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.As recently as 6 to 7 million years ago, the hominin lineage split off from the rest of the great ape (hominid) lineage 1 and consequently shares many biological traits and behavioral and cognitive similarities with great apes.
2,3Nevertheless, humans also exhibit remarkable differences from our closest relatives. First, we not only live far longer lives and reproduce at faster rates than do the other great apes, but our offspring take much longer to mature and women cease reproduction well before somatic senescence sets in.4 Second, our ecological niche involves specialization on large, valuable food packages that have to be acquired together, as well as shared, and mandatory reliance on techniques acquired through cumulative culture.5 This niche and our social relationships are based on an un-apelike selflessness, a degree of hypersociality reflected in a concern for others, eagerness to share food and information with others, and cooperation in a wide array of contexts, even with nonrelatives and near-strangers.5-8 Our mode of life facilitates our spread into new habitats, resulting in a ubiquitous geographic distribution.9 Third, with regard to intellectual performance, humans differ from the other great apes, which, as a group, show relatively homogeneous cognitive abilities. [10][11][12]
