A Re-examination of the Sinodonty/Sundadonty Dental Complex and the Peopling of Japan

The primary theory for the peopling of Japan is the dual origin hypothesis that states there were two separate migrations into Japan separated by more than 10,000 years. The early migration involved the ancestors of the Jomon who in turn were ancestral to the Ainu populations in Hokkaido and Sakhalin. A later migration dating to about 2200 BP was comprised of Neolithic farmers known as the Yayoi. There is debate over the origins of both the Jomon and Yayoi, with the dual origin hypothesis positing that the Jomon are Southeast Asian in origin while the Yayoi are East Asian. Others postulate that Jomon origins could lie in Northeast Asia and the Yayoi in Southeast Asia. To re-examine this debate, dental morphological data were analyzed for Jomon (n=643) Ainu (n=285) individuals categorized by island: Honshu, Hokkaido, and Sakhalin. Trait frequencies were compared to East Asian, Southeast Asian, Polynesian, Micronesian, Melanesian, New Guinea, and Australian samples to explore population relationships. Sinodont/Sundadont traits were analyzed through ANOVA for Jomon, Ainu, East Asian, and Southeast Asian populations to determine which traits showed significant differences among the groups. Analysis shows the modern Japanese are quite distinct from, and not related to, the Jomon and Ainu. The most likely geographic origin for the Jomon is Southeast Asia. Gene flow between East and Southeast Asians from the early Holocene on may contribute to some of the problems of interpreting Jomon and Yayoi origins. A lack of significant differences for UI1 double shoveling, UM1 enamel extensions, UP1 root number, LM1 deflecting wrinkle, and cusp number LM2 shows the possibility of Holocene gene flow between East Asia and Southeast Asia. UI1 shoveling most strongly characterizes the migration patterns in the dual origin hypothesis. UI1 shoveling is a trait associated with the EDAR 370A gene variant as well as Sinodonty, meaning that the gene could aid in characterizing migrations into Japan