Cooperative breeding as a likely early catalyst of human evolution

Unlike any other great ape, humans give birth to large, secondarily altricial babies, show precocial social development, have bigger brains that require a long maturation period, and engage in cooperative breeding (CB). These traits, which characterize the human adaptive complex, are intricately linked and must have mutually reinforced each other over evolutionary time. Here, we use recent evidence from paleontology, developmental psychology, and pediatrics, complemented with comparative analyses to ask what may have triggered this coevolutionary feedback loop: bipedality, direct selection on altriciality, a higher-quality diet, or cooperative breeding. An early adoption of extensive allomaternal care during human evolution, i.e. the CB-first model, best accommodates the available data. In particular, CB was a catalyst enabling further increases in brain size, because even though larger brains slow down life history and neurodevelopment and thus lead to a demographic dilemma, CB enabled the necessary increase in birth rates.