Genetic differentiation and the evolution of cooperation in chimpanzees and humans

It has been proposed that human cooperation is unique among animals for its scale and complexity, itsaltruistic nature and its occurrence among large groups of individuals that are not closely related or areeven strangers. One potential solution to this puzzle is that the unique aspects of human cooperationevolved as a result of high levels of lethal competition (i.e. warfare) between genetically differentiatedgroups. Although between-group migration would seem to make this scenario unlikely, the plausibilityof the between-group competition model has recently been supported by analyses using estimates of geneticdifferentiation derived from contemporary human groups hypothesized to be representative of thosethat existed during the time period when human cooperation evolved. Here, we examine levels ofbetween-group genetic differentiation in a large sample of contemporary human groups selected to overcomesome of the problems with earlier estimates, and compare them with those of chimpanzees. We findthat our estimates of between-group genetic differentiation in contemporary humans are lower than thoseused in previous tests, and not higher than those of chimpanzees. Because levels of between-group competitionin contemporary humans and chimpanzees are also similar, these findings suggest that theidentification of other factors that differ between chimpanzees and humans may be needed to providea compelling explanation of why humans, but not chimpanzees, display the unique features of humancooperation.