Cashmore, Lisa Anne
The expression of bilateral asymmetry in the hands and humeri: a methodological comparison.
University of Southampton, School of Humanities,
The population-level preference for the use of the right hand is one of the defining characteristics of Homo sapiens and as such, its evolutionary origins within the human genus remain a topic of interest. Identifying hand preference in extinct hominin species is complicated by the difficulty in distinguishing markers of laterality in the bones of the upper limb. These difficulties are further compounded by the range of osteological methods available to study asymmetry and the under-representation of the bones of the hand in such studies.
To better understand the evolution of handedness within the hominin lineage, the effect of methodology on asymmetry expression must first be clarified, as this in turn influences our notions of handedness in individuals and groups. The current study took an inclusive approach to the measurement of upper limb asymmetry in both modern human and non-human primate samples. To assess the contribution that the bones of the hand can make to asymmetry research, data from the metacarpals and phalanges were compared with that from the humerus, a more commonly-studied region of the upper limb. Both metric and musculoskeletal stress marker (MSM) data were collected and compared in order to assess the comparability of asymmetry profiles generated by contrasting methodological approaches. Asymmetry was determined for a sample of modern human skeletons and a non-human primate sample comprising Pan troglodytes schweinfurthii and Gorilla gorilla gorilla.
Two key findings emerge from these analyses: 1) The expression of asymmetry differs, in terms of both direction and magnitude, between the bones of the hand and the humerus. Differences are also apparent between the metacarpals and phalanges. 2) Metric and MSM methods differ in the asymmetry profiles they generate, with the MSM method generally underestimating the magnitude of asymmetry present in a sample, relative to that identified by a metric approach. In addition, the various skeletal samples studied exhibit variation in their relative asymmetry profiles which can be attributed to potential differences in functional recruitment patterns in the upper limbs of these individuals.
Together, these findings clearly highlight the care that must be taken in analyses of asymmetry, due to the level of methodological variation present in currently inter-changeable approaches. The relatively neglected region of the hand has an important contribution to make to our understanding of asymmetry in the upper limb. The results of this study recommend the adoption of a more inclusive approach to the study of upper limb bilateral asymmetry, particular when inferences are to be made regarding handedness. By combining methodological approaches and incorporating data from across the upper limb, a more accurate picture of asymmetry expression will emerge and allow us to better understand the evolutionary development of this trait in our hominin ancestors.
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