• cortico-cerebellar system;
  • allometry;
  • brain evolution;
  • primates

Although it is commonly accepted that brains work as functionally distributed systems in which interconnected structures work together in processing particular types of information, few studies have investigated the evolution of functionally specialized neural systems across many different lineages. MR-related research has provided in-depth information on connectivity patterns, but because of its focus on particular species, it has given only indicative clues about evolutionary patterns shaping brain organization across primates. Here, we combine depth with breadth of analysis by investigating patterns of covarying size evolution in substructures of the cortico-cerebellar system across 19 anthropoid species spanning 35 million years of divergent evolution. Results demonstrate two distinct patterns of size covariation in substructures of the cortico-cerebellar system, suggesting neural systems involving profuse cortico-cerebellar connections are a major factor in explaining the evolution of anthropoid brain organization. We set out an evolutionary model of relative cortico-cerebellar expansion and provide a detailed picture of its branch-specific evolutionary history suggesting the ape radiation is the clade with the strongest and most consistent evolutionary history in relative (frontal) cortico-cerebellar expansion.