Abstract
Significant differences in facial form between the Jomon and the post-Jomon series of the Yayoi, Medieval, and Early Modern period have been interpreted in terms of the gene flow from the Asian continent or the reduction of masticatory stress through the Jomon to the Early Modern period. However, the developmental process that produced these differences in facial form between the two groups has remains unknown. In this study we examined the factors contributing to differences in facial form by studying the developmental patterns of facial form using data from 311 subadult individuals from the middle-late Jomon (c. 5000–2500 BP) to the Early Modern period (1900–1950 AD). We found statistically significant differences in facial form among the temporally defined groups at early stages of development. Differences in upper facial height between the Jomon and the post-Jomon series were significant during both childhood (3–6 years, P < 0.05) and adolescence (12–20 years, P < 0.01). Differences in simotic index and bigonial breadth between the Jomon and the post-Jomon series were significant, beginning in childhood (P < 0.05), and differences in symphyseal height between the series were significant during both infancy (0–3 years, P < 0.05) and adolescence (P < 0.01). Based on previous ethnographical data, studies on the wear of the deciduous dentition in the Jomon and post-Jomon populations, and stable isotope analyses for reconstructing weaning diets in Jomon and Medieval children, it seems to be the first-stage juvenile stage (6–9 years) when the differences in masticatory load among the chronological groups become conspicuous enough to act on the functional adaptation of facial bones. Therefore, the patterns of group differences which begin in infancy or childhood and become remarkable in adolescence do not contradict the former explanation that the gene flow from the Asian continent largely contributed to the change of facial form during the transition between the Jomon and Yayoi periods.
