What is the evolutionary basis behind laughter?

Laughter is, as the poster ahead of me mentioned, important as communication. In humans laughter developed as a response to humor which is a rather sophisticated aspect of our communication. Communication is so important to humans evolutionarily because depending on who you follow in biological theory it may very well be the reason we were able to evolve at all. While the Raymond Darts of the world will say it was because we were predators, the Sussman's of the world will say it was because we were pray. In either theory communication was key. Hunters needed to be able to choreograph attacks against larger animals, and if Sussman was correct, early hominids would have needed to be able to communicate warnings to each other about possible predators.In both cases you can see how communication would have helped chances of survival. Even if the theory that strikes you is man the dancer (read enough criticism of Sussman and you'll get the joke) communication is still a key evolutionary feature, and laughter is just one complex section of that (along with all the other subtle cues such as body language that we have developed).

Laughter (or equivalent communication of emotions) is very important in social animals like us. In a social species a group of individuals that communicates well has a better chance of functioning well, and hence has a better chance of surviving.

rats laugh and so do dogs. In different ways though. Apes laugh too. Animals do laugh just like us.