• allomaternal care;
  • prosociality;
  • callitrichids;
  • shared intentionality;
  • food sharing


Despite sharing a recent common ancestor, humans are surprisingly different from other great apes. The most obvious discontinuities are related to our cognitive abilities, including language, but we also have a markedly different, cooperative breeding system. Among many nonhuman primates and mammals in general, cooperative breeding is accompanied by psychological changes leading to greater prosociality, which directly enhances performance in social cognition. Here we propose that these cognitive consequences of cooperative breeding could have become more pervasive in the human lineage because the psychological changes were added to an ape-level cognitive system capable of understanding simple mental states, albeit mainly in competitive contexts. Once more prosocial motivations were added, these cognitive abilities could also be used for cooperative purposes, including a willingness to share mental states, thereby enabling the emergence of shared intentionality. Shared intentionality has been identified as the original source of many uniquely human cognitive abilities, including cumulative culture and language. Shared intentionality rests on a fundamentally prosocial disposition that is strikingly absent in chimpanzees, but present in cooperatively breeding primates. Thus, our hypothesis is that while chimpanzees and perhaps all great apes exhibit many of the important cognitive preconditions for uniquely human mental capacities to evolve, they lack the psychological preconditions. In humans, we argue, the two components merged, the cognitive component due to common descent from ape ancestors and the motivational component due to convergent evolution of traits typical of many cooperative breeders.