The evolution of the parietal lobes in the genus Homo

Abstract

The parietal areas have always been of major interest in paleoneurology because of their remarkable variation among hominids. In neuroanatomy, difficulties in defining their blurred boundaries and their complex functions have delayed a proper quantitative study of their organization and evolution. Paleoneurological evidence indicates the upper parietal cortex, including its deep medial folds, as a probable area of evolutionary change. In modern humans, the intraparietal sulcus shows species-specific features when compared with other primates. The size and proportions of the precuneus represent a determining factor of variability among adults, and a major difference between human and chimpanzee midsagittal brain morphology. This medial element is a relevant connectivity hub of the whole brain, is central for the frontoparietal system, and has an important role in the default mode network. When compared with extinct human species, modern humans display a marked enlargement of the parietal bone and of the parietal lobes, inducing the longitudinal bulging of the whole parietal surface. This morphological change is very similar to the pattern associated, among living humans and between humans and apes, with the size variation of the precuneus. It remains to be understood to what extent such evolutionary variations are due to genetic selection or to environmental and physiological factors. These cortical areas are involved in many complex cognitive functions, but most of all they are central for visuospatial integration, coordinating body management, the eye-hand system, the interaction between body and environment, and the integration between body and inner cognitive levels including self-awareness, egocentric memory, social perception, and mental imagery.