Cerebellar Volume

The volume of the human cerebellum is smaller than expected for an ape of human size, but the cerebellum of non-human apes is much larger than expected in monkeys. It has previously been suggested that developmental programming constrains brain growth of individual neural components, such that size of individual components scales predictably with allometric trends. A principle components analysis of 131 mammalian species demonstrated that brain size did indeed account for over 95% of differences in size of neural components, and that the cerebellum size of humans and other primates followed the expected trend (Findlay and Darlington 1995). However, others suggested that the huge amount of variance that the allometric equation accommodated for in this study (up to 250% structure size difference in two species with the same brain weight) masked phylogenetic grade shifts of certain evolutionarily coordinated neural systems (Barton 2006). To examine this question in the cerebellum, one group measured overall brain volume and cerebellar volume through in vivo MR scans in 44 primates representing 11 haplorhine species (Rilling and Insel 1998). They found that cerebellar volume relative to brain volume was 45% larger than expected in non-human apes, while the relative cerebellar volume in humans was smaller than expected. These results suggest that the evolution of cerebrocerebellar circuit diverged in human and non-human apes, such that the cerebellum became more enlarged in non-human apes, while the cerebral cortex became more enlarged in humans (Rilling and Insel 1998).