What use is a human being — to a tapeworm?
A mature tapeworm has a very simple lifestyle. It lives in the gut of a host animal, anchoring itself to the wall of the intestine with its scolex (or head), from which trails a long string of segments (proglottids) that contain reproductive structures. The tapeworm absorbs nutrients through its skin and gradually extrudes more proglottids, from the head down; as they reach the end of the tape they mature into a sac of fertilized eggs and break off.
The adult tapeworm has no knowledge of what happens to its egg sacs after they detach; nor does it know where it came from. It simply finds itself attached to a warm, pulsing wall, surrounded by a rich nutrient flow. Its experience of the human being is limited to this: that the human surrounds it and provides it with a constant stream of nutrients and energy. A hypothetical intelligent tapeworm might well consider itself blessed to have such a warm and comforting environment, that gives it all the food it needs and takes away anything that it excretes. And if it were of a philosophical bent, it might speculate: what is the extent of my environment? Is it infinite, or are there physical limits to it? And, eventually, are there other tapeworms out there? And finally, the brilliant polymath-level Enrico Fermi of tapeworms might ask, if there are other tapeworms, why aren't they here?
Our tapeworm-philosopher gets its teeth into the subject. Given that the human is so clearly designed to be hospitable to tapeworm-kind, then it follows that if there are more humans, other humans out there beyond the anus, then they, too, must be hospitable to tapeworm-kind. Tapeworm-kind has become aware of itself existing in the human; it is logical to assume that if other humans exist then there must be other tapeworms, and if travel between humans is possible—and we infer that it might be, from the disappearance of our egg sacs through the anus of the human—then sooner or later humans interacting in the broader universe might exchange eggs from these hypothetical alien tapeworms, in which case, visitors! Because the human was already here before we became self-aware, it clearly existed for a long time before us. So if there are many humans, there has been a lot of time for the alien tapeworm-visitors to reach us. So where are they?
Welcome to the Fermi paradox, mired in shit. Shall we itemize the errors that the tapeworm is making in its analysis?
The first and most grievous offense our tapeworm logician has committed is that of anthropocentrism (or rather, of cestodacentrism); it thinks everything revolves around tapeworms. In reality, the human is unaware of the existence of the tapeworm. This would be a good thing, from the worm's point of view, if it had any grasp of the broader context of its existence: it ought by rights to be doing the wormy equivalent of hiding under the bed covers, gibbering in fear.
It has inferred the existence of other humans, but it doesn't know about cooking, or the other arcane processes by which food makes its way into the gut for the tapeworm to absorb. Or about the sanitary facilities that kill tapeworm eggs before they get to another, intermediate host. There are vast, ancient, alien intellects in the macrocosm beyond the well-known human, and they are unsympathetic to tapeworms. Intrepid tapeworm cosmonauts seeking to make their way beyond the anus and across the universe to colonize other humans are in for a rough ride indeed, for they are intimately evolved to thrive in one particular environment, and that environment (the mammalian gut) is sparsely distributed throughout the universe. Much of the cosmos is inherently hostile to tapeworms. This is why tapeworms have not, in fact, colonized the universe and converted all available biomass into a constantly spawning Gordian knot of Platyhelminthic life, contra the prognostications of some teleologically-inclined tapeworm-philosophers of yore.
The human does not owe the tapeworm a living, or even a comfortable home. The tapeworm's existence is contingent on it not damaging its human, resulting in an undesirable human/tapeworm interaction with fatal consequences for the tapeworm. Some of the tapeworm's descendants might be able to find another new human to claim as their home, but the same constraints will apply. Only if the tapeworm transcends its tapewormanity and grows legs, lungs, and other organs that essentially turn it into something other than a tapeworm will it be able to make itself at home outside the human.
(Note: I picked tapeworms, rather than the bacterial gut fauna, because nobody much cares what happens to an E. coli. Tapeworms, on the other hand, are multicellular eukaryotic organisms with differentiates tissues, have nervous systems and genitalia, and are probably much closer to us—practically kissing-cousins to our form of vertebrate life—than anything we might discover on other planets. Perhaps the biggest weakness in this metaphor is its reliance on humans. While we may attribute intentionality to many natural phenomena—the supernatural stance—those of us tapeworms who are hard-headed materialists must surely concede that the
human Earth is not a sentient being, Sir Arthur Conan Doyle's fiction aside. On the other hand, if you want to traffic with the ghost-infested depths of the simulation hypothesis, then anything is possible. Even tapeworm-cosmonauts flying out of my arse ...)