The 82nd Annual Meeting of the American Association of Physical Anthropologists (2013)

Masticatory and non-masticatory spatial explanations for mandibular symphyseal morphology in extant Homo sapiens


Department of Anthropology, University of Iowa

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The chin is considered a uniquely derived feature of Homo sapiens; however, causal explanations for its presence are debated. While adaptive dynamics related to masticatory biomechanics have been used to explain the presence of the chin, recent preliminary studies on extant H. sapiens have demonstrated an inverse relationship between chin size and facial prognathism, indicating that the size of the chin likely results, at least partially, from differential growth of the maxilla and mandible.

This study tests two sets of hypotheses that have been proposed to account for chin prominence: (1) masticatory biomechanics, and (2) craniofacial spatial dynamics. For the two sets of hypotheses, it was predicted that (1) both symphyseal shape and orientation will affect the strain caused by both lateral transverse and vertical bending stresses in the symphyseal region, and (2) chin prominence should be associated with the relative anterior-posterior placement of the maxilla and dentoalveolar complex. Coordinate landmark data were collected from 3D laser scans of associated Euro-American crania and mandibles, and data were analyzed in MomentMacroJ, Morphologika, and R.

Symphyseal shape and orientation were associated with the strain caused by both lateral transverse and vertical bending stresses. Furthermore, chin prominence was positively associated with the anterior-posterior position of the dentoalveolar complex and maxilla. This suggests that chin development in extant Homo sapiens is, in part, a function of both masticatory stresses experienced at the symphysis, as well as lower anterior maxillary growth reduction, both of which may influence chin morphology across the wider range of Homo.

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