Inside and Out: Virtual anatomy and cerebrocranial evolution of the temporal regions in anthropoid primates

Abstract

The brain does not preserve in the fossil record making any study of brain evolution reliant on endocasts or moulds of the inside of the skull. While endocasts are not the brain but retain neuroanatomical features preserved as imprints on the endocranium including the middle cranial fossae (MCF) which is in close spatial proximity to the temporal lobe of the brain where many cerebral features are retained on the surface of the MCF. Unfortunately for palaeoneurologists, the superior-posterior border of the temporal lobe cannot be defined on endocasts or the external surface of the brain but rely on internal neuroanatomy only observable from dissections or magnetic resonance imaging (MRI) of the brain. The temporal lobe its important in anthropoid evolution where it functions as a multimodal association cortex and is associated with many memory-related tasks, auditory and visual processing, and speech comprehension in modern humans. The methodology includes virtual anatomy techniques such as computed tomography (CT) and micro-computed tomography (uCT) of the skull in extant anthropoids (n= 135) and magnetic resonance imagining (MRI) of the brain in extant anthropoids (n = 78) and CT and uCT in extinct anthropoids (n= 38) with a total sample size for extant and extinct anthropoids (N = 251). I present, a new approach to predict temporal lobe volume (TLV) in extant and extinct anthropoids from the middle cranial fossa (MCF) using seven linear metrics I developed which are biologically homologous in the extant and extinct anthropoid taxa studied to allow for the first time, the quantification of temporal lobe size and middle cranial fossa (MCF). The findings presented in this thesis relate to five research aims. The prediction of (TLV) from any of the seven MCF metrics within +/- 1. S.E in the anthropoid taxa studied (Aim 1); for the first time, the ability to predict TLV in fragmentary fossil remains noting species-specific differences between Homo erectus and Homo ergaster (Aim 2); a reassessment of a foundational study using updated imaging and phylogenetic comparative analyses found contrary that modern humans did not have disproportionately large temporal lobe size but fit the general anthropoid trend (Aim 3); the prediction TLV relative to brain size in fossil catarrhines and extant cercopithecines indicated a relative increase in TLV associated with paleoenvironmental shifts from arboreal fossil catarrhines to semi-terrestrial cercopithecoids with potential increased social complexities (Aim 4); the comparison of absolute and relative TLV and proportions in extant hominids and fossil hominins where the greatest changes occurred in Australopithecus, Paranthropus and extant Pan species potentially reflecting the absolute, relative and temporal lobe size and proportions of the last common ancestor between chimpanzees and fossil hominins (Aim 5). This thesis examined the anthropoid trend over 40 million years of cerebrocranial temporal region evolution, the eco-behavioural trajectories for some anthropoid lineages and the application of phylogenetic comparative methods and virtual anatomy to examine the cerebrocranial temporal regions and enable future study directions for cerebral reorganisation in anthropoid primate evolution.