I assume this is "human" in the ontological sense? As I understand it, realism claims that the nature of a given thing can be recognized(though never fully understood) by perceiving the whole being. We can't make any claims about the "whole" of an ancient species by looking at the skeleton, or very pretty and very misleading artists' conceptions of the fleshed-out creature.
I asked brought up the following point on another thread months ago, but you wiggled out of it. Darwinism presupposes that biological species are in flux. We are simply another transitional form between our distant ancestors and our distant descendants. Our distant descendants might be similar to us, as certain creatures are the same as very ancient ones. But it is also possible in evolutionary theory that mankind can evolve into a superrational state in which we have sense organs to perceive stuff we can't even imagine now or mental faculties inconceivable by our little minds(this has been absorbed into many flaky new age philosophies). Likewise, it's possible for humanity to evolve into a subrational state(imagine us evolving in symbiosis with a computer system that does all our thinking for us).
Given that man is an incredibly contingent and unstable concept when seen "from the perspective of the Darwinian universe," how can the statement "God became man" be coherent?
I agree with your analysis of Neanderthals. We would have to see a live one to determine whether they are truely human, but it seems to mee that even if we were presented with one, it would be very difficult to judge whether it were fully human. And yes, I mean "human" in the ontological not biological sense.