Sounds like a non-answer. Doesn't explain why there aren't other half-human, half-ape, semi-reasoning/moral creatures around.
Why would you expect there to be? Our ancestors evolved in a particular direction. All of our hominid non-homo sapiens forbears are extinct. The other apes evolved in a different direction from us. Gorrillas and Chimps aren't likely to evolve into half-man, half-ape.
Where do you expect such half-man, half-ape beings to have evolved from?