Phylogenetic development of brain and brain sulci in primates

The purpose of this study was to investigate the phylogenetic development of the cerebrum and cerebral sulcus in primates. The species selected were Macacus, Hylobates, Pan and Homo sapiens. These samples are classified as old world monkeys (Cercopithecidae), anthropoid apes (Pongidae), and Man (Hominidae). Although these four species divided up and went their separate ways from about the Oligocene era, the pattern of the cerebral sulci is similar. Of various cerebral sulci, the cingulate and calcarine sulci were selected, because they run on the medial surface of the cerebrum. The length of these sulci and fronto-occipital (FO) length were measured by a "cotton-thread" method. With the increase of size (FO-length) and weight of the brain, these sulci became longer, but there were no significant differences in the ratio of the calcarine sulcus to the FO-length among these four species. On the contrary, the ratio of the cingulate sulcus to the FO-length in Pan and Homo sapiens was significantly higher than in the other species, indicating that this ratio becomes higher with the phylogenetic development. The results of the present study suggest that the ratios of these sulci to the FO-length can be used as good indices to assess the degree of the phylogenetic development of the brain.