How about both?

Really the UN could work like that... but only if western governments grew more of a consious.

Also well... aliens. If aliens show up the world will unite together and be predjudiced against them.

Well that is where the conspiracy of the "New World Order" comes from. The fear that the world will be ruled by one government body.

We're all already governed by the laws of nature and physics. So in that regard it has been that way for as long as the earth has existed.

As for human government more than likely any world government would be built on the basis of establishing basic human rights and then allowing "states" within that structure to set themselves up as they see fit. I would actually really enjoy seeing what happens when you let people choose between the major political ideologies they wish to live under.

It would be quite revealing to see which weaknesses appear and which philosophy was able to sustain itself the longest. 4 states - whenever a state collapses or loses X% of its population to other states over Z years it would be dissolved and reformed anew (ideally learning from mistakes) under whoever remained. If the state ever drops to less than 5% of world population it is dissolved and its land is split amongst the remaining states proportional to population.

This is of course far to cold to test on real people - but the concept of the experiment is fascinating.

http://www.youtube.com/watch?v=yHbnM_42mQE
ANSWER!

humanity will unite when there is a super,major, extraordinary crisis, like aliens or a event of extintion.