Scientists reckon that human beings in one form or another (albeit probably quite primeval and wayne Rooney-esque) have been around for about half a million years. Nature has thrown everything in it's arsenal at us up until now and we've survived so there's no reason to believe that we can't go on in some capacity for millions more years. The only thing that could wipe us out completely would be a meteor hitting the planet. Nuclear war could've put a significant dent in the population if it had broken out in the back end of the cold war, but there will always be far out of the way places like far Northern Europe or deepest Africa that wouldn't have been affected.