"Laughter that occurs during everyday social interaction in response to banal comments and humorless conversation is now being studied," write Matthew Gervais and David Sloan Wilson. "The unstated issue is whether such laughter is similar in kind to laughter following from humor."

Using empirical evidence from across disciplines, including theory and data from work on mirror neurons, evolutionary psychology, and multilevel selection theory, the researchers detail the evolutionary trajectory of laughter over the last 7 million years.

Evolutionarily elaborated from ape play-panting sometime between 4 million years ago and 2 million years ago, laughter arising from non-serious social incongruity promoted community play during fleeting periods of safety. Such non-serious social incongruity, it is argued, is the evolutionary precursor to humor as we know it.

However, neuropsychological and behavioral studies have shown that laughter can be more than just a spontaneous response to such stimuli. Around 2 million years ago, human ancestors evolved the capacity for willful control over facial motor systems. As a result, laughter was co-opted for a number of novel functions, including strategically punctuating conversation, and conveying feelings or ideas such as embarrassment and derision.

"Humans can now voluntarily access the laughter program and utilize it for their own ends, including smoothing conversational interaction, appeasing others, inducing favorable stances in them, or downright laughing at people that are not liked," write Gervais and Wilson.

Gervais, Matthew and David Sloan Wilson "The Evolutions and Functions of Laughter and Humor: A Synthetic Approach." Quarterly Review of Biology, Dec. 2005.

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