I read an article on Seed Magazine's website today about why the Uncanny Valley exists. The term refers to almost humanlike robots and CGI. When they're very cartoony they're cute, when they're extremely realistic we react to them like other humans, but in between they repulse us. The article explains some theories on what is driving our subconscious to dislike these creations (we think they're dead bodies, we really don't want to mate with them, they confuse our understanding of what man is). I'd like to propose an alternative theory.
Imagine you're a rabbit who has suddenly evolved a poisonous bite. What would you do? You could hunt down all the wolves and dogs in the area, and save rabbit-kind from constant harassment. At first, all the other bunnies would treat you as a savior. Without predators, the bunny society would grow and flourish. And then suddenly, all the plants have been eaten and everybunny starves. If the bunny population doesn't die off, it'll do so by evolving to better regulate their population. And if predators return, then suddenly they won't be able to have babies fast enough.
Part of evolution is adapting to your environment. Part of evolution is being able to adapt when that environment changes. Mammals exist because they could deal with a new threat the dinosaurs couldn't. But part of evolution is avoiding having to adapt to a new environment. If you're well suited for the world around you, its in your best interest not to mess with that world.
Thus even if an herbivore could kill all the predators around it, it probably shouldn't. Similiarly, dogs don't eat plants, but if they kill plants they'll be indirectly killing their food supplies as well. An environment is filled with many niches, and animals who leave other niches alone are generally going to have a better shot at survival.
But leaving other niche's alone says nothing about your own niche. When predators outperform prey, the prey dwindle, the predators starve, and suddenly the prey are safe to grow again. There's a feedback loop that seeks equilibrium. If a new herbivore enters the scene, the situation is different for the rabbits. If they're better at eating the mutual food source, there will be less for the rabbits, and the rabbits will die. If they overeat, the new creatures will die off, but so will the rabbits. Thus competition within a niche will tend to have equilibrium points where one of the two species die off.
If an animal looks very different from you, it probably fills a different niche. If it looks exactly like you, you can mate with it. Even if it outcompetes you, it's got enough genes in common with you that your species is still succeeding evolutionarily. But in between, you've got a competitor. Thus there's incentive to help those like you, ignore those different from you, and kill the middle ground. Humans were not always the only hairless ape. If we're so well equipped for survival that we now occupy every corner of the globe, why didn't any other survive? Some evidence suggests humans may have actively killed off neanderthals, not just by outcompeting it for food but by stabbing it with spears.
Returning to the uncanny valley, this would mean that we're bothered when CGI doesn't quite look human because unconciously we think we're looking at a new creature that could somebody replace us. And interestingly, in evaluating a robot, that's a not altogether irrational opinion to hold.