Is a man-eating plant realistic?

A plant chasing humans would be unrealistic, but I think a plant generating a trap for human-sized animals could be possible in principle, given the right circumstances (maybe it evolved from one eating smaller animals, but those got extinct, and the trap size grew to capture ever larger animals).

The most obvious way would IMHO be if the plant managed to create a concealed hole in the floor where animals would fall in, breaking through the thin cover (which would regrow while the animal would be digested inside the trap). Of course the natural countermeasure would be to be always careful where you go, so animals in a region with many such plants would usually always test the floor before they step; running would probably be quite unusual. If the substance in the trap kills quickly, a careless human (or one not knowing about that danger) could also be caught by such a plant.

Another possibility could be using many small barbed hooks growing out of the floor, and unnoticeable to the animal (looking basically just like the normal floor). When the animal steps on the floor, the hook enters its skin, and if the hook is strong enough, it cannot leave any more. Such a plant could normally be completely under the earth except for the hooks; after an animal is caught by the hooks, it might grow special roots relatively quickly into the caught animal to digest it from the inside. Coevolution would probably make the animal's feet thicker (so they are less easily pentrated by the hooks) and the animals lighter (so they are pressed into the hooks with a lower force), or to avoid the floor as much as possible (like apes living in the trees). Humans living in those regions would probably develop shoes early on as protection against those plants. You certainly should not go to sleep where such a plant grows; maybe humans would predominantly live on trees, too. Humans not knowing about that danger might fall prey to such plants because they walk onto such plants with their bare foots or insufficiently strong shoes, or lie down to sleep on one. Also stumbling on such a place can be deadly (the plants might also grow tripping hazards in order to make animals "” and humans "” trip on such places and thus involuntarily expose less-protected parts of their body to the hooks).

Here's another option, which is actually a worked-out scenario how an actually man-eating plant could have evolved.

Imagine a nutrient-low environment, and a plant which happens to produce a substance that happens to be very beneficial for some man-ancestor animal. Members of that species will then, of course, start harvesting that substance from the plant, which actually harms the plant, therefore it will start to evolve counter measures. Those counter measures could initially have been a contact poison on the bark, which over time got more effective. However, since the substance was so advantageous to those species, they also co-evolved to become more skilled in getting at the substance while avoiding that contact poison. There still were some individuals dying from the poison, but the substance was so advantageous to them, that it more than made up the evolutionary disadvantage of some individuals dying.

However, at the same time those individuals dying turned out to be an evolutionary advantage for the plant, since the rotting corpses delivered sorely needed nutrients to it. So the evolutionary pressure changed on the plant: Now it was not just reducing harm from the harvesting of the substance, but also increasing the number of individuals killed by the poison, to deliver more nutrients.

Of course there's a delicate equilibrium involved here, as the chance to survive the harvesting attempt must be high enough that the advantage of harvesting outweighs the danger of being killed. So the evolutionary pressure is to increase the absolute number of individuals killed, while at the same time not increasing the relative number, at least not beyond a certain point. That means, attracting more individuals. Thus, the substance in question will be produced in more amount, and also in a way that is more easily accessible (which also means that after some time the harvesting doesn't harm the plant any more, because it is offered at a place where it can be gotten at with little harm (but high probability to get into contact with the poison).

At the same time, it will also evolve means to more effectively harvest the nutrients from the corpses, like growing roots into it.

Of course, for the human-precursor species, there's an evolutionary pressure to get at the substance while losing as few individuals as possible, therefore getting better and better at avoiding being killed by the plant; this will be countered by the plant evolving better and better strategies of killing. At this point, both species my already be dependent on each other, so that's the only way evolution may proceed.

Now imagine that the plant, due to some mutation, produces a bit more of the advantageous substance while consuming a killed individual. Now this changes the evolutionary pressure inside the pre-human species: Now it is not only advantageous to avoid getting killed, now it is also advantageous if a competing individual does get killed. Not only does that other individual get removed from the gene pool (that would have been the case even before), but in addition it means more of the advantageous substance. So there's now an evolutionary pressure not only to avoid getting killed by the plant, but also to cause competing individuals getting killed (for example, by pushing them to the tree). This means (according to the egoistic gene assumption) that such a behaviour will evolve. Also it means that individuals get an evolutionary advantage from recognizing times when the plant produces more of the substance.

Now individuals being actively pushed to the plant for being consumed by it is, of course, of advantage of the plant, so it itself will evolve to encourage that behaviour; it will make the difference between "hungry" and "consuming" state more pronounced, eventually stopping to provide any substance while "hungry", and probably over time stop to kill during "consuming" phases. At that point, it will not just be advantageous, but actually mandatory that some individuals are pushed to the plant, in order to get the substance which the species is long dependent on.

For members of the species this means an evolutionary pressure to get more intelligent: First, it is of advantage to understand in which phase the plant is, so you can avoid it while it is hungry, and harvest it otherwise. But at the same time you need social intelligence to both trick others to visiting the plant when hungry, and to avoid being pushed to the plant yourself. So this specific situation should give an evolutionary pressure to become an intelligent species.

Now fast forward, and see how that species turns into (that world's equivalent of) humans. As the intelligence evolves, they will become aware of the fact that they are killing others at the plant, but also that this is absolutely essential for them to be able to live on. Probably they'll rationalize it that the plant is from a god who wants them to sacrifice humans, but who gives them the substance in return.

The specific way they evolved will probably mean they will naturally mistrust each other very much. The main goal of the emerging power structure (with a priest or similar on the top) will likely to be to regulate sacrifices to the plant, so individuals don't have to live in constant danger. I'd also expect tribal wars to be common, in order to get people from other tribes to sacrifice to the plant.

All in all, you'd get a man-eating plant which is explicitly fed by an inherently mistrusting and martial humanity depending on that plant.

posted about 10 years ago

celtschk‭

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