Can diet be inferred from the biomechanical response to simulated biting in modern and pre-historic human mandibles?

Abstract

Differences among mandibular remains of past and present populations might be expected to reflect differences in loading history and so, diet. This is because evolutionary and experimental studies and orthodontic observations in modern humans indicate that adult mandibular form is influenced by genetic and loading history. In this study, we apply geometric morphometrics and biomechanical modelling to the mandibles of Upper Palaeolithic, Mesolithic hunter-gatherers and recent and living humans in order to assess if and how differences in adult form reflect subsistence strategies and so, masticatory system loading history. We show, using analyses of size and shape variation, that mandibular form in humans varies in a way that is consistent with the differences among subsistence groups. In particular mandibles from individuals who habitually fed on prepared and softened foods are small and show relative shortening of the mandibular body, among other differences. Using finite element analysis to simulate central incisor and first molar loading, we demonstrate that the performance of the human mandible in terms of resisting deformation covaries with mandibular form (size and shape). However, biomechanical performance in incisor or molar bites reflects only a proportion of the total variance in mandibular morphology; different aspects of morphology contribute to resisting different bites. Nevertheless, differences in biomechanical performance do reflect subsistence mode to some extent, especially for anterior bites. These differences are most strongly associated with mandibular size, the relative length of the body and the form of the gonion; which in turn reflect the degree of mandibular development, and so, loading history. While small, modern mandibles are more efficient at converting muscle to biting forces because of their shortened out lever arm (the body) they are not as capable of withstanding these loads and, for the same input force, deform more relative to upper Palaeolithic and Mesolithic individuals. Thus, we conclude that the differences between modern and prehistoric humans principally arise due to reduced mandibular loading during ontogeny rather than as adaptations to softer diets; they reflect underdevelopment. As such, it is unlikely that morphological and functional comparisons of mandibles across cultural transitions can differentiate anything other than broad aspects of loading history at a population level.