Hmm...good question. When I think about it, they all sound like possibilities. Humanity has a history of wars, so I could never exclude that we destroy one another that way. An asteroid has already hit the Earth, and when it did, it wiped out nearly all life-forms that inhabited this place. Who is to say that can't happen again? An infection, I doubt one could spread around the entire globe and deliver a 100% mortality rate, but I can't say that it's impossible either. Over population, I don't think that'll cause our doom. It could cause problems, for sure. But not the end of the world. Basically, what I'm saying is that those are all possible candidates, and if one doesn't happen, then one of the others will...eventually.
