Factors influencing cranial variation between prehistoric Japanese forager populations

Abstract

Understanding the factors shaping human crania has long been a goal of biological anthropology, and climate, diet, and population history are three of the most well-established influences. The effects of these factors are, however, rarely compared within a single, variable population, limiting interpretations of their relative contribution to craniofacial form. Jomon prehistoric foragers inhabited Japan throughout its climatic and ecological range and developed correspondingly varied modes of subsistence. We have previously demonstrated that a large sample of Jomon crania showed no clear climatic pattern; here, we examine variation in Jomon crania in more detail to determine if dietary factors and/or population history influence human intrapopulation variation at this scale. Based on well-established archaeological differences, we divide the Jomon into dietary groups and use geometric morphometric methods to analyse relationships between cranial shape, diet, and population history. We find evidence for diet-related influences on the shape of the neurocranium, particularly in the temporalis region. These shape differences may be interpreted in the context of regional variation in the biomechanical requirements of different diets. More experimental biomechanical and nutritional evidence is needed, however, to move suggested links between dietary content and cranial shape from plausible to well-supported. In contrast with the global scale of human variation, where neutral processes are the strongest influence on cranial shape, we find no pattern of population history amongst individuals from these Jomon sites. The determinants of cranial morphology are complex and the effect of diet is likely mediated by factors including sex, social factors, and chronology. Our results underline the subtlety of the effects of dietary variation beyond the forager/farmer dichotomy on cranial morphology and contribute to our understanding of the complexity of selective pressures shaping human phenotypes on different geographic scales.

Supplementary information: The online version contains supplementary material available at 10.1007/s12520-023-01901-6.

Keywords: Diet; Foragers; Jomon; Morphological variation.

Fig. 1

Map showing sampled Jomon sites and dietary regions used in this paper. The main islands of Japan (Hokkaido, Honshu, Shikoku, and Kyushu) are labelled. There are no Jomon from Shikoku included in this study. Coloured pentagons indicate dietary group; numbers indicate site. Dark blue: Hokkaido, purple: northeastern Honshu, grey: Coastal central Honshu, black: Inland central Honshu, orange: South/West Japan (Table 1). 1: Funadomari; 2: Takasago; 3: Irie; 4: Kotan Onsen; 5: Kitakogane; 6: Miyano; 7: Ebishima; 8: Wakaumi; 9: Yoshigo; 10: Ikawazu; 11: Tochibara; 12: Tsukumo; 13: Yamaga; 14: Einomaru; 15: Todoroki; 16: Goryo. Figure composed in Inkscape (Inkscape project, 2020)

Fig. 2

A craniofacial landmark set (Table S2); B facial landmark set (Table S3); C neurocranial landmark set (Table S4); D temporalis region landmark set (Table S5); E bite-force landmark set (Table S6); this landmark set is not shown from front because all landmarks are clearly visible in side or base views. In A–D, all landmarks are true landmarks; in E, landmarks 1–12 are true landmarks, and 13–26 are semilandmarks, denoted by smaller size. For explanatory text for D and E, see “Dietary groups” below. Figure composed in Inkscape using images from AVIZO

Fig. 3

Boxplot with jitter showing bite force by sex. Figure created using PAST and edited for aesthetics using Inkscape

Fig. 4

Left: CVA showing neurocranial shape variation amongst dietary groups. Dark blue: Hokkaido; purple: northeastern Honshu; grey: coastal central Honshu; black: inland central Honshu; orange: South/West Japan. Wirefames (see right-hand image) show extremes of respective CVs in lateral views. Figure composed in Inkscape using images from MorphoJ and AVIZO

Fig. 5

Left: CVA of temporalis shape amongst dietary groups. Dark blue: Hokkaido; purple: northeastern Honshu; grey: coastal central Honshu; black: inland central Honshu; orange: South/West Honshu. Wirefames (see right-hand image) show extremes of respective CVs in lateral views. Figure composed in Inkscape using images from MorphoJ and AVIZO

Fig. 6

Boxplot with jitter showing bite force by dietary group. NEH, northeastern Honshu; SWJ, South/West Japan; HOK, Hokkaido; CCH, coastal central Honshu; ICH, inland central Honshu. Figure created using PAST and edited for aesthetics using Inkscape

Fig. 7

Correlograms showing spatial autocorrelation in size (left column; PC1 scores) and shape (right column; centroid sizes) between sites for different landmark sets. Figures created in R