Abstract :
[en] Modern human and Neanderthal faces present clear morphological differences at all ontogenetic stages. Their post-natal ontogenetic allometric trajectories diverge [1] and in both species, as the various components of the mid-face develop and grow, bone facial remodelling is thought to play a key role in adapting them to their final adult form. In modern humans, maxillary growth is characterised by bone resorption on the anterior surface, whereas in Neanderthals extensive bone deposition is the common finding [2]. This morphogenetic difference is present by approximately 5 years of age [2].
During development, crania are loaded by forces applied to the masticatory system in feeding and manipulation. These change over time, as diet (e.g. weaning) and paramasticatory behaviour change. Differences are known to exist in adults between Homo sapiens and Neanderthals, with Homo sapiens relatively more efficient at generating bite forces but less able to support the strains and deformations produced by such forces [3,4].
Differences in mid-facial form between these species might be expected to be influenced and to influence the distribution and magnitude of strains experienced during masticatory system loading. Since bone adapts to loads, such differences might underlie and contribute to the distinctive distributions of facial remodelling fields in both species and so, to differences in craniofacial growth.
The present study explores the ontogeny of modern human and Neanderthal biting resistance as a preliminary to assessing potential associations between the distributions of facial strains arising from biting and facial remodelling among hominins. Our aim is to see if any differences in strains exist during post-natal ontogeny. We test the null hypothesis that modes and magnitudes of cranial deformation do not differ between modern humans and Neanderthals at each age stage when exposed to similar constraints.
We used ontogenetic series of Neanderthals and modern humans ranging from newborn to adult. Using 44 landmarks and 201 sliding semilandmarks multivariate regressions of cranial shape on size were used to create three surfaces representing the mean infant, juvenile, and adult stages. These surfaces were converted into finite element models and constrained and loaded in a standardised way to simulate right first incisor and P4/dm2 biting. Applied forces and material properties were identical among models to control all variables except craniofacial form. We compared the resulting deformations, maps of von Mises strains and tensile and compressive strains in the maxilla.
The resulting deformations differ in both mode and magnitude between modern humans and Neanderthals. In both incisor and P4/dm2 biting simulations, modelled strains decrease between infants and adults, as is to be expected given differences in size. The infant modern human presents higher strains than the infant Neanderthal over the anterior and inferior maxilla in both biting simulations. This is reversed in the juvenile models and the strains are more similar in adults. Finally, for both biting simulations, modern humans and Neanderthals deform differently, reflecting the differences in developed strains at each age stage.
These findings reflect differences in the dynamics of facial growth between modern humans and Neanderthals. Moreover, the differences in strains in the infant, juvenile maxillae in modern humans and Neanderthals model may to some extent underlie and explain the differences in maxillary surface remodelling in these two species. Further work on a wider range of models and loading scenarios is needed to explore this issue further.
Acknowledgments:
We would like to thanks D. Shapiro, Joan T. Richtsmeier, G. Holoborow, S. Black and L. Scheuer for the information and access to Bosma and Dundee-Scheuer human skeletal collections. For the access and permission to their fossil materials, we would like to thanks the different institutes and their collaborators: Musée national d’Histoire naturelle, Musée de l’Homme (Paris, France), Musée national de la Préhistoire and his director J.J. Cleyet Merle, Museum of Natural History (London, UK), Patrick Semal and the Institut Royal des Sciences naturelles de Belgique (Bruxelle, Belgique), Jean Jacques Hublin and Philipp Gunz from Max Planck Institute for Evolutionary Anthropology (Leizpig, Germany), Luca Bondioli and the Pigorini Museum (Università di Padova, Italy).We would also like to thank the Dan David Center of Human Evolution and Biohistory Research, Shmunis family anthropological institute, Sackler Faculty of Medicine, Tel Aviv University (Tel Aviv, Israel) for granting access to Amud 1. Finally, we thank the NESPOS platform for access to modern and fossil material.
References:
[1] Krovitz, G. E., 2003. Shape and growth differences between Neandertals and modern humans: grounds for a species-level distinction?. Cambridge Studies in Biological and Evolutionary Anthropology, 320-342.
[2] Lacruz, R.S., Bromage, T.G., O’Higgins, P., Arsuaga, J.L., Stringer, C., Godinho, R.M., Warshaw, J., Martínez, I., Gracia-Tellez, A., De Castro, J.M.B. and Carbonell, E., 2015. Ontogeny of the maxilla in Neanderthals and their ancestors. Nature communications, 6(1), 1-6.
[3] Godinho, R. M.,Fitton, L. C., Toro-Ibacache, V., Stringer, C. B., Lacruz, R. S., Bromage, T. G., O'Higgins, P., 2018. The biting performance of Homo sapiens and Homo heidelbergensis. Journal of Human Evolution, 118, 56-71.
[4] O'Connor, C. F., Franciscus, R. G., Holton, N. E., 2005. Bite force production capability and efficiency in Neandertals and modern humans. American Journal of Physical Anthropology: The Official Publication of the American Association of Physical Anthropologists, 127(2), 129-151.
