Comparative Neuroanatomy in Primates

Abstract

Comparative neuroanatomy across primate taxa provides a uniquely powerful framework for understanding the evolutionary origins of enlarged neocortex, enhanced prefrontal connectivity, and the neural substrates of complex cognition. This study quantified 18 neuroanatomical variables -- including absolute and relative brain mass, neocortex ratio, cerebellar volume, prefrontal cortex fraction, primary visual cortex (V1) area, and corpus callosum cross-sectional area -- in 28 primate species spanning prosimians, New World monkeys, Old World monkeys, lesser apes, and great apes, using high-resolution 3T MRI volumetric analysis of 312 post-mortem specimens held in seven European comparative neuroanatomy collections. Encephalisation quotient (EQ) ranged from 1.14 +/- 0.18 (Microcebus murinus) to 6.84 +/- 0.42 (Pan troglodytes), with humans excluded from the primate comparison to avoid confounding. Neocortex ratio (neocortex volume / rest-of-brain volume) was most strongly predicted by social group size across species (Pearson r = 0.82, p < 0.001), consistent with the Social Brain Hypothesis, and by dietary frugivory index (r = 0.74, p < 0.001). Prefrontal cortex fraction increased significantly with phylogenetic proximity to hominids (F4,23 = 18.4, p < 0.001). Cerebellar volume scaled allometrically with neocortex volume (slope = 1.14 +/- 0.06 on log-log axes, p < 0.001), suggesting coordinated expansion of cerebellar and neocortical circuits across primate evolution. Corpus callosum area scaled sub-linearly with brain volume (slope = 0.82), indicating relatively reduced interhemispheric connectivity in larger-brained species. These results quantify the mosaic nature of primate brain evolution and provide a phylogenetically controlled reference dataset for comparative neuroscience and clinical neuroimaging studies.