The effects of environmental stress on third molar agenesis in humans and its population differences.

Third molar (M3) agenesis has become prevalent since the terminal Pleistocene in humans. Population-level studies suggest that M3 agenesis is related to genetic factors and to environmental stresses such as malnutrition and infectious diseases. In addition, the fact that there are populations, such as the Jomon people, that have a high frequency of enamel hypoplasia (EH), an indicator of stress, but a low M3 agenesis rate, suggests that there are population differences in how the effects of environmental stress on M3 agenesis are manifested. In this study, I used EH to investigate the effects of environmental stress on M3 agenesis in three populations with different genetic bases: recent Japanese (1868-1926), Jomon people, and Islamic Mesopotamians. The results showed the effects of environmental stress on M3 agenesis in recent Japanese and Islamic Mesopotamians, but not in Jomon people. Furthermore, it was shown that M3 agenesis of recent Japanese and Islamic Mesopotamians are similarly susceptible to environmental stress. Thus, the results showed that the extent to which environmental stress affects M3 formation varies among populations. Since M3 agenesis was observed even in Jomon people, whose M3s are not thought to have been affected by environmental stress, it is reasonable to assume that M3 agenesis can be congenital or caused by environmental stress, and each cause of agenesis occurred independently in human populations.