Debates about altruism are often based on the assumption that it is either unique to humans or else the human version differs from that of other animals in important ways. Thus, only humans are supposed to act on behalf of others, even toward genetically unrelated individuals, without personal gain, at a cost to themselves. Studies investigating such behaviors in nonhuman primates, especially our close relative the chimpanzee, form an important contribution to this debate.

This week in PLoS Biology, Felix Warneken and colleagues from the Max Planck Institute for Evolutionary Anthropology present experimental evidence that chimpanzees act altruistically toward genetically unrelated conspecifics. In addition, in two comparative experiments, they found that both chimpanzees and human infants helped altruistically regardless of any expectation of reward, even when some effort was required, and even when the recipient was an unfamiliar individualall features previously thought to be unique to humans. The evolutionary roots of human altruism may thus go deeper than previously thought, reaching as far back as the last common ancestor of humans and chimpanzees. In a related article, Frans de Waal discusses the issues brought out by this discovery.