The Fossil Record of Anthropoid Brain Evolution

Objectives: This study examines brain evolution in early fossil anthropoids using virtual reconstructions of endocranial morphology. Materials and Methods: We estimated the size of the brain and the proportions of its components using new estimates of body mass and a broad sample of virtual endocasts of extant primates and Oligocene–mid-Miocene anthropoids. Results: Brain size relative to body mass and brain proportions relative to brain size were evaluated. Compared with body mass, (1) Brain size enlargement has occurred convergently in many anthropoid lineages. (2) Tarsiers and anthropoids evolved smaller olfactory bulbs and larger neocortices than strepsirrhines. (3) Enlargement of tarsier and anthropoid non-frontal regions (occipital, parietal, and temporal lobes), not of the frontal lobe, accounts for larger neocortex size. Considering brain proportions relative to brain size yields similar findings: (1) Tarsiers and extant and fossil anthropoids have smaller olfactory bulbs and larger neocortices than strepsirrhines. (2) Tarsier and anthropoid neocortical expansion occurred principally in regions where visual signals are processed. (3) Correspondingly, species with relatively more visual input also have larger non-frontal neocortex. Discussion: Shifts in brain proportions found in the stem anthropoid Simonsius, and stem platyrrhines and catarrhines were established before brain enlargement. The neocortex was enlarged, with disproportionate expansion of regions where visual signals are processed, emphasizing the heightened importance of the haplorhine visual system. Depending on the phylogenetic position of omomyoid haplorhines, large shifts in neocorticalization either occurred separately in stem tarsiids and anthropoids or once in the stem lineage leading to tarsiids and anthropoids.