Psychology. Beyond a joke: from animal laughter to human joy?

be consistently associated with specific DNA sequence motifs (17). These comparisons suggest a number of potentially useful studies. Although technically challenging, it may prove fruitful to examine regional variation in chro-matin accessibility in mammalian meiotic cells. How does this affect the action of recombination-related proteins such as Spo11? In addition to Spo11, at least 11 other proteins are involved in the initiation of double-strand breaks and recombination in yeast (21), and many of the responsible genes have orthologs in humans (such as, RAD50, RAD51, and MRE11). Comparisons of these genes in humans and chimpanzees could reveal differences that affect recombination patterns. In yeast, recombination hotspots can be eliminated by the insertion of the Ty transposable element , which suppresses recombination in nearby sequences (22). Thousands of Alu and LINE1 mobile elements have been differentially inserted in humans and chimpanzees since their divergence 5 million to 6 million years ago (23). Could these elements act in a fashion similar to yeast Ty, contributing to the rapid divergent evolution of recombination hotspots in humans and chimpanzees? Studies such as that by Winckler et al. demonstrate the value of comparative genomic analysis for understanding basic biological processes such as recombination, and for potentially improving the design of genetic association studies. Their work also demonstrates the utility of analyses of within-species diversity and underscores the need for DNA sequence information from large samples of humans and other species. As this information accumulates, our understanding of biology, as well as our ability to design well-conceived gene-mapping studies, will continue to evolve and improve. I n the beginning was the word…but was the word funny? 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 (1), and ancestral forms of play and laughter existed in other animals eons before we humans came along with our hahahas and verbal repartee. Recent studies in rats, dogs, and chimps (2, 3) are providing evidence that laughter and joy may not be uniquely human traits. The capacity to laugh emerges early in child development, and perhaps in mam-malian 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 …