A deformation-based approach to the frontal lobe morphology in OH9, UA 31 and Bodo

Abstract

Frontal lobes are considered key cerebral areas in human brain evolution, particularly because of the role of the frontal cortex in executive functions and language. Apart from general brain size variation, the morphology of the frontal lobes did not display major differences among species of the human genus. Nonetheless, some changes in frontal proportions have been described in modern humans and Neanderthals, deserving further enquiries. In this study we describe the frontal lobe morphology of three Early to Middle Pleistocene African key specimens: OH 9 (H. erectus, c. 1.4 Ma, Tanzania), UA 31 (H. erectus-ergaster, 1 Ma, Eritrea), and Bodo (H. heidelbergensis, c. 0.6 Ma, Ethiopia). Using digital endocasts, frontal lobe form comparison is performed through a landmark-free registration method based on surface deformation, and both local and global information are used to compute topological mapping of shape differences. When compared with a modern human endocast, the three specimens display some differences. The frontal lobes of OH 9 are generally flatter, throughout their dorsal surface, while in the case of UA 31 and Bodo flattening is more restricted to the upper lateral areas. Whenever, at the Eastern African regional scale, these three fossils express some time-related morphoarchitectural evolutionary trends, therefore we must assume that frontal areas underwent first a general bulging of the whole dorsal surface, then a further dilation of the upper lateral surface. Interestingly, in the modern cast this method does not evidence any consistent relative widening of the lower frontal areas, including the third frontal circumvolution.