Metric and nonmetric dental variation and the population structure of the Ainu

Ethnic derivation of the Ainu inferred from ancient mitochondrial DNA data

Objectives

The Ainu, the indigenous people living on the northernmost island of Japan, Hokkaido, have long been a focus of anthropological interest because of their cultural, linguistic, and physical identity. A major problem with genetic studies on the Ainu is that the previously published data stemmed almost exclusively from only 51 modern‐day individuals living in Biratori Town, central Hokkaido. To clarify the actual genetic characteristics of the Ainu, individuals who are less influenced by mainland Japanese, who started large‐scale immigration into Hokkaido about 150 years ago, should be examined. Moreover, the samples should be collected from all over Hokkaido.

Materials and methods

Mitochondrial DNA haplogroups of 94 Ainu individuals from the Edo era were successfully determined by analyzing haplogroup‐defining polymorphisms in the hypervariable and coding regions. Thereafter, their frequencies were compared to those of other populations.

Results

Our findings indicate that the Ainu still retain the matrilineage of the Hokkaido Jomon people. However, the Siberian influence on this population is far greater than previously recognized. Moreover, the influence of mainland Japanese is evident, especially in the southwestern part of Hokkaido that is adjacent to Honshu, the main island of Japan.

Discussion

Our results suggest that the Ainu were formed from the Hokkaido Jomon people, but subsequently underwent considerable admixture with adjacent populations. The present study strongly recommends revision of the widely accepted dual‐structure model for the population history of the Japanese, in which the Ainu are assumed to be the direct descendants of the Jomon people.

Reconstructing human population history from dental phenotypes

Dental phenotypic data are often used to reconstruct biological relatedness among past human groups. Teeth are an important data source because they are generally well preserved in the archaeological and fossil record, even when associated skeletal and DNA preservation is poor. Furthermore, tooth form is considered to be highly heritable and selectively neutral; thus, teeth are assumed to be an excellent proxy for neutral genetic data when none are available. However, to our knowledge, no study to date has systematically tested the assumption of genetic neutrality of dental morphological features on a global scale. Therefore, for the first time, this study quantifies the correlation of biological affinities between worldwide modern human populations, derived independently from dental phenotypes and neutral genetic markers. We show that population relationship measures based on dental morphology are significantly correlated with those based on neutral genetic data (on average r = 0.574, p < 0.001). This relatively strong correlation validates tooth form as a proxy for neutral genomic markers. Nonetheless, we suggest caution in reconstructions of population affinities based on dental data alone because only part of the dental morphological variation among populations can be explained in terms of neutral genetic differences.