Model replicates brain and body sizes of seven hominin species from australopithecine scale. Brain expansion driven solely by positive socio-genetic covariation with developmentally late follicle count. No direct selection exists for brain size, body size, or skill level. Direct selection targets only follicle count, decreasing with age. Challenging ecology and cumulative culture generate covariation by altering energy extraction challenges and learning returns. Social arms races fail to evolve human-sized brains due to metabolic costs. Cooperation disfavors brain growth as individuals rely on partners. Development alone canalizes high brain-body allometry under sapiens conditions but rarely reaches hominin sizes without evolution. Evolution orthogonal to direct selection – evolvability decreases to zero. Ancestral genotypes dictate evolutionary outcomes via persistent phylogenetic constraints. Follicle count evolves later onset with higher adult fertility. Body size evolves slower growth to larger adult mass with multiple spurts. Adult skill expands from 2TB to 4TB via learning. Brain metabolic costs affect covariation but not direct fitness. Population size expands with brain size. Socio-genetic covariation propagates developmentally, diverting fertility selection to brain growth. Weakly diminishing learning returns from culture enable human-scale brains. Sharp ecological challenge increase triggers plastic brain size jump toward habilis. EQ evolves from 4x to 6x expected for body size.