Human beings often trade with strangers. When chimpanzees, our closest evolutionary cousins, meet strangers, they routinely kill them. What accounts for humanity's greater penchant for cooperation?
Anthropologists traditionally have argued that our ancestors evolved while living in small bands of close kin, so cooperation within the band helped their genes survive. A study reported in the March 11 issue of Science casts doubt on this theory, known as "inclusive fitness." A research team led by the anthropologists Kim Hill of Arizona State University and Robert Walker of the University of Missouri analyzed data on band composition for 32 contemporary hunter-gatherer societies. "In our sample of 32 societies," they found, "primary kin generally make up less than 10 percent of a residential band."
If this finding holds for our Stone Age ancestors, it suggests that "inclusive fitness cannot explain extensive cooperation in hunter-gatherer bands." The researchers speculate that tribes whose members cooperated more easily eventually out-competed less cooperative groups and thus succeeded in passing along their genes and behaviors to their descendants. If so, the division of labor and peaceable trade are what make us human.