While I write this, my husband sits in the adjacent room breathing hard and stimulating his zygomatic major muscle. No silly, he's not doing THAT, he's watching "Evan Almighty" and laughing his head off.
Until this evening, I, a surprising un-curious writer, hadn't thought about the 'act' of laughing. According to "How Stuff Works"
Laughter is the physiological response to humor. Laughter consists of two parts -- a set of gestures and the production of a sound. When we laugh, the brain pressures us to conduct both those activities simultaneously. When we laugh heartily, changes occur in many parts of the body, even the arm, leg and trunk muscles.The Encyclopedia Britannica describes laughter as: "rhythmic, vocalized, expiratory and involuntary actions." I think it is much easier to do it, than describe it.
The first laugh may have been in response to the passing of danger. A couple of cave men staring up at the gruesome teeth of some hungry dinosaur and then run like hell for their lives. They meet up in a cave, look at each other and in relief, they vocalize, breathe deeply and out it comes. The world's first human laugh. Of course hyenas and most animals have been enjoying a good joke since the first moment they saw that relatively hairless wonder called: Man.
One of my favorite songs not only tells about laughing, but makes those hearing it want to laugh. The whole movie is wonderfully laugh-riddled: Mary Poppins.