Quantifying the lateralization in the upper limb of historical human populations: a detailed study of the cortical bone distribution in the humeral diaphysis

The study of long bone morphology in modern human and fossils hominins plays a central role in understanding the pattern of physical activity during the life of individuals and, by extension, to the more general living conditions of populations [1]. The investigation of long bones robusticity and shape allows to reconstruct long-term repeated stress, due to occupational or physical activities. The remodelling of the cortical bone of the diaphysis is due to a skeletal response to biomechanical loadings during lifespan. Several studies [2],[3] have noted males showing a higher degree of bilateral asymmetry in the humerus than females; this observation has been related to sex-specific differences in activities and division of labour. In this study we investigate the pattern of distribution of the cortical bone in the humeral diaphysis in an explorative sample including 12 individuals (10 male, 2 female and 1 indeterminate) with both right and left humeri acquired via Computerized Tomography. It represents a preliminary approach to the study of the variability between population from early Middle Ages in Italy. On each humerus we extracted 61 cross sections along the diaphysis from the 20% to the 80% of the total biomechanical length. On each cross section we calculated 48 equiangular semilandmarks: 24 on the periosteal and 24 on the endosteal contour by using the morphomap R package [4]. Then, we determined the thickness between paired semilandmarks (periosteal and endosteal) and we built morphometric maps of cortical thickness for each individual. Eventually, we computed the difference between right and left sides and mapped the differences in morphometric maps. In this way, the diaphysis is unrolled and the differences in thickness along the direction (anterior-lateral-medial-posterior) and the longitudinal axis are reported respectively on the x and y axis. Our exploratory examination confirms the hypothesis that this sample, mainly composed of males of the working class, shows different levels of robusticity. We also notice that, in right humeri, antero-medial part of the diaphysis is thicker than in left humeri; particularly, in five individuals we found the dominance of a side (2 right dominance and 3 left). We hypothesize that the observed bilateral asymmetry is probably due to specialization in occupational activities. These preliminary results, in turn, give support to the adoption of the methodology we tentatively used here. Therefore, it can be now applied to a wider sample of populations between the Roman times and the Middle Ages.