*
http://www.webscription.net/chapters/0743498747/0743498747___6.htm

Charlie had a nice 1920s take on that in "The Atrocity Archives."

*I appreciated the "Surface Tension" reference, Charlie.

181:

Brendan @ 180:

I remember "A Pail of Air." It's definitely one of my favorite stories ever. Fritz Leiber was great!

182:

Cat Faber @ 179:

Whether experiment A or experiment B is run first makes no difference whatsoever as long as both experiments are run.
I'm not sure this is true, especially because in fact the experiments weren't strictly sequential.

Oxygenation wasn't a point event, it took place over a very long time, in which organisms with reducing metabolisms were slowly pushed out of increasingly many habitats. I think that makes the relative timing of the evolution of oxygen-producing prokaryotes versus the potential evolution of reducing-metabolism multicellular organisms¹ important.

One point that's important to note is that the contingent nature of evolution makes its history chaotic: actual outcomes can be affected in large amount by very small events, especially when gene pools are small or large-scale changes of the environment occur on time scales smaller than the number of generations required for a species to adapt to them.

1. That is to say, the time at which some primitive multicellular reducing organism had evolved and could be selected for or against in competition.

183:

@182: Bruce, a couple of issues. The boffins (http://www.sciencenews.org/view/generic/id/47153/title/Atmospheric_rollercoaster_followed_Great_Oxidation_Event) suspect that there was at least one episode around 1.9 byr where oxygen levels dropped again. You are right about the chaos of evolution, but I think the gradual switch from a system dominated by anaerobes (which were probably running on sulfide metabolism) to aerobes (definitely running on oxygen) is seeming more and more like a multi-million year conflict. If you believe the stories about most mass extinctions in the last 300 myr, they may have been caused by anaerobic organisms dominating the deep ocean, followed by hydrogen sulfide outbursts.

That said, as Cat pointed out, the evidence is quite good that multicellularity and oxygen are linked. I'd add one more point: most of the life on Earth appears to be in the deep crust, which is a low oxygen environment and apparently dominated by unicellular organisms. Also, aside from strict anaerobes, there are huge numbers of facultative anaerobic archaea in the ocean. Oxygen doesn't kill them, and they get by.

Cat's mostly right, but I'd add that anaerobes have had more time than aerobes to produce multicellular life on this planet, and all sorts of divided, insular mini-ecosystems in which to evolve. Where is anaerobic multicellular life?

184:

heteromeles@183

That said, as Cat pointed out, the evidence is quite good that multicellularity and oxygen are linked. I'd add one more point: most of the life on Earth appears to be in the deep crust, which is a low oxygen environment and apparently dominated by unicellular organisms. Also, aside from strict anaerobes, there are huge numbers of facultative anaerobic archaea in the ocean. Oxygen doesn't kill them, and they get by.

Is there any evidence that doesn't boil down proximity in time?[1] Because I haven't seen much so far other than that. And you know what they say about arguing from a sample size of one . . . Is this also a prediction that no multicellular life will be found in extremely cold environments, i.e., does this rule out slow but complicated - even intelligent - life? I would say that if one wants to make some sort of pure numbers argument, about the only way to do a halfway decent job would be a sort of multiplication of improbable events affair. And even then, this would only imply linked in the broadest sense, not any sort of causality.

Cat's mostly right, but I'd add that anaerobes have had more time than aerobes to produce multicellular life on this planet, and all sorts of divided, insular mini-ecosystems in which to evolve. Where is anaerobic multicellular life?

Eh, surely you mean that I'm mostly right :-) But that is just the post hoc fallacy. I think a lot of people want an answer to this question when there isn't enough evidence or expertise yet to say either way. You're either going to have to be able to have much more detailed biochemical modelings, or you're going to have to actually go out and do some looking[1]. Someone had a great throwaway line that went approximately life appears every fifty light years, multicellularity every 500, higher animals every 5,000, and so on up the line of hierarchical complexity.

[1]These are really bad arguments for evolutionary phenomena. I've also seen people use this logic to argue that only mammals, and of the mammals, only the great apes are capable of developing intelligence. Obviously: there were no intelligent animals until humans evolved. Reptiles, birds, and for that matter, monotremes and marsupials, are forever doomed to nonsentience. As are entire orders of the infraclass Eutheria, the chiroptera, the carnivora, the even and odd toed ungulates, etc.

[2]A creationist once told me that even if life were created in test tube, and even if life were found on other planets, and even if life spontaneously arose in cultured prebiotic media . . . it still wouldn't prove that God was not responsible for life arising on Earth.

185:

@149 Chris L

"Look up papers by a guy called Phillip Bell. He argues (with good evidence) that eukaryotes could have arisen by a symbiosis of multiple prokaryotes inside the fringes of a big membrane-bound virus."

Just based what I thought I knew on relative sizes of prokaryotes and viruses, this kind of boggled me for a moment. My access to a lot of the journals is pretty limited right now, but I did do a little (very little) googling and saw this name attached to the idea that a membrane-bound virus inside a prokaryote might have given rise to the first eukaryotes.

Figuring for the moment that's what you had in mind, wow :-) I would like to learn more about it. My education was more toward the biochemical/molecular biology side than the cell biology side, and we either didn't cover this in class (it was long enough ago that this idea may not have been well established) or I have forgotten it over the ensuing years.

Also @58 "If I've ever heard a firm date for the rise of eukaryotes (cf multicellular life), I can't recall it. I assume Cat's talking about the Ediacaran fossils with the ~500 million years ago, but they're a bit... enigmatic."

Actually I was making a distinction between eukaryotes (organisms with a well-defined nucleus and possibly, but not necessarily, mitochondria and chloroplasts as well)* and multicellular life (a subset of eukaryotes). We know that yeast, for example, can live without oxygen, by fermentation; it just doesn't get much energy out of the process. But it's not multicellular.

(Comparing the net energy output of the fermentation cycle to the net energy output of glycolysis that included the Krebs cycle (a required part of undergraduate biochemistry courses) first gave me the idea that oxygen was important for energy. I grant I was overgeneralizing from a single example at the time, but know of no examples of high-energy biochemical cycles that don't use oxygen as a terminal electron acceptor. However I would expect such cycles to be of interest mostly to microbiologists, since as far as I know only prokaryotes use non-oxygen terminal electron acceptors. I only had a couple of microbiology courses, so perhaps my education was a trifle eukaryo-centric, and there's something out there I am missing.)

In any case, while I don't know of any eukaryote that is an obligate anaerobe or merely aerotolerant, as one might expect in an organism that is adapted to a reducing atmosphere like that of pre-oxygenation earth, I wasn't intending to say that none existed. However if one did, I don't see why that would necessarily show that oxygen is not important to multicellularity (unless such a eukaryote turned out to also be multicellular, of course).

Regarding the time I gave, yes, I was thinking of the Edicarian biota (though not well-informed enough to recall the name at the time :-). I agree they're enigmatic, but is there any school of thought that proposes they weren't multicellular?

I agree that eukaryotes were around for a while, possibly a long while, before multicellular life. However the original comment I was responding to (a long way back in the thread, I admit) was talking about some non-terrestrial and presumably intelligent (starfairing) creature finding pre-oxygen-atmosphere-earth a hospitable place, and I kind of assumed such a creature must be multicellular.
------------------------------
*Err... just in case there's any kind of misunderstanding, I included the explanatory asides for the benefit of anyone reading this far who doesn't have your background. I can see perfectly well you know this stuff at least as well as I do.

186:

@ 182 Bruce Cohen

"I'm not sure this is true, especially because in fact the experiments weren't strictly sequential."

Absolutely I agree with you that they weren't, and that the "anaerobic experiment" continues to run, without evidence of multicellularity, to this day.

But I think that only strengthens the case that this is not a "post hoc ergo propter hoc" argument.

"Oxygenation wasn't a point event, it took place over a very long time, in which organisms with reducing metabolisms were slowly pushed out of increasingly many habitats. I think that makes the relative timing of the evolution of oxygen-producing prokaryotes versus the potential evolution of reducing-metabolism multicellular organisms¹ important."

Sure. I agree with you that the starting point for experiment B requires a reducing atmosphere and life adapted to it, and that oxygen must be introduced only gradually, because free oxygen ruins the molecules composing any life that can't detoxify it, and evolving a method to detoxify it can only take place if naive organisms can survive at a reduced rate long enough for random variation to throw up some variant that can detoxify it somewhat. If you poison all your "seed life" in one fell swoop, of course multicellular life has no chance to emerge.

However I don't see how these starting requirements for experiment B make the combination of the results of the two experiments a "post hoc ergo propter hoc" argument either.

"One point that's important to note is that the contingent nature of evolution makes its history chaotic:"

Very much so. I agree with you that randomness in the form of the founder effect, genetic drift, and the available pool of heritable variation affects evolution. And absolutely I agree with you that evolution can only proceed from what is already present and this can profoundly affect the ways in which a particular environmental challenge can be met. And I agree that for these reasons evolution is not predictable and that a second run won't come out with the same answers as the first.

I suppose that means it's possible that if evolution on earth were re-run from the beginning some rare event might lead to the rise of multicellular life in a reducing atmosphere. My curiosity about what that life would be using for energy would not be reduced by the fact that it was terrestrial rather than extra-terrestrial. If anything, it would be greater, since at that point exotic environments of hydrogen slush would presumably not apply.

More generally, however, this is not a single example. Life evolved multicellularity multiple times. Nor is it a series of experiments run in a single environment--it is a range of all the environments available on this planet now and in the past, from deep sea mid-ocean ridges with their "smokers" to anoxic sediments in bogs and warm shallow seas, to windswept plains and mountaintops and deep chilly caves, with and without oxygen.

187:

@184: SoV, I should be following my own advice to ignore this part of the argument, but it's time for you to get some numbers out and start proving that multicellular anaerobic life is *POSSIBLE*

Here's how you do it: compare the net energy output of glycolysis (per Cat. You can look this up on the web), and all forms of fermentation. There is an energy difference, trust me.

Multicellular life works on the principle of specialization. My simple guess is that the energy provided by all forms of fermentation is so low that any cell running on fermentation alone has to perform all functions just to stay alive. It does not produce enough surplus energy to support another cell in any specialized function, such as respiration, propulsion, or even more specialized functions of nutrient and energy uptake, support, information networking (i.e. nerves) and so forth.

Your job, to further pursue your argument, is to demonstrate that any fermentation system produces enough energy for one or more cells to support another specialized cell, so as to make a multicellular organism possible.

Arguing from rhetorical logic is lazy. Do some work for a change. We're checking our references, and so should you.

188:

Cat@185: I'm not much of a microbiologist either, I just happened to have shared a department with the dude for a few years.

The standard endosymbiotic hypothesis has been around for years, and it's probably the one you learned in undergrad. It requires the existence of an obligately anaerobic eukaryotic predator, something like an amoeba with no mitochondria, that acquired mitochondria by a failed predation event. There are more modern examples of these kinds of things happening -- some of the more obscure groups of single-celled algae have a quadruple chloroplast membrane, because their chloroplasts are actually evolved from other single-celled algae that already had chloroplasts. They even have remnants of the original host genome trapped in between the sets of double membranes. So the potential for that process is uncontroversial.

Phillip's idea is that lots of the more peculiar aspects of eukaryotic cells, like linear chromosomes, and membrane-bound genomes that can only communicate with the cytoplasm by poking RNA out through a membrane pore, are primitively found in viruses. There are some HUGE membrane-bound viruses, some of them are actually larger than prokaryotes. Apparently (and I'm reduced to repeating what I've been told here) you find assemblages in the wild with various species of bacteria snuggled around folds of these big viruses, engaging in a facultative mutualism to exploit local nutrient pools or what have you. It's a relatively small step from there to the virus enclosing the prokaryotes entirely (viruses make their own membrane-fusion proteins) and moving some of the genes out of the now-organelle into their own genome (again, viruses are pre-equipped for those kinds of shenanigans). Well, relatively small step/make several million tries until the right circumstances arise... you know what I mean :)

I'm afraid I'm database-challenged myself at the moment, so I can't dig up the examples I'm thinking of. The beauty of the idea is it's so parsimonious: all the ingredients are things we know exist, and we know they can do all the individual tricks required to make the process happen.

Some people seem to think that the Ediacaran fauna were multi-cellular, but not the ancestors of modern multicellular life. I have no idea what that concept is based on, other than the fact that they don't look much like modern multicellular life. Or like anything from the Burgess Shales.

189:

I just want to share with you that this is a wonderfully illustrative portrait of the evolutionary change that has brought us to this Earth in *this* time.

I look forward to sharing this with my wife and son at the dinner table. I only wish that science textbooks could take advantage of the same rhetorical technique you use to get your point across.

I haven't pushed through all the comments, so I will only say that - minute details aside - it is a compelling look at the basic processes that made this world.

190:

heteromeles@187:

I should be following my own advice to ignore this part of the argument, but it's time for you to get some numbers out and start proving that multicellular anaerobic life is *POSSIBLE*.

Blink. That's absurd. If you really believe that, then I'll come by your house with an investment proposition and a request for $50,000. I won't have to convince you that the investment is sound, of course, you'll have to prove why it isn't; and if you can't, well, where's my 50K. I suspect that this isn't what you really believe ;-) Now listen, and listen well: what I happen to believe or disbelieve[1] is completely immaterial to the requirement that the obligation to support your statements is on you. Nobody else. And while I might agree with, say, the proposition that the sum of two even numbers is always an even number, if your proof is that this is how it's worked every time you've tried it so far, well I'll let you know where you messed up, trust me.

That I even have to bring such basic notions to your attention - and this burden of proof thing is very basic in both senses of the word to the scientific method - is jaw-dropping. To put it mildly.

Your job, to further pursue your argument, is to demonstrate that any fermentation system produces enough energy for one or more cells to support another specialized cell, so as to make a multicellular organism possible.

Arguing from rhetorical logic is lazy. Do some work for a change. We're checking our references, and so should you.

No, if you want to claim that multicellarity is impossible without some sort of aerobic respiration, it's your job to support it, not my job to disprove it. I don't know whether I'm more shocked than appalled that you would even experience an instant where you would think otherwise, or more disgusted than either that you would actually post such drivel.

This is a basic science FAIL. I'm sorry to be so blunt, but there isn't any way I know of to sugarcoat this truth. If you honestly don't know this, you really shouldn't be employed in any sort of teaching or supervisory capacity. That you would even suggest that requiring you to support your own statements is some sort of 'rhetorical strategy', well, words fail me. That's what libertarians do.

[1]If you'd bother to actually read what I wrote, my position, stated multiple times, is simply that I don't know. The latest such:

I think a lot of people want an answer to this question when there isn't enough evidence or expertise yet to say either way.

And while I don't know if every even number can be expressed as the sum of two primes, I do know that saying that's the way it's worked the couple of dozen times you've tried it isn't much of a proof. What you've given so far falls into that category.

191:

@189: You are asking me to prove a negative to your satisfaction? Impossible. I'm asking you to prove a positive, as a way to assert that your claim has validity.

I told my niece a joke last night:

Q: Why do elephants paint their toenails red?
A: To hide in cherry trees.
Q: Ever seen an elephant in a cherry tree?
A: No? See how well it works?

Why should we run around attempting to demonstrate that, in the universe of all possible cherry trees, there isn't an elephant hiding in any single one? That's proving a negative, and as you know full well, it ain't possible.

Instead, support your positive: demonstrate that an anaerobic elephant has sufficient energy to climb up into the cherry tree of life, and then we'll talk about its toenail colors. That is a positive test of your hypothesis, that anaerobic multicellular life is possible.

192:

heteromeles@190: !?!?!?! What part of "I don't know" do you not understand? I'm not trying to assert anything in particular, you are.

So basically, what's happened is you've given what I regard as extremely poor reasons for your rather dogmatic beliefs, and when called on it, say that I have to disprove your idle theorizing. This is epic FAIL. End of story, and no further discussion is possible because now you're just playing the "If you can't make me say I'm wrong I win game." To say I have nothing but contempt for that sort of strategy is putting it mildly; that's precisely what's put the United States in the world of hurt it is now. Thanks, bud.

193:

@191 heteromeles

Good joke about the elephant.

@188 Chris L

Thanks for giving more details about the virus hypothesis. I appreciate it.

Both:

I've found your posts interesting and informative, even on those few where I've disagreed with the details.

If you're ever in the same city as me, I'll buy you a beer, or a coffee or something--it would be fun to hang out and talk for a bit.

194:

ChrisL, heteromeles and in general.

I think I have figured it out. The secret lies at the bottom of post 190, where ScentofViolets writes:

"And while I don't know if every even number can be expressed as the sum of two primes, I do know that saying that's the way it's worked the couple of dozen times you've tried it isn't much of a proof."

What we have here is a basic cultural disconnect.

Anyone who understands science knows that a theory is the best known explanation for a large body of observed fact. This is a pretty high standard, and I agree that an argument can be made that the idea that oxygen is probably necessary for multicellular life may not rise to that standard and may be a mere hypothesis (an explanation for a small body of observed fact). Note, however that this is an argument that ScentofViolets has *not* been making.

So whether the oxygen/multicellularity argument rises to the level of a theory doesn't matter. There is a wide gap between the best known explanation for a large body of observed fact, and a proof, like a mathematical proof, which demonstrates something is true in all possible cases.

And *this* is why ScentofViolets won't listen no matter how many examples you bring up: ScentofViolets is demanding a proof. No finite number of examples will ever provide proof, just as he was pointing out with the "even numbers are the sum of two primes" example. By those standards, we could find a thousand life-bearing worlds and demonstrate that in every single case multicellularity arose only if and when oxygen was available and all multicellular organisms on every world absolutely required oxygen for life and it wouldn't be enough.

This is a cultural disconnect between a few people who have internalized the standards of science and someone who doesn't.

Time to shrug, and go our separate ways.

195:

A similar discussion, in comic strip form, at The Stonemaker Argument!

196:

I have to wonder who in their right mind would want to settle on THIS planet. We, as the primary inhabitanats, have amost destroyed the entire ecostructure. What advanced race would want to waste time and resources re-terraforming a planet its own inhabitants care little for?
Avoid the planet earth at all costs....

197:

Bill: no we haven't. We could destroy 90% of the remaining species on the planet -- make that 99% -- and then drink the Jim Jones Kool-Aid ourselves, and I guarantee you that within another five million years the Earth would have a whole bunch of ecosystems that would be as fascinating as anything that was out there say, 50,000 years ago (before our ancestors really got going).

You say mass extinction, I say lots of vacant niches for opportunist species to move into (and lots of foreign species that have recently been redistributed hither and yon by our activities, too). The resulting ecosystems might look a little sparse -- all those first-cousin species, no really weird divergencies -- but then, our background would look a bit sparse from the point of view of a sentient descendant of anomalocaris.

It might be a bit tough for anyone who wants to set up a civilization running on readily-accessible liquid hydrocarbons in the next fifty megayears.

198:

I trust you've noticed that the New York Times Ideas blog has linked to this post.

199:

I just discovered your blog, Charlie. I feel like a little kid on Christmas morning. Thanks for making me think.

Here's my take on the Drake Equation - http://blogs.myspace.com/index.cfm?fuseaction=blog.view&friendId=433257619&blogId=505997302

200:

#198
And Daily Kos noticed that, so there may be some strangers wandering through looking at labels and kicking the tires ....

201:

(Re Daily Kos strangers - I'm guilty.)

Charlie, fascinating post. It certainly puts things into perspective. I think we have to stop thinking of ourselves as something outside of the natural life of the planet. Just as with any other life form, Gaia is subject to infections. We won't necessarily kill her, but we could very well put her into an ICU if there was one large enough to accommodate her. I'm afraid that since there isn't, she'll have to ride out this fever until the source of the infection is eradicated.

Since there is three hundred million years of potential left, let's hope it peaks quickly so her recovery can start soon.

Given our current state of technological development (and I have to confess, I do not possess any scientific background, only an enthusiasm for science fiction (or more accurately, speculative fiction since I hate rules) I don't know that it will ever be possible for humans to move out into space. This whole shielding our biology from harmful radiation thing may keep us close to home.

What we have on our human doorstep right now is a moral responsibility toward the other life forms we share this planet with. Our instinct for survival trumps that moral responsibility so I don't hold out much hope on that front.

The bottom line seems to be that everyone and everything on this planet is in for a Great Suffering. Given my family history, I am unlikely to survive for another twenty years, so for me this is largely a frivolous speculation, but I long for some sense of redemption after being witness to the excesses of our species.

So what can I do? What can WE do? Nothing and everything.

202:

(Re Daily Kos strangers - I'm guilty.)

Charlie, fascinating post. It certainly puts things into perspective. I think we have to stop thinking of ourselves as something outside of the natural life of the planet. Just as with any other life form, Gaia is subject to infections. We won't necessarily kill her, but we could very well put her into an ICU if there was one large enough to accommodate her. I'm afraid that since there isn't, she'll have to ride out this fever until the source of the infection is eradicated.

Since there is three hundred million years of potential left, let's hope it peaks quickly so her recovery can start soon.

Given our current state of technological development (and I have to confess, I do not possess any scientific background, only an enthusiasm for science fiction (or more accurately, speculative fiction since I hate rules) I don't know that it will ever be possible for humans to move out into space. This whole shielding our biology from harmful radiation thing may keep us close to home.

What we have on our human doorstep right now is a moral responsibility toward the other life forms we share this planet with. Our instinct for survival trumps that moral responsibility so I don't hold out much hope on that front.

The bottom line seems to be that everyone and everything on this planet is in for a Great Suffering. Given my family history, I am unlikely to survive for another twenty years, so for me this is largely a frivolous speculation, but I long for some sense of redemption after being witness to the excesses of our species.

So what can I do? What can WE do? Nothing and everything.

203:

Wow, great to see Andrew Sullivan linking to you. Two of my more favorite bloggers! :)

204:

@201 Mark Mc:
"Just as with any other life form, Gaia is subject to infections. We won't necessarily kill her, but we could very well put her into an ICU if there was one large enough to accommodate her."

Or, on the other hand, Gaia got tired of intermittent ice ages and occasional asteroid impacts, and bred an intelligent biped to handle that sort of thing, taking the minor loss of some species and a fairly minor warming to prevent the once-in-a-while loss of almost the whole biosphere...

(The Gaia hypothesis cuts both ways - if Mother Earth is so pissed at us, a quick viral infection and that's it for Homo Sapiens.)

205:

The real point, I think, is that we co-evolved with a whole bunch of other organisms, as part of a dynamic process of change within a biosphere. We're not in a stable setting, we're intimately dependent on many other species for our own survival -- and in many cases we don't even realize it. (Crops and cattle are obvious; the deep ocean phytoplankton that provide about 80% of the oxygen in the air? Less so.)

Take us out of our biosphere and we're unlikely to fit neatly (or comfortably) in another setting. We're creatures of a very specific time and place, and if we want to find somewhere else to live it's not just a matter of finding other rocky planets orbiting their stars in the liquid-water zone (and then figuring out how to get there).

206:

Interesting post, Charlie. It highlights the point that human beings are essentially a tropical animal. We can survive in our 'natural' state (i.e., without clothing...) in air temperatures above 35 Celsius (if I remember correctly) and a humidity range that I forget, but is enough that we don't sweat too fast or too slowly. (It's good to have thermal control to keep your body cool, but it's also good not to dehydrate.) Surprisingly enough, those conditions are most closely met in parts of Africa - and surprisingly enough, that's where we evolved.

It shows IMHO that the anthropic principle has it exactly backward. The universe isn't in any way 'configured' for us, we're configured for (a very small part) of it.

207:

Wow - this is really eye-opening. But what about Mars? I know you said that a randmom sampling would yield a lower probability of finding the planet hospitable, but how much lower?

Also, Charlie, you write that:

"Take us out of our biosphere and we're unlikely to fit neatly (or comfortably) in another setting. We're creatures of a very specific time and place, and if we want to find somewhere else to live it's not just a matter of finding other rocky planets orbiting their stars in the liquid-water zone (and then figuring out how to get there)."

Excellent point, but what is the last half of this sentence? In other words, if it's not just about finding other rocky planets, what is it about? Are we not destined for space? Are humas destined to go extinct on earth? Recently I watched a panel, "Are we Bound for Space" - http://www.q2cfestival.com/play.php?lecture_id=8011 - with Chris McKay, Chris Hadfield, Lawrence Krauss, Donna Shirley and others, and it actually made me feel optimistic about our future colonizing Mars. But reading this makes me think about all of the variables that are actually involved with survival and what makes a planet hospitable. In way one, it highlights our adaptability - but in another way it reveals our vulnerability.

208:

In the Wikipedia there is an article that says the Earth will probably be too hot for liquid or vapor water in 1Bill.yrs. In 2.2Byrs. the Andromeda will crash into the Milky Way. The black holes may combine into who knows what. There are a pair of neutron stars within range of us capable of killing all life in 150..light years. None of us owns a home that will with stand earthquakes, strong winds, floods, tsunamis, fires, poisonous volcanic or otherwise gases, but we ship stuff aboard ship, via trains, planes, and trucks in steel shipping containers that could. The biggest mall in the world is in Russia made of steel shipping containers. Maybe we should begin to learn how to live on the moon by learning to live encapsulated in some thing besides the wood our media moguls make their newspapers out of. Imagine thinking about the box, instead of out or inside of it, or how much better we are than monkeys because we've got wheels.

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