Complex language would seem to be beneficial to the survival of other species, so why are humans the only species with this trait?

Because it didn't evolve in any other species.

That wasn't very helpful. More to the point, it may not even be true. For all we can say for sure, other hominid species (perhaps Neanderthal?) had language, but didn't survive. In any case, the question of just why a particular trait did or didn't show up more than once in evolutionary history may not have any clear or uniform answer.

The philosophical issue here, I suppose, might be whether the fact (if it is one) that as useful a trait as language only appeared in one species makes some sort of difficulty for the theory of evolution. Someone might claim: if the evolutionary picture really is correct, we would expect to see many species with this trait. Being neither a biologist nor a philosopher of biology, I can't say for sure. But I'm strongly inclined to suspect that this just isn't a good reading of evolutionary theory. Given the complexity of language-capable brains, what might be surprising is that the ability appeared even once. But it could also be that in a few eons, there will have been many species that developed some sort of linguistic capacity. Language has probably been around for a rather short amount of time from the evolutionary perspective. It's not clear why we would expect a relatively new biological trait to be more widely distributed than it is.

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