During evolution, the cerebral cortex has undergone tangential expansion and an increase in the number of cortical areas. Besides its extensive tangential expansion, the hallmark of primate cortex is the selective enlargement of the supragranular layer compartment that is considered to underlie the highly developed computational abilities of the human brain. Major non primate-primate differences in cortical development are underlined by differences in the dimensions, cellular composition and developmental timing of the germinal zones. The sub-ventricular zone (SVZ) enlarges along with cortical expansion and primates have evolved a specialized SVZ that is split into the inner SVZ (ISVZ) and the outer SVZ (OSVZ). The emergence of the OSVZ is arguably a key factor underlying evolutionary adaptive change in the development of primate cortex.The OSVZ of area 17 - the primary visual area- of the primate is highly developed and can be further subdivided into internal and external layers.

We laser dissected brain regions from developing area 17 and 18 of embryonic day 80 Maccaca fascicularis and deep sequenced the microRNAs (miRNAs) in the germinal layers and in the cortical plate. Each of these developing anatomical regions could be distinguished by their miRNAs profiles. miRNAs which are present in primate and in some cases carnivore lineages, but not in rodent, contributed to the differential expression in the most recent anatomical innovations within the germinal zones. Prominent among the targets of these novel miRNAs, as identified by RISCtrap, were regulators of the cell cycle and neurogenesis. Co-evolution between the emergent miRNAs and the target was apparent. We conclude that multiple regulatory events involved in the control of neural precursor proliferation underlie the emergence of specific features of the cortex in primates.