  Expansion of the neocortex is a hallmark of human evolution. However, it
remains an open question what adaptive mechanisms facilitated its expansion.
Here we show, using gyrencephaly index (GI) and other physiological and
life-history data for 102 mammalian species, that gyrencephaly is an ancestral
mammalian trait. We provide evidence that the evolution of a highly folded
neocortex, as observed in humans, requires the traversal of a threshold of 10^9
neurons, and that species above and below the threshold exhibit a bimodal
distribution of physiological and life-history traits, establishing two
phenotypic groups. We identify, using discrete mathematical models,
proliferative divisions of progenitors in the basal compartment of the
developing neocortex as evolutionarily necessary and sufficient for generating
a fourteen-fold increase in daily prenatal neuron production and thus traversal
of the neuronal threshold. We demonstrate that length of neurogenic period,
rather than any novel progenitor-type, is sufficient to distinguish cortical
neuron number between species within the same phenotypic group.
