First of all, this is the BEST definition of laughing ever: "A laugh is characterised by a series of short (75 msec) vowel-like syllables that are regularly repeated at a rate of about 5 per second". (1) (If this doesn't make you laugh, I don't know either! ;)
Secondly, apparently, apes laugh as well, and laughter plays an important role as a kind of social glue both in human and ape society. (2)
Thirdly, there are plenty of physiological benefits to laughter, such as lowering blood pressure and reducing stress hormones. (3)
I would reckon, that laughter evolved from the need to relax, because even our ancestors probably could not maintain sky-high adrenaline levels all the time. Also, it helped to signal: Danger is over, or something similar. But these are just my guesses now.
Hope this helps a bit!
1. Human Evolution by John L. Bradshaw