This gets posted a lot, but nobody ever seems to post what the thing was.
The answer is probably "other hominids". Humans (Homo sapiens specifically) co-existed with them for a long time and competed with them over resources.
Edit: and the genetically deformed (with whom it would be beneficial to not breed, at least from an evolutionary standpoint) and corpses or people with disease
To elaborate more, I think that it's because if humans have an aversion to other hominids and corpses, we wouldn't try to waste resources attempting to breed with them.