Secular Change of Third Molar Agenesis in the Japanese Population

Abstract

The tooth germs of the third molars grow up at the latest stages of ontogeny in the human dentition, and the teeth show the highest frequencies in congenitally missing teeth. Moreover, the third molar in the dentition is subjected to change easily in the course of microevolution. The frequencies of the third molar agenesis were investigated in this study to evaluate their secular changes in the microevolution of the Japanese populations.
The results are as follows: the third molar agenesis are considerably low in incidence for the peoples of the Jomon Era, but the incidence in the people from the Yayoi Era suddenly turned to increase, although higher food intake due to rice cultivation may be expected as compared with that of the Jomon Era. This rapid change in frequency is considered to be caused by the gene flows from the immigrants from the Asian Continent with the congenitally missing third molar teeth at a higher rate of incidence. From the Yayoi Era, the frequencies of the third molar agenesis increased gradually and reached to the peak in the percentage at the beginning of the Showa Era. After that period, the frequencies decreased rapidly up to the present. The reasons for this rapid change are the food intake with high nutrition and high protein as well as the change of life environments, which have also caused the higher stature and the acceleration of sex maturation.