I think the problem with coming to this conclusion is simply the fact that we are all alone in our genus, and we are even all along in our general bipedal method of locomotion among primates. It's possible that there's something in the Homo genus that simply doesn't appear in any extant primate species, but would have appeared loud and clear in the Australopithecines. Likewise, there are sometimes species-specific oddities whereas other traits are more species-general.
Sort of like how odd it is that we have slightly webbed fingers and toes when no other primate has that.. I know plenty of Ancient Astronaut types think this is proof that we are a product of alien genetic engineering, but the problem is that this assumes you go straight from chimpanzees and gorillas straight to anatomically and behaviorally modern humans when, in reality, it didn't happen anything like that.
It's one of the bigger tragedies of history, that our ancestors and brethren could not survive to the present alongside us. If Australopithecines, Paranthropus, and archaic Human species still lived, I'm sure several of these theories wouldn't be.