Abstract
Brain size tripled in the human lineage over four million years, but why this occurred remains uncertain. To advance our understanding of what caused hominin-brain expansion, I mechanistically replicate it in-silico by modelling the evolutionary and developmental (evo-devo) dynamics of hominin-brain size. I show that, starting from australop-ithecine brain and body sizes, the model recovers the evolution of brain and body sizes of seven hominin species, the evolution of the hominin brain-body allometry, and major patterns of human development and evolution. Analysis shows that in this model the brain expands because it is “socio-genetically” correlated with developmentally late preovulatory ovarian follicles, not because brain size is directly selected for. The socio-genetic correlation causing the recovered hominin brain expansion is generated over development by ecology and possibly culture. Thus, in this model, direct selection that does not favour brain expansion provides a force that developmental constraints divert causing hominin-brain expansion.
Competing Interest Statement
The authors have declared no competing interest.
Footnotes
This is a major revision. The whole text has been revised for clarity and conciseness. The Result section has been reordered to read more naturally. Additional results for the afarensis scenario have been included. Terminology that can be misinterpreted has been removed (e.g., ``drive'' or ``creative''). ``Human'' in the context of brain expansion is now ``hominin'', including in the title, for added precision. The original version contained a Box 1, which has been removed and the new Fig. 5 more accurately describes the fitness landscape. The choice of ancestral genotypic traits used to initialise the numerical solutions has been improved. All figures and supplementary files have been revised for conciseness and clarity. A new Table 1 has been added, listing the values of key parameters.
