Endocranial proportions and postorbital morphology of the Minatogawa I and IV Late Pleistocene Homo sapiens crania from Okinawa Island, Japan

Abstract

The purpose of this paper is to clarify the endocranial and postorbital morphologies of the terminal Pleistocene Minatogawa people, and obtain insights into their evolutionary background and genealogical relationships. The Minatogawa I and IV crania were compared with 83 Homo sapiens specimens (including 19 prehistoric Jomon). Metric comparative analyses and observational evaluations revealed that the two Minatogawa endocrania are characterized by a common suite of features (or tendency), including a small endocranial volume, relatively low endocranial shape, distinctly broad temporal region (qualitatively associated with the strong temporal bulge), weak parietal boss, and weakly swollen frontal bulge. Postorbital constriction was confirmed to be strong, relative to both upper facial breadth and maximum cranial breadth. Partial correlation analysis and bivariate comparisons were performed to examine the possible associations of Minatogawa’s strong postorbital constriction. The results suggest that constriction relative to the face is predominantly due to a large facial breadth (frontal endocranium not so narrow in Minatogawa I), and also in part to strong neuro-orbital disjunction, while constriction relative to the posterior cranium is largely attributable to endocranial shape. Comparisons with modern/recent H. sapiens materials and limited but informative outgroup specimens (Skhul V, and an example of Homo erectus, Daka) suggests that some of the features characteristic of Minatogawa are possibly ancestral retentions of early H. sapiens (e.g. strong temporal bulge, marked postorbital constriction relative to the face), while others are probably derived population features (e.g. a small endocranial volume, weakly swollen frontal bulge, marked postorbital constriction relative to the posterior cranium). In overall endocranial proportions, the Jomon tend to lie closer to Minatogawa than does the modern Japanese, but such morphologies were also found in individuals of other populations, and thus the similarity does not necessarily support the hypothesis of Minatogawa–Jomon genealogical closeness.