What Is Human?
A lot of people are talking about the immediate liklihood of humanity soiling and depleting the planet so horribly that we all end up dying of famine, plague, and war. Call this the bioapocalyptic side, in the question of whether or not humanity survives the coming century or so.
Many others have pointed out the possibility that the planet gets whacked by a big rock, or the Sun decides to do something unexpected, or a random burst of gamma rays from ten million light years away just happens to intersect our planet and boil off the oceans. Or, really, any of a hundred other natural catastrophes that could wipe out the species in a geological eyeblink. There's not much to be done about it: no matter how advanced the species (or what descends from it) becomes, there will always be natural disasters sufficiently bad to wipe it out. I will, however, point out the 65 million year extinction cycle, stretching back hundreds of millions of years and due, oh, any time now. Lets call this scenario the cosmocalypse, because in the end it includes the possibility that the universe itself simply ceases to be.
Then there are those who have been discussing the imminent possibility that mankind might begin drastically accelerating its evolution. So drastically that in a few hundred years, while intelligent activity is unquestionably far more abundant than it is even now, there won't be much that we today would recognize to be human, exactly. Most of those who talk about this (I'm guilty of being one of them myself, occasionally) call themselves transhumanists, but over on the extreme you can find, as always, the most entetainingly loony, at the technocalypse Yahoo! group. It's not even necessary to read the posts: just look at the collection of memes gathered under their keywords. At any rate, because we're talking about the possibility of humanity becoming extinct here, I'm going to go ahead and label this the technocalyptic side of the argument.
So, assuming we avoid a bioapocalypse and a cosmocalypse, we're left with a best-case scenario of a technocalypse. To a certain extent the latter of course comes down to a question of what is human. Is it Homo sapiens sapiens, and nothing else? Or, going backwards, would we include Neandertal man? What about Homo erectus, which innovated the use of both watercraft and fire, used tools throughout its history, and almost certainly possessed a rudimentary language of some sort? I myself would draw the line when the Australopithecines were the only hominids around. Any further back and you're in ape country.
What if we apply the same principle looking forward? What are the traits that define 'human'? Despite the fact that H. sapiens and H. erectus are morphologically almost identical, the human is unlikely to be purely taxonomic. Ask yourself if a person who just happened to have four arms, while being utterly normal otherwise, would be considered human. Most would say yes. Then start adding extra body-parts, sensory capabilities, and anything else you can think of. At what point does the creature stop being human? Or does it, so long as it also continues to talk and use tools? Morphology, in and of itself, could be completely beside the point when it comes to what matters about humanity.
My anthropology prof maintained that the single thing that separates humans from animals is fire. Or, more properly, the harnessing of energy outside the body in order to process matter, also outside the body (using this definition we can throw nuclear reactors into the same pile as camp-fires.) Everything else - language, tool use - is a matter of degree rather than kind. Her argument was that fire was unique, and extremely powerful as an adaptation. It is also, crucially, cultural rather than biological.
What if purely memetic criteria were to be used? Take the wholly cultural man, one who consists only of tools: an upload, utterly faithful to the mind of the original but in fact software running on manufactured circuitry. Is that entity human? Would a society composed solely of them be a human civilization? There are those who would answer it would be, so long as the entities inhabiting the computers (and probably robots) thought of themselves human, and expect to survive for millenia in just such a state. But what about when you start tinkering with the entity's thought processes? Increasing memory, speed, intelligence, and connectivity with others? Does a point come at which the structure of its mind is so different that, although the principle things defining our species (fire, and to a lesser degree language and tool use) are more abundant than ever before, there's nothing remotely human left? There's a compelling argument that there is, and to those who expect to achieve immortality through uploading I have this to say: how long do you think you'd remain you, living life inside an environment you control utterly, your mind running at such speeds that a year can last a subjective millenia ... or a subjective million years.
Of course it's also possible that not everyone will choose to transcend their humanity through technology. In the Orions arm scenario, humanity swarms by the trillions, like bacteria in an ecology of intelligence.
So in the end, there's no way of really knowing how long, or whether, the species will survive, and maybe it's best to act as we've always acted, as though ours (or our kids', or our grandkids') is the last generation that will ever be. But it sure is fun to think about, eh?

