This Wikipedia article defines laughter in many terms, such as...
"a visual expression of happiness, or an inward feeling of joy"
"a part of human behavior regulated by the brain, helping humans clarify their intentions in social interaction and providing an emotional context to conversations".
Note: the emphasis was added by myself.
The article also states that laughter is "probably genetic", and that
"Scientists have noted the similarity in forms of laughter induced by tickling among various primates, which suggests that laughter derives from a common origin among primate species."
According to this report which the Wikipedia referenced, the expression of laughter in general is present among other great apes. Phylogenetic trees were reconstructed to represent the evolution of this trait among primates (which concludes that the ability to laugh must have a genetic basis, at least to some degree).
Note: I also found this WikiAnswers post, but it clearly can't be that reliable.
A search on the OMIM database yielded this result, where children with Angelman syndrome were characterized with "excessive laughter", this result, in which Charles Bonnet syndrome was characterized with "inappropriate laughter", as well as many other genetic mutations which resulted in some sort of uncontrollable laughter.
I understand that laughter is a complex psychological expression of emotion likely associated with more abstract thought, and that there simply can't be "a laughter gene", but my question is:
Is there any known genetic or physiological origin of laughter? What is biologically different in great apes which allows for laughter, in comparison with a panoply of other animals? I'm looking for more of a molecular answer rather than an ecological or psychological one.