I had a similar thought myself not too long ago, and one of the Repliers said that there's a good chance that the uncanny valley comes from our fear of death, the recognition of a dead body.

I don't know if to them it was an off the cuff comment, but I think there might be some truth in that.

There were indeed many non-human apes.

Also corpses

There were also other Homo genus species that were genetically incompatible and wouldn't produce viable offspring. And disfiguring diseases you wanted to avoid.

IDK why people try to being Eldrich horrors into this, it's a very reasonable and common evolutionary adaptation. Most other mammals just mainly use scent for this adaptation, our noses are shit but our eyes are decent so use sight.

Because its fun! The rational explanation is fine but pliestocene vampire fantasy is fun.

Maybe? Or maybe it's just a byproduct of a very finely tuned system used to tell individual humans apart.