The following points are made by Jaak Panksepp (Science 2005 308:5718):
1) 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 we humans came along. Recent studies in rats, dogs, and chimps [2,3] are providing evidence that laughter and joy may not be uniquely human traits.
2) The capacity to laugh emerges early in child development, and perhaps in mammalian brain-mind evolution as well. Indeed, young children, whose semantic sense of humor is marginal, laugh and shriek abundantly in the midst of their other rough-and-tumble activities. If one looks carefully, laughter is especially evident during chasing, with the chasee typically laughing more than the chaser. As every aspiring comedian knows, success is only achieved if receivers exhibit more laughter than transmitters. The same behavior patterns are evident in the “play panting” of young chimps as they mischievously chase, mouth, and tickle each other .
3) Laughter seems to hark back to the ancestral emotional recesses of our animalian past [3,4]. We know that many other mammals exhibit play sounds, including tickle-induced panting, which resembles human laughter [2,4,5], even though these utterances are not as loud and persistent as our sonographically complex human chuckles. However, it is the discovery of “laughing rats” that could offer a workable model with which to systemically analyze the neurobiological antecedents of human joy . When rats play, their rambunctious shenanigans are accompanied by a cacophony of 50-kHz chirps that reflect positive emotional feelings. Sonographic analysis suggests that some chirps, like human laughs, are more joyous than others.
4) Could sounds emitted by animals during play be an ancestral form of human laughter? If rats are tickled in a playful way, they readily emit these 50-kHz chirps . The tickled rats became socially bonded to the experimenters and were rapidly conditioned to seek tickles. They preferred spending time with other animals that chirped a lot rather than with those that did not . Indeed, chirping in rats could be provoked by neurochemically “tickling” dopamine reward circuits in the brain, which also light up during human mirth. Perhaps laughter will provide a new measure for analyzing natural reward/desire circuits in the brain, which are also activated during drug craving.
1. K. Poeck, in Handbook of Clinical Neurology, P. J. Vinken, G. W. Bruyn, Eds. (North Holland, Amsterdam, 1969), vol. 3
2. T. Matsusaka, Primates, 45, 221 (2004)
3. J. Panksepp, J. Burgdorf, Physiol. Behav. 79, 533 (2003)
4. G. M. Burghardt, The Genesis of Animal Play (MIT Press, Cambridge, MA, 2005)
5. R. R. Provine, Laughter (Viking, New York, 2000)