Limb segment proportions of prehistoric Jomon hunter-gatherers from fetal life to adolescence: comparison with four other chronological groups from Japan

Abstract

Adult individuals from the Jomon period are known to have upper- and lower-limb skeletons characterized by relatively longer distal segments (radius and tibia) than their proximal counterparts (humerus and femur, respectively), in contrast to later historical and modern Japanese individuals. However, it is currently unclear at what age the characteristics of Jomon limb segment proportions emerged and reached adult-equivalent levels. To address these issues, we compared representative intersegmental ratios of limb bone length (e.g. brachial index, crural index, and intermembral index) between Jomon and four other Japanese groups from fetal life to adolescence, and described the limb proportions and growth patterns characteristic of Jomon at different stages of growth. The specimens included humeri, radii, femora, and tibiae from 143 prehistoric Jomon, 22 protohistoric Yayoi, 79 medieval Kamakura-Muromachi, 76 early modern Edo, and 311 modern Japanese skeletons, ranging in estimated age from the fifth fetal month to 19 years. The results confirmed that fetal Jomon individuals had relatively longer distal segments than their proximal counterparts in both the upper- and lower-limb skeletons compared with modern Japanese individuals, and that the Jomon maintained their comparatively long forearm and lower leg bones from childhood through adolescence. Furthermore, for all chronological groups, including the Jomon, the respective distal/proximal ratios of the upper- and lower-limb skeletons reached adult equivalence at approximately 1.5 years of age and remained nearly constant throughout subsequent growth. These findings suggest that the elongated forearm and lower leg bones of the Jomon may be derived primarily from the morphogenetic pattern of the limb cartilage primordium during early development, and that group-specific limb proportion characteristics may be established by the first two years of life at the latest.