Genetic aspects

The discussion of genetic aspects initiated in lecture 4 was extended with an example of gene duplication followed by the evolution of a duplicated element. This was the SRGAP2A gene for Slit-Robo Rho-GTPase activating protein, a protein involved in cell migration and arborization morphogenesis. In Homo sapiens and Homo neanderthalensis (but not in chimpanzees, orangutans or gorillas), SRGAP2A has two duplications that generate closely related truncated forms, SRGAP2B and C, whose expression affects the speed of neuroblast migration (formation of the six cortical layers) and the formation of dendritic spines, slowing down maturation (cell neoteny). The lecture continued with a description of genetic coexpression modules specifically present in the human cortex due to a high percentage of genomic rearrangements. A human cortical module, virtually absent in chimpanzees, contains a large number of genes involved in energy metabolism, synaptogenesis and cytoskeleton regulation. We can infer that the spectacular increase in parallel data analysis power, linked to the increase in cortical size, has imposed a concomitant increase in energy demand. We must therefore link the two parameters of the explosion in "cortical circuitry" and metabolic demand, and insist once again on the role that the evolution of nutrition may have played in the ability of humans to develop a brain that consumes 20% of their daily energy. The lecture concludes with a discussion of the excessively prolonged time of cortical development in sapiens and the consequences of this postnatal maturation for our species' cognitive performance.