Here's what I have so far...

(got this from an published abstract)
Research suggests that the capacity for human laughter preceded the capacity for speech during evolution of the brain. Indeed, neural circuits for laughter exist in very ancient regions of the brain and ancestral forms of play and laughter existed in other animals eons before humans came along with hahahas and verbal repartee. The capacity to laugh emerges early in child development and perhaps in mammalian brain-mind evolution as well. Research on rough-housing play in mammals, both sapient and otherwise, clearly indicates that the sources of play and laughter in the brain are instinctual and subcortical.

From animal laughter to human joy
(/end up abstract)

(my partner came up with this)
Laughter is a birthright, a natural part of life. The part of the brain that connects to and facilitates laughter is among the first parts of the nervous system to some one line after birth. Infants begin smiling during the first weeks of life and laugh out loud within months of being born. Even if you did not grow up in a household where laughter was a commong sound, you can learn to laugh at any stage of life. Eventually, we want to incorporate humor and laughter into the fabric of our lives, finding it naturally in everything we do.
(/end of my partner's contribution)

Humor is a coping mechanism to make light of unpleasant events. As the old saying goes, "it is only funny until someone gets hurt, then then it is hilarious."

I've seen chimps and other primates laughing their baldy asses off when something falls out of a tree onto their friends heads, or other comical moments. This obviously precedes the ability to talk.

I had a theory that laughter is a subconscious act. I think it has some effect on memory. Perhaps certain chemicals are released in our brains during laughter to faciliate future recall of a particular event. This is just a theory at this time and I don't have any accredditted research to support it.

It must have some biological use for animals - we have all had the giggles at one point or another, and our inability to control what we laugh at points to it being an effect of some unknown system.

This reminds me...

http://www.youtube.com/watch?v=eXgdSOxaCGI

As Dutch84 noted, children smile and laugh long before they learn to speak.

IMO, there is a world of difference between laughing with others--and, sometimes at oneself--than laughing at and ridiculing others--using others as the butt of the "joke". I am sure we have all read stories in the media about how making "fun" of people, and their beliefs, can often trigger reactions which are anything but funny. Depending personality types--what I call the pneumatological factors--ridicule, sarcasm and put-down jokes will drive some to suicide, others to homicide.

Philosophically speaking, it is said that getting a joke is simply the sudden apprehension of incongruity.

Have you ever noticed that when people are uncomfortable they get super giggly. As if their laughter and ridiculousness is going to distract you from the issue at hand. maybe laughter evolved in ancient man as a means to confuse and distract predators...

The first onsets of LSD also cause either extreme giggling or extreme unsettled behaviour. This feels like theres a huge ball of energy in your stomach and chest and the only alleviation you get is when you laugh. I don't know whether thats important or not, but I've always found it a strange property that could be worth more investigation.

I'll add my small denomination coinage here... No references, just spilling brains:

My understanding is that feral children, while extremely impaired socially, laugh in a socially acceptable way; in the case of a child allied with wolves or wild dogs the laughter seems to be associated with teeth baring. If it can be assumed that such a response would be learned during the normal course of integration into the pack through play fighting and dominance games, what does this reveal about the nature of laughter in a normally developed human being? If it can be said that wolves at play are either in training for battle or recovering from it, it would seem that the laughter of wolves prevents them from tearing one another apart during stressful or uncertain situations involving close relatives or companions.

I would think laughter served the same purpose in human beings until very recently in our history. Though these days we don't usually attack our close relations tooth and nail when under stress, I believe the physiological components of laughter have turned it into a desirable behavior highly valued by the modern brain. Just as human sexual behavior has become a year-round recreational pursuit, so it seems laughter has become a prized luxury of our advanced mental capacity.

What I think I'll spend some more time considering are the actual effects of laughter and their part in a social situation, i.e. irregular breathing, loss of fine motor control, total paralysis.. is this a mechanism for forced submission? Also, what is the difference between laughter resulting from perceived humor and the same behavior resulting from tactile discomfort? One is clearly more severe and less enjoyable.. Actually, someone brought up LSD.. It seems to me the effects mentioned mimic being severely tickled.