The Origins of Human Cooperation Through Intergroup Relations in Our Closest Living Relatives

Luke Daniel Bakic-Pawlak, Stuart Wigby

Cooperation is the foundation of human society, yet our understanding of its evolutionary origins and underlying mechanisms remains limited. This review draws on intergroup encounters in our closest evolutionary relatives, chimpanzees (Pan troglodytes) and bonobos (Pan paniscus), to better understand the evolutionary pressures and processes that shaped human cooperation. In chimpanzees, parochialism is demonstrated by in-group bias and coalitionary aggression against rival groups. Chimpanzee parochialism is reinforced by social behaviors within groups, such as fission-fusion dynamics and strong social ties, driven by the fitness benefits – in terms of increased female fecundity– of territorial expansion. By contrast, bonobos cooperate with unrelated strangers from different groups, supported by female coalitions and sociosexual behavior within and between groups, because reduced aggression increases mating opportunities. Physiologically, the oxytocinergic system is crucial in regulating parochialism in chimpanzees, but does not regulate out-group cooperation in bonobos. Instead, the serotonergic pathway plays an important role in reinforcing bonobos’ cooperative tendencies. Human cooperation encompasses extreme forms of both parochialism and cooperation with strangers, raising the question of how we evolved these seemingly contradictory tendencies. The self-domestication hypothesis offers a plausible evolutionary framework, suggesting that selection against reactive aggression has led to increased prosocial and cooperative tendencies. Further research into the cognitive basis of cooperation in bonobos and chimpanzees, as well as genetic comparisons between humans and known domesticated species, can help clarify the roots of our uniquely cooperative nature.