The Embryonic Origins of Primate Encephalization

Encephalization is one of the defining characteristics of the primate Order. Unlike other mammalian radiations, primates exhibit exceptionally high relative brain sizes at birth and across prenatal development. This indicates that the shared degree of adult encephalization in primates is the developmental product of changes to early brain or body growth that have never been fully characterized. This dissertation examines brain and body growth relationships across prenatal ontogeny in a wide range of primate and non-primate mammals in order to reexamine the developmental origins of primate relative brain size.A review of allometric brain/body growth over fetal development shows that primate prenatal encephalization is shared by all primate radiations but not the closest out-groups, and begins during embryonic development. Fetal rates of exponential brain growth acceleration in primates are within the range of eutherian values; species with larger adult brains or isocortical proportions do not exhibit faster fetal brain growth. Neither allometric nor acceleration data support theories proposing faster fetal brain growth in mammals according to physiological or life history variables. Rates of fetal body and visceral organ growth acceleration are exceptionally slow in primates, consistent with slow postnatal body growth rates and life history schedules. Embryonic development is characterized by high brain/body proportions in many non-primate mammals; however, only primates retain this high allometric proportion into later fetal stages of development. This novel feature of primate growth is likely a consequence of slower post-cranial body growth, rather than any particular feature of primate brain growth and development.This study provides developmental evidence that increases in relative brain size at the origin of the primate Order may have been a consequence of body size reduction, possibly as an adaptation to locomotion within an arboreal niche.