Spatial relationship between the mental foramen and mandibular developing teeth in modern humans, chimpanzees, and hamadryas baboons.

The mental foramen (MF) of adult human mandibles is characterized by its high position and posterosuperior opening orientation, compared with that of nonhuman primates. In this study, to examine when and how such interspecies variations in MF position/orientation are manifested in the context of dental development, positional relationships between the MF and nearby forming teeth (dc, dm1, C, P3) were assessed using CT-scanned growth-series mandibles of the following three species with various MF positions and anterior dental sizes: modern humans, chimpanzees, and hamadryas baboons. Results showed that modern humans have more antero-superiorly located MF and dc than the two nonhuman samples during growth, whereas the MF and dm1 of hamadryas baboons are the most inferiorly positioned. Considering that the mandibular canal generally reaches the dc/dm1 position during infancy, the species differences in MF position can be attributed largely to the positions where the anterior deciduous teeth first appear. Specifically, the distinctive dental position of modern humans should stem eventually from the comparatively small anterior dental size. Furthermore, human MF position shifted slightly upwards with alveolar development, unlike in the nonhuman samples, accompanied by strong curvature and vertical elongation of the anterior mandibular canal. Meanwhile, the human MF shifted from a forward to a lateral/backward orientation in association with human-specific growth-related alveolar recession. The findings of this study, thus, collectively indicate that taxonomic variations in adult MF position/orientation can be interpreted partly by the positions of the surrounding developing teeth.