How did altruism and reciprocity evolve in humans? Perspectives from experiments on chimpanzees (Pan troglodytes)

The evolution of altruism and reciprocity has been explained mainly from ultimate perspectives. However, in order to understand from a proximate perspective how humans evolved to be such cooperative animals, comparative studies with our evolutionary relatives are essential. Here we review several recent experimental studies on chimpanzees' altruism and reciprocity. These studies have generated some conflicting results. By examining the differences in the results and experimental paradigms, two characteristics of prosociality in chimpanzees emerged: (1) chimpanzees are more likely to behave altruistically and/or reciprocally upon a recipient's request, than without request, and (2) chimpanzees also show a tendency to regard others and help in contexts not involving food. Supposing that these two characteristics of altruism, recipient-initiated altruism and non-food altruism, were present in the common ancestor of chimpanzees and humans, it is possible that increased social cognitive abilities, capacity for language, necessity for food sharing, and enriched material culture favored in humans the unique evolution of cooperation, characterized by voluntary altruism and frequent food donation.