I think that humanity will become extinct either through an epidemic caused by a supervirus or perhaps when AI becomes so smart that they decide to eliminate humanity.
The first one is highly probable. The second one is questionable on philosophical grounds.
The first thought that comes to mind is this. If we define “consciousness” as “humaness,” and computers become conscious, then they would be human in all respects. Why? Well, whats the atomic difference between an organic body and a silicone body? Not much if any. Perhaps, then, the defining characteristic of the human being is consciousness, not necessarily a body with arms, legs, brains, etc.
Exhausting our resources is a good canidate.
A meteorite crashing into Earth. (there is a belt that is right beside our solar system, I think)
Nuclear war.
FutureMan going beserk.
Those are the most likely.
I don’t think we will go extinct, call me idealistic but I think we’re always able to keep just ahead of the brink
no, we will evolve into cockroaches
-Imp
I think we’re guarented extinction when the sun turns into a red giant unless we can start some planet hopping; becoming the parasites of the universe.
Wouldn’t that be de-evolving?
Considering that we got here from scratch in few hundred milion years, i think that in few bilion years (when the sun is supposed to burn out) there will be a small portion of humans living on earth, and the rest on different planets. that, of course, if we fail to make earth unliveable till then.
Either someone will say, “I wonder what happens when I do this…”
[size=200]Boom!!![/size]
Or misguided love will spell our doom.
no, we are already cockroaches 
Hmm… How about medical nano-tech experiments going bananas, creating all-new smartplague.
Life-extension flatworm research succeeding and creating near immortal super-rich people who subjugate the ‘norms’ to the point where world-wide civil wars cause new stone-age.
Humans disappear into their own navels en-masse when virtual-reality gets perfected.
I’m with you on the AI option really - we’re going to make one sooner or later… Probably by accident. 
a meteor or other cosmic catastrophy, unless we get off this doomed planet and populate the cosmos.
Suicide after too much philosophical contemplation.
“I don’t want to know too much; wisdom sets limits to knowledge too.”- Nietzsche
I just wanna frolic through the fields, naked and carefree.
Who’s in?