When we try to define what it means to be human, we look for features that are 1. universal among humans and 2. not found in any other animals. Laughter falls into both categories, and is also a collective activity that binds us together. It's even contagious -among humans.
Building on these insights, scores of theorists have attempted to explain why humans evolved to be the species that laughs. One classic idea is the Superiority Theory, according to which the loudest laughs were originally cries of triumph made at the expense of the enemy. Another is the Relief Theory, in which laughter is thought to have evolved long before words or grammar, as an instinctive way of signalling that danger had passed and everyone could relax. Finally, the Ambivalence Theory holds that laughter erupts as a means of escape from contradictory emotions or perceptions.
What these ideas have in common is their focus on individual psychology. In each case, the thinking is that tension is released with the sudden realisation that there is nothing to fear. For supporters of the Superiority Theory, the initial threat comes from other people who are suddenly exposed as harmless. The Relief Theory agrees that we laugh upon realising we are safe. The Ambivalence theory also proposes that laughter arises when a mental or physical challenge or paradox suddenly dissolves.
Then again, to be honest, we once thought big brains made us human until we studied dolphins, and then though tool use made us human until we met crows. But laughter as a particularly human trait offers an intriguing theory, which you can read about at Aeon. -via Digg