The brain and its main anatomical subdivisions in living hominoids using magnetic resonance imaging

Primary comparative data on the hominoid brain are scarce and major neuroan atomical differences between humans and apes have not yet been described sa tisfactorily, even at the gross level. Basic questions that involve the evo lution of the human brain cannot be addressed adequately unless the brains of all extant hominoid species are analyzed. Contrary to the scarcity of or iginal data, there is a rich literature on the topic of human brain evoluti on and several debates exist on the size of particular sectors of the brain , e.g., the frontal lobe. In this study we applied a non-invasive imaging technique (magnetic resonan ce) on living human, great ape and lesser ape subjects in order to investig ate the overall size of the hominoid brain. The images were reconstructed i n three dimensions and volumetric estimates were obtained for the brain and its main anatomical sectors, including the frontal and temporal lobes, the insula, the parieto-occipital sector and the cerebellum. A remarkable homogeneity is present in the relative size of many of the lar ge sectors of the hominoid brain, but interspecific and intraspecific varia tion exists in certain parts of the brain. The human cerebellum is smaller than expected for an ape brain of human size. It is suggested that the cere bellum increased less than the cerebrum after the split of the human lineag e from the African ancestral hominoid stock. In contrast, humans have a sli ghtly larger temporal lobe and insula than expected, but differences are no t statistically significant. Humans do not have a larger frontal lobe than expected for an ape brain of human size and gibbons have a relatively smaller frontal lobe than the rest of the hominoids. Given the fact that the frontal lobe in humans and great apes has similar relative size, it is parsimonious to suggest that the rel ative size of the whole of the frontal lobe has not changed significantly d uring hominid evolution in the Plio-Pleistocene. (C) 2000 Academic Press.