The failure of the gyrification index (GI) to account for volumetric reorganization in the evolution of the human brain
The gyrification indices (GI) recently compared for chimpanzee and human brains by Armstrong et al. (1991) ignores important volumetric differences in primary visual striate cortex (area 17 of Brodmann) and the lateral geniculate body which provides the optic radiation to the visual cortex. Allometric relationships for these structures are very strong in non-human primate brains based on the data of Stephan et al. (1981). In human brains, however, their observed empirical values are over 121 % less than expected for a primate of such a brain size. The regions that show the same overlap of values between chimpanzee and human cortex of the GI are not homologous given the sharp reduction in lateral extent of primary visual striate cortex in the human brain. The GI as used by these authors ignores important reorganizational changes between chimpanzee and human despite volume differences in brain size. Thus, it is doubtful that the GI can be used to assess the position of primary visual striate cortex in the Taung hominid endocast, or to argue for the primacy of brain enlargement before brain reorganization. To assess the simultaneity of volume and reorganizational changes in early hominid evolution, it will be essential to have a better understanding of the paleoneurological evidence. Although controversial, the Hadar AL 162-28 endocast of the 3 + MY Australopithecus afarensis provides evidence for morphometric reorganization of the occipital and parietal lobes of the brain prior to any dramatic increase in overall brain volume.
