Childhood First: Prolonged Development Before Big Brains in Human Evolution

The prevailing account of human life-history evolution holds that selection for larger brains drove the prolongation of childhood: because neural tissue is metabolically costly, encephalization necessarily slowed developmental pace. Recent fossil evidence calls this causal sequence into question. Across multiple hominin lineages spanning over three million years (including Australopithecus afarensis, early Homo at Dmanisi, and Homo naledi), prolonged developmental timing appears in species with chimpanzee-sized brains. Moreover, Neanderthals exhibited faster dental development than Homo sapiens despite comparable or larger brain volumes, and the prolongation of childhood in our lineage, including a distinctive phase of postnatal brain globularization, continued after maximum brain size had been attained. Together, these patterns indicate that brain size and developmental pace are decoupled and that the extension of human childhood was not a passive consequence of encephalization. I propose the Childhood-First Hypothesis, which reverses the conventional causal arrow: the emergence of a cooperative childcare niche, in which allomaternal provisioning and early weaning extended juvenile dependency, may have preceded and facilitated brain expansion rather than resulting from it. Drawing on fossil geochemistry, comparative primatology, computational modeling, and developmental neuroscience, I argue that the evolutionary foundations of human cognition were social before they were neural. If this cooperative childcare niche (a self-reinforcing caregiving ecology, continuously reconstructed across generations) created the conditions under which encephalization became viable, then current theoretical frameworks may overstate the causal primacy of brain size and understate the role of social organization in shaping human developmental trajectories.