The development of bite force resistance and cranial form in Neanderthals and modern humans

The general aim of the thesis is to understand how biting mechanics interact with cranial form to impact post-natal craniofacial ontogeny in modern humans and Neanderthals. To this end, CT scans of ontogenetic samples of 12 Neanderthal and 63 modern human crania were collected and a series of reconstructions of Neanderthal crania were carried out. Geometric morphometric and multivariate regression approaches were used to create a craniofacial growth model for each species. Using these two models, 3D virtual crania representing the mean adult, juvenile, and infant were extracted in each species. These 6 mean crania were then converted into finite element models and used to conduct two biting simulations: at the right second premolar or second deciduous molar (RP2/RdM2) and right first incisor (RI1), applying the same muscle forces for all models because these are unknown especially for Neanderthals. This study compared modes and magnitudes of deformation, and the distribution and magnitude of tensile and compressive strains between the mean infant, juvenile, and adult models within each species and between the two species at each age stage.
The morphometric analyses indicate that cranial ontogenetic trajectories differ be-tween modern humans and Neanderthals. The finite element analyses (FEA) in both biting simulations indicate that, within each species, the mean infant juvenile and adult models deform differently. Further, in both biting simulations, the highest strains are localised over similar regions of the cranium; over the anterior maxilla, orbits, and anterior subnasal surface. Modern humans and Neanderthals deform differently and show differences in the development of biting forces during RI1 and RP2/RdM2 biting simulations at each stage. These findings confirm that modern human and Neanderthal crania have divergent postnatal developmental trajectories and manifest differences in the resistance of masticatory system loadings throughout life. Differences in modes of deformation and so, strain distributions are considered in light of known differences in craniofacial bone growth remodeling between Neanderthals and modern humans. The findings show some correspondence with the remodeling maps for both species, particularly during RP2/RdM2 biting simulations. They do not falsify the hypothesis that facial remodeling differences arise because of differences in load resistance, and so, in the strain environment during post-natal development. As such, how differences among adult crania arise through post-natal interactions between form and functional loadings merits further investigation through more detailed analyses of a wider range of loading scenarios.