The only feasible way I could see humanity uniting for a sinlge goal would be in a Watchmen esque scenario where an alien force invaded to conquer our planet as you mentioned. And even then, we'd most likely fuck it up given our inferior technology and venal, crass, skeptical and trecherous nature. As for the point of life, I believe there is none. Existential Nihilism sure is a time saver, makes it much easier to just enjoy our pointless existence rather than try and analyse it.
Edit: The Hell? Where'd the thread go?
Edit: The Hell? Where'd the thread go?