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Abstract
The more than 1000-fold increase in the cortical surface without a comparable increase
in its thickness during mammalian evolution is explained in the context of the radial-unit
hypothesis of cortical development. According to the proposed model, cortical expansion
is the result of changes in proliferation kinetics that increase the number of radial
columnar units without changing the number of neurons within each unit significantly.
Thus, mutation of a regulatory gene(s) that controls the timing and ratio of symmetric
and asymmetric modes of cell divisions in the proliferative zone, coupled with radial
constraints in the distribution of migrating neurons, could create an expanded cortical
plate with enhanced capacity for establishing new patterns of connectivity that are
validated through natural selection.