Living anthropoid primates - monkeys, apes and humans - have proportionally larger brains than lemurs and lorises (Strepsirrhini). The size differences are concentrated in certain brain regions, including the olfactory bulb, neocortex, and cerebellum. These shifts in size and proportions are often highlighted in discussions of differences in adaptive responses to social and environmental pressures between anthropoids and strepsirrhines. Shifts in brain size and shape also are used to explain some differences in cranial shape among living primates. Most of these comparative analyses, however, are based only on studies of the brain of living primates. Fossil evidence for parallel brain size increase between anthropoid clades provides a valuable opportunity to test developmental and spatial constraints on brain size and cranial organization from both a comparative and historical perspective.
Many details of brain size and proportions in extinct primates can be estimated from the size and proportions of endocasts, a cast of the internal braincase. Endocasts are virtually rendered using micro-CT scans of crania. This research, conducted by doctoral student Kari Allen, under supervision by Dr. Richard Kay at Duke University, explores the evolution of brain size, shape, and proportion, and its relationship to cranial form, by examining virtual endocasts of both living and extinct anthropoids to identify trends through geologic time in various primate groups. Three-dimensional geometric techniques are used to characterize global and regional differences in endocast shape that may carry a functional or phylogenetic signal. The reconstructed size and shape of selected regions of the endocasts are used to evaluate the timing of cerebellar, cerebral and frontal lobe expansion and their relationship to aspects of cranial form. The paleontological dataset includes early fossil anthropoids from Africa and South America, and European Eocene basal primates. These new data provide useful information to test adaptive explanations about brain evolution that were developed from comparative studies of living primates. Undergraduate student training will be incorporated into the research activities, and the archived, freely-available micro-CT scans of living primates will serve as teaching materials.
