I think humans will be around for awhile, even though modern society and our current world system wont last forever.
Nuclear War would be an easy way to end everything, but I honestly think that the world in general has enough common sense to realize that every country would effectively be killing themselves to do so. We fight wars better when we know we can win them.
We've eventually evolve or die off naturally I'm guessing. We'll change into something else, perhaps gaining a bigger brain or nature shrinking it based on the situation. Plagues and such will hit, but by the sheer power of genetics its near impossible for an outbreak of almost anything to kill us. Hell, there are people who are immune to AIDS.
I doubt we'll ever branch of into space though. Physics will probably hold us back on that one.
But, unless like the show we just disappear in an instant, the world will change with us and we'll eventually get to the point when we aren't human anymore, and then our chapter will have closed.