Developmental structure in brain evolution

How does evolution grow bigger brains? It has been widely assumed that grow th of individual structures and functional systems in response to niche-spe cific cognitive challenges is the most plausible mechanism for brain expans ion in mammals. Comparison of multiple regressions on allometric data for 1 31 mammalian species, however, suggests that for 9 of 11 brain structures t axonomic and body size factors are less important than covariance of these major structures with each other. Which structure grows biggest is largely predicted by a conserved order of neurogenesis that can be derived from the basic axi tl structure of the developing brain. This conserved order of ne urogenesis predicts the relative scaling not only of gross brain regions li ke the isocortex or mesencephalon, but also the level of detail of individu al thalamic nuclei. Special selection of particular areas for specific func tions does occur, but it is a minor factor compared to the large-scale cova riance of the whole brain. The idea that enlarged isocortex could be a "spa ndrel," a by-product of structural constraints later adapted for various be haviors, contrasts with approaches to selection of particular brain regions for cognitively advanced uses, as is commonly assumed in the case of homin id brain evolution.