Cooperative care as a candidate mechanism linking prolonged childhood and brain expansion in human evolution

Two traits distinguish modern humans from all other primates: an exceptionally large brain and an exceptionally prolonged childhood. What mechanism produced this combination, and why are the two traits dissociated in other hominins? Standard models predict that brain size and developmental duration should co-vary tightly, yet the fossil record reveals that they do not: small-brained hominins at Dikika, Dmanisi, and Rising Star show slow development, while large-brained Neanderthals matured fast. Evolutionary simulations show that cooperative care can offer a coherent account of these dissociations. In Experiment 1, cooperative care extends childhood and, as an emergent consequence, enhances social learning independent of brain size (+32.5% childhood extension with no change in brain size), creating a knowledge–storage mismatch that generates selection pressure for brain expansion. In Experiment 2, when the metabolic ceiling is lifted through allomaternal provisioning, cooperative care populations show significant brain expansion (+34%) co-occurring with further childhood extension and enhanced social learning. In Experiment 3, when populations lacking cooperative care achieve equivalent brain expansion through dietary improvement alone, they instead mature faster, producing brain expansion decoupled from developmental prolongation. The simulation thereby identifies two emergent routes to large brains: a cooperative-care route producing coupled brain–childhood expansion, and a nutrition-only route producing brain expansion without developmental prolongation—a dissociation that parallels the comparison between modern humans and Neanderthals. These results suggest that the distinctly human developmental package may reflect not brain expansion per se, but the cooperative care system within which that expansion occurred.