Cortical folding, the lunate sulcus and the evolution of the human brain
Controversy over the placement of the lunate sulcus on the Taung australopithecine endocast has been central in the debate as to whether cortical reorganization occurred independently of and preceded the expansion of the human brain. A new technique, the gyrification index, measures the degree of cortical folding. A comparison of the index among extant great ape and human brains shows that the values of the two taxa overlap only in the caudal cortex where the lunate sulcus is found. The similarity in degree of cortical folding in the caudal cortex contrasts to differences in all the other portions where the human cortex has a higher degree of folding than found in ape brains. The conservation in the degree of caudal cortical folding is unexpected if, during human evolution, the parietal association cortex had enlarged independently of brain size so much that the lunate sulcus moved posteriorly. The findings are more consistent with interpretations that the major differences in folding between human and ape parietal and occipital cortices arose concurrently with changes in brain size. Although these data do not localize the position of the lunate sulcus on the Taung endocast, the similarity in degrees of folding among hominoids is consistent with interpretations that, given its small cranial capacity, the lunate sulcus of the Taung specimen is in a pongid position.