If it's unclear whether each of us is made up of cells of more than one species right now (the subject of my last post), it turns out that it's also unclear how many species each of us is made up of as a matter of genetics. In an article ("Annals of Evolution: Sleeping with the Enemy") in the August 15-22, 2011 New Yorker, Elizabeth Kolbert reports that it now appears that all humans except those whose ancestors never left Africa interbred with the Neanderthals they encountered as they (the humans) spread out, with the result that "all non-Africans, from the New Guineans to the French to the Han Chinese, carry somewhere between one and four per cent Neanderthal DNA" (page 71).
From this we learn two things. First, it is possible, in fact typical, for human beings to have within them DNA from another species. Does that make these human beings not human? Obviously not; these DNA-borrowing humans are most of the humans there are. (Whether the Neanderthals should be described as "non-human," by the way, is itself part of what's at issue -- what's the line between humans and non-humans? But as I understand it, even if Neanderthals are humans they're not our particular species of humans, homo sapiens.)
On the other hand, does the lack of this admixture of another species' DNA make someone not actually a human in the sense we should today understand the term? Again, clearly not. Hundreds of millions of members of our species don't have any Neanderthal DNA.
Matters seem likely to grow more complex -- that is, it seems likely that we will find more of these borrowings from other species. Kolbert reports the recent discovery of another non-homo sapiens species, the Denisovans (named for the cave in which a finger bone from the species was found) -- and it turns out that one group of modern humans, the New Guineans, share "up to six per cent Denisovan DNA" (page 74). Presumably this six per cent is in addition to their 1 - 4 % of Neanderthal DNA. And who knows who else we interbred with? As Kolbert emphasizes, it appears that we routinely interbred with those we were exterminating. (Did we exterminate the Neanderthals deliberately, or just as a byproduct of taking the fruit of the land away from them? We don't know. But it's hard not to think that the circumstances of our "borrowing" of DNA were very unpleasant.)
So it is quite possible for members of our own species to carry within them DNA from one, two, perhaps more other, closely related species. And different members of our species carry different mixes of DNA from these other species. Kolbert's article focuses on the search for whatever it may be that makes us distinctively human -- the genes that we have to have, and that those other species lacked. That's a useful search, but the fact remains that even if "we" are the beings who share this as yet unidentified characteristic, there are many things that we don't share with each other, and do share, apparently, with the members of other species, long extinct.
One more point: Suppose there is some genetic essence of humanity that distinguishes us from our close relatives -- the Neanderthals and Denisovans of the past, or the clearly not-human, but very smart, chimpanzees of the present. But once we've identified this essential spark of humanity, what about all the inessential details? Would genetically green hair take someone out of the human race, if he or she held the human spark? That seems implausible. What about webbed feet? Again, that change by itself doesn't seem to make a human being inhuman. But what's the boundary? It's clearly not genetic identity; after all, as Kolbert notes (at 72), there's a lot of genetic variation between members of our species (presumably even leaving aside the heritage of borrowings from other species by some but not all of our ancestors).
In short, it seems as though the definition of human is going to turn out to be an act of will rather than a fact of biology.