Cortical folding and the evolution of the human brain

Abstract

The degree of cortical folding can be measured by the gyrification index (GI) and has undergone a previous comparative analysis. In this paper the ontogenetic features of cortical folding that are of interest for an analysis of human brain evolution are examined. The human-specific rostro-caudal distribution of GI maxima arises early in ontogeny. There is no evidence that cortical folding in the parietal region changes prior to the rest of the cortex. Increases in GI values and brain weight are significantly correlated during the first half of brain weight growth. Following that, the brain continues to enlarge, but changes in absolute cortical folding only match that of brain weight, thus producing a constant degree of cortical folding. When the human brain evolved, normal developmental processes could generate a bigger brain containing a region with the same degree of cortical folding as found in great apes providing that the two features, gyrification and absolute brain size, changed together. Both ontogenetic and comparative data suggest that during human evolution, changes in cortical organization occurred when the brain enlarged.