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Bio-mechanical organisation of the modern human mandible in transition from foraging to agriculture

Final Report Summary - BIOMAN (Bio-mechanical organisation of the modern human mandible in transition from foraging to agriculture.)

The work performed since the beginning of the project
The BIOMAN project explored the biological changes that human groups went through with the transition from foraging to an agriculturalist life style. The following work has been performed:

The raw data constituted CT scans of mandibles and, in some cases, mandibles with crania, raw data were obtained from three different sources: The Institute and Museum of Anthropology, Moscow State University (Russia); the “Francis I. Rainer” Anthropology Institute of the Romanian Academy (Romania). Equally, an important part of the project involved collecting archaeological and anthropological literature on the studied populations published in local literature.

The analytical part of the project first of all explored sensitivity of the Finite Element analysis to material properties, physical constraints, muscle orientation and variation in individual morphology and established a reliable path for analyses. Reconstruction of any minor missing surfaces in CT scans and creation of models for further analysis was carried out with the help of available software and mathematical procedures. After that, 3D coordinate data were collected from the complete surfaces and Geometric Morphometric analysis of morphological differences between individuals and between groups was carried out.

Further analysis included regression of mandibular form (i.e. shape and size) on age, geographical coordinates and mode of subsistence. This allowed us to estimate the proportion of variation explained by each of the external factors.

Finally, Finite Element models were created. The FEA analysis has been performed on the following data:
a. Individual fossils and modern subjects: this is a typical approach to finite element analysis of biological subjects.
b. Average shapes for each of the available groups using a single surface to warp to all group averages. In so doing we ensured full control of all variables in the analysis, except for the form (i.e. size and shape) of the mandible.
c. Two forms representing the extremes of the regression of the mandibles on the mode of subsistence. In so doing we identified any differentiation in the biomechanical response of a human mandible that might be attributed to the mode of subsistence.

Results of the FEA for loadings on several individual teeth in the mesial and distal parts of the mandibular arch were export used in the subsequent analysis, including methods of geometric morphometrics and multivariate statistics.

The main results
We have shown that the way the mandible is constrained has a large effect on the mode and magnitude of mandibular deformation while changing muscle vectors or Young’s modulus of teeth, cancellous bone and periodontal ligament to that of cortical bone impacts significantly, but to a lesser degree especially in comparison with individual variation. However, a comparative analysis among individuals is possible if all models are constrained and loaded similarly, given identical material properties throughout and if the internal structure of the mandible is ignored. This allows one to concentrate on any differences in biomechanical performance conferred by the differences in the form of the mandible alone.

Following this approach we found that the Upper Palaeolithic mandible from Oase, Romania, despite appearing more ‘robust’ likely was used to generate similar bite forces to those in modern humans, rather than significantly greater ones, however, it could generate these over more posterior teeth without distraction of the temporo-mandibular joints. This individual appears well adapted to intraoral processing of tough foods by repetitive chewing.

Further statistical analyses of mandibular form and shape of the sampled populations demonstrated considerable spread of individual variation in groups. However, it was still possible to determine a morphological trend that distinguishes between ‘hunter-gatherers’ and ‘agriculturalists’ in our data. When comparing average group shape, a clear morphological trend is defined where mandibles differ by the total size and the shape of the mandibular ramus, with hunter-gatherers tending to have a longer alveolar process and a wider ramus than agricultural groups contributing to more space for the teeth development and for the origin of the temporal masticatory muscles (Figure 1).

FEA analysis of the forms obtained due to the regression on cultural subsistence showed larger deformation in hunter-gatherer mandibles both during the bites on anterior and posterior teeth. However, the total magnitude of developing strains was clearly larger in the ‘modern - agriculturalist’ mandible (Figure 2). Therefore, modern mandibles were less adapted to withstanding large biting forces, which is in contrast with their increased biting efficiency, due to the short mandibular body.

Partial least squares analyses indicate that 18% of the total variance in mandibular deformation arising in biting on M2 is correlated with both subsistence and mandibular form with 45% being correlated with both form and subsistence when biting on I2 (Figure 3). The main feature driving the correlation between subsistence and biting performance was the ratio of body length to ramus height.

Regression analysis of average forms established that geographical location explains 34.4% of variation. At the same time, the age is responsible for 36.3% and mode of subsistence for 42.6% of variation. There is nearly no autocorrelation between variation explained by geography and variation explained by either age or mode of subsistence. However, the age and mode of subsistence in our data shows 34.8% of autocorrelation, suggesting a further direction for research into the biomechanics of recent hunter-gatherer populations.

Final results and their potential socio-economic impact
The present study makes a considerable contribution in understanding the biomechanics of the modern human mandible. The most important results include:
I. Large individual variation in deformation of the mandible when compared with variation in material properties and orientation of muscle vectors;
II. The presence of a relatively weak but noticeable trend in mandibular morphology that separates hunter-gatherers from agriculturalists and modern people.
III. A relatively large percentage of variation in the mandibular form can be explained by the mode of subsistence. Geographical variation, and therefore genetic distances, explain a comparable proportion of variation in morphology. This conclusion is important for understanding micro-evolutionary processes and their velocity in modern humans.
IV. A larger deformation is produced in mandibles of hunter-gatherers. However, the total amount of developing strains is clearly larger in the ‘modern - agriculturalist’ mandible. The modern mandibles are, therefore, less adapted to withstanding large biting forces, which contrasts with their increased efficiency in generating due to the short mandibular body. Rather than adaptation, this implies ‘de-adaptation’ due to under-loading in childhood,

The results of the present study should be used for public education and information on the consequences of soft and highly processed diet for our health and development. They are also applicable as a scientific basis for developing public health recommendations by appropriate official authorities and health organisations.