Dr Mona Le Luyer
Dr Mona Le Luyer is a biological anthropologist studying teeth to assess late human evolution and interactions among biological, environmental and cultural modifications. She defended her PhD at the University of Bordeaux in January 2016 on dental evolution in Late Pleistocene and Early Holocene human populations from southwest France, assessing both external (crown dimensions, occlusal wear and nonmetric variations) and internal aspects (enamel thickness, dental tissue proportions, enamel-dentine junction morphology) using 3D-imaging methods (microCT) and geometric morphometrics.
Dr Le Luyer is a Fyssen Postdoctoral Research Fellow at the School with a research project to study enamel biorhythms and childhood growth trajectories.
Dr Le Luyer's research focuses on the evolution of human teeth during the Late Pleistocene and Holocene boundary, which was a key period in our social and cultural evolutionary history associated with major environmental modifications and the transition to agriculture. Based on observations and external crown metrics, a reduction in tooth size and morphological complexity has been recorded at the end of the Pleistocene and it is discussed in relation to the generalisation of the farmer and breeder ways of life. The influence of these changes that have influenced the biological evolution of human groups is still poorly understood, especially on internal dental tissue morphology and structural organisation.
At the meso- and microstructural levels, teeth provide a wealth of unique information about growth trajectories and life-history parameters. This unique amount of biological data is stored at the external level, but mostly internally in enamel and dentine microstructures. Thus, a key for understanding the dental variations observed between Late Pleistocene and Early Holocene human groups is to retrieve accurate information on their development by moving from the external surface deep into the internal structure. Accordingly, by combining meso- and microstructural analyses, Dr Le Luyer aims to progress in the subtle assessment of tooth development variation and its causes in human children.
Le Luyer, M. (2018). La réduction des dents humaines à la transition Pléistocène-Holocène dans le sud-ouest de la France : réévaluation et apports de l’analyse de la structure interne par imagerie 3D. Paleo [Online]:77-86. Available at: https://journals.openedition.org/paleo/.Une réduction de la taille des dents humaines et une simplification de leur forme ont été observées en Europe et débattues en lien avec des changements culturels et environnementaux de la fin du Pléistocène et du début de l’Holocène. Entre autres facteurs, les techniques de cuisson ou la pression démographique ont été proposées comme causes de ces modifications biologiques. Cependant, ces modèles sont toujours débattus et les mécanismes qui ont contribué à la réduction des couronnes ne sont pas entièrement compris à ce jour. Grâce à de nouvelles découvertes et la révision des contextes chronologiques et archéologiques de certains gisements du sud-ouest de la France, une réévaluation la nature des variations des dimensions dentaires a été effectuée pour les individus datés du Magdalénien supérieur, de l’Azilien et du Mésolithique ancien. Des acquisitions microtomographiques d’une partie de l’échantillon ont été réalisées afin de caractériser de manière non invasive la structure interne des dents (épaisseur de l’émail, proportions des tissus dentaires). Si une réduction globale des dimensions dentaires est constatée pour tous les types de dents, elle est plus marquée entre l’Azilien et le Mésolithique ancien. Cette réduction est principalement liée aux changements de volume et d’épaisseur de l’émail, paramètre plastique et écosensible témoignant de variations dans la biomécanique de la mastication. Ces résultats soulignent l’impact des changements environnementaux et culturels du début de l’Holocène. Une révision des modèles impliqués dans ce phénomène de réduction dentaire est proposée. Ainsi, cette approche intégrée, combinant les aspects externe et interne des dents, a permis de dégager des facteurs potentiellement responsables de la variation dentaire observée à la transition Pléistocène-Holocène.
Mahoney, P. et al. (2018). The Biorhythm of Human Skeletal Growth. Journal of Anatomy [Online] 232:26-38. Available at: http://onlinelibrary.wiley.com/doi/10.1111/joa.12709/full.Evidence of a periodic biorhythm is retained in tooth enamel in the form of Retzius lines. The periodicity of Retzius lines (RP) correlates with body mass and the scheduling of life history events when compared between some mammalian species. The correlation has led to the development of the inter-specific Havers-Halberg Oscillation (HHO) hypothesis, which holds great potential for studying aspects of a fossil species biology from teeth. Yet, our understanding of if, or how the HHO relates to human skeletal growth is limited. The goal here is to explore associations between the biorhythm and two hard tissues that form at different times during human ontogeny, within the context of the HHO. First, we investigate the relationship of RP to permanent molar enamel thickness and the underlying daily rate that ameloblasts secrete enamel during childhood. Following this, we develop preliminary research conducted on small samples of adult human bone by testing associations between RP, adult femoral length (as a proxy for attained adult stature), and cortical osteocyte lacunae density (as a proxy for the rate of osteocyte proliferation). Results reveal RP is positively correlated with enamel thickness, negatively correlated with femoral length, but weakly associated with the rate of enamel secretion and osteocyte proliferation. These new data imply that a slower biorhythm predicts thicker enamel for children but shorter stature for adults. Our results develop the intra-specific HHO hypothesis suggesting that there is a common underlying systemic biorhythm that has a role in the final products of human enamel and bone growth.
Zanolli, C. et al. (2017). Is the deciduous/permanent molar enamel thickness ratio a taxon-specific indicator in extant and extinct hominids? Comptes Rendus Palevol [Online] 16:702-714. Available at: http://dx.doi.org/10.1016/j.crpv.2017.05.002.In Primates, enamel thickness variation stems from an evolutionary interplay between functional/adaptive constraints (ecology) and the strict control mechanisms of the morphogenetic program. Most studies on primate enamel thickness have primarily considered the permanent teeth, while the extent of covariation in tooth enamel thickness distribution between deciduous and permanent counterparts remains poorly investigated. In this test study on nine extant and fossil hominids we investigated the degree of covariation in enamel proportions between 25 pairs of mandibular dm2 and M1 by a so-called “lateral enamel thickness diphyodontic index”. The results did not provide an unambiguous picture, but rather suggest complex patterns likely resulting from the influence of many interactive factors. Future research should test the congruence of the “diphyodontic signal” between the anterior and the postcanine dentition, as well as between enamel and the enamel-dentine junction topography.
Le Luyer, M. and Bayle, P. (2017). Microevolution of outer and inner structures of upper molars in Late Pleistocene and Early Holocene humans. Comptes Rendus Palevol [Online] 16:632-644. Available at: http://dx.doi.org/10.1016/j.crpv.2016.11.009.In this study, we investigate outer and inner variations of upper second molars (UM2) for Late Pleistocene and Early Holocene modern humans, at a key-period in our evolutionary history associated with major sociocultural, economic and environmental changes. Non-metric traits have been recorded on 89 UM2 of 66 Upper Paleolithic, Mesolithic and Neolithic individuals, and 40 UM2 have been microscanned to assess variations in enamel thickness (ET) distribution and enamel-dentine junction (EDJ) shape. Major changes are found between Mesolithic and Neolithic periods: a decrease of the metacone expression combined with an increase of the hypocone development; an increase of the heterogeneity of ET distribution between lingual and buccal cusps; and an increase of the development of the dentine horn tips corresponding to the hypocone and, to a lesser extent, to the metacone. These morphological modifications could be linked to the masticatory functional changes associated with the transition to agriculture.
Le Luyer, M. et al. (2016). Internal tooth structure and burial practices: insights into the Neolithic necropolis of Gurgy (France, 5100-4000 cal. BC). PLoS ONE [Online] 11:e0159688. Available at: http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0159688.Variations in the dental crown form are widely studied to interpret evolutionary changes in primates as well as to assess affinities among human archeological populations. Compared to external metrics of dental crown size and shape, variables including the internal structures such as enamel thickness, tissue proportions, and the three-dimensional shape of enamel-dentin junction (EDJ), have been described as powerful measurements to study taxonomy, phylogenetic relationships, dietary, and/or developmental patterns. In addition to providing good estimate of phenotypic distances within/across archeological samples, these internal tooth variables may help to understand phylogenetic, functional, and developmental underlying causes of variation. In this study, a high resolution microtomographic-based record of upper permanent second molars from 20 Neolithic individuals of the necropolis of Gurgy (France) was applied to evaluate the intrasite phenotypic variation in crown tissue proportions, thickness and distribution of enamel, and EDJ shape. The study aims to compare interindividual dental variations with burial practices and chronocultural parameters, and suggest underlying causes of these dental variations. From the non-invasive characterization of internal tooth structure, differences have been found between individuals buried in pits with alcove and those buried in pits with container and pits with wattling. Additionally, individuals from early and recent phases of the necropolis have been distinguished from those of the principal phase from their crown tissue proportions and EDJ shape. The results suggest that the internal tooth structure may be a reliable proxy to track groups sharing similar chronocultural and burial practices. In particular, from the EDJ shape analysis, individuals buried in an alcove shared a reduction of the distolingual dentin horn tip (corresponding to the hypocone). Environmental, developmental and/or functional underlying causes might be suggested for the origin of phenotypic differences shared by these individuals buried in alcoves.
Le Luyer, M. et al. (2016). 3D models related to the publication: Internal tooth structure and burial practices: insights into the Neolithic necropolis of Gurgy (France, 5100-4000 cal. BC). MorphoMuseuM [Online] 2:e1. Available at: http://dx.doi.org/10.18563/m3.2.1.e1.The present 3D Dataset contains the 3D models of external and internal aspects of human upper permanent second molars from the Neolithic necropolis analyzed in the following publication: Le Luyer M., Coquerelle M., Rottier S., Bayle P.: Internal tooth structure and burial practices: insights into the Neolithic necropolis of Gurgy (France, 5100-4000 cal. BC).
Le Luyer, M., Rottier, S. and Bayle, P. (2014). Brief communication: Comparative patterns of enamel thickness topography and oblique molar wear in two early neolithic and medieval population samples. American Journal of Physical Anthropology [Online] 155:162-172. Available at: http://dx.doi.org/10.1002/ajpa.22562.Enamel thickness has been linked to functional aspects of masticatory biomechanics and has been demonstrated to be an evolutionary plastic trait, selectively responsive to dietary changes, wear and tooth fracture. European Late Paleolithic and Mesolithic hunter-gatherers mainly show a flat wear pattern, while oblique molar wear has been reported as characteristic of Neolithic agriculturalists. We investigate the relationships between enamel thickness distribution and molar wear pattern in two Neolithic and medieval populations. Under the assumption that dietary and/or non-dietary constraints result in directional selective pressure leading to variations in enamel thickness, we test the hypothesis that these two populations will exhibit significant differences in wear and enamel thickness patterns. Occlusal wear patterns were scored in upper permanent second molars (UM2) of 64 Neolithic and 311 medieval subadult and adult individuals. Enamel thickness was evaluated by microtomography in subsamples of 17 Neolithic and 25 medieval individuals. Eight variables describing enamel thickness were assessed. The results show that oblique molar wear is dominant in the Neolithic sample (87%), while oblique wear affects only a minority (42%) of the medieval sample. Moreover, in the Neolithic molars, where buccolingually directed oblique wear is dominant and greatest enamel lost occurs in the distolingual quadrant, thickest enamel is found where occlusal stresses are the most important—on the distolingual cusp. These results reveal a correlation between molar wear pattern and enamel thickness that has been associated to dietary changes. In particular, relatively thicker molar enamel may have evolved as a plastic response to resist wear.