Researchers at the University of Tokyo conducted a census of the Japanese population of approximately 2,500 years ago using the Y chromosomes of men living on the main islands of modern-day Japan. This is the first time that analysis of modern genomes has estimated the size of an ancient human population before they were met by a separate ancient population. The research was published online on June 17, 2019 in Scientific Reports. The open-access article is titled “Analysis of Whole Y-Chromosome Sequences Reveals the Japanese Population History in the Jomon Period.” "Evidence at archaeological dig sites has been used to estimate the size of ancient human populations, but the difficulty and unpredictability of finding those sites is a big limitation. Now we have a method that uses a large amount of modern data," said Associate Professor Jun Ohashi, PhD, an expert in human evolutionary genetics and leader of the research team that performed the analysis. The current theory on human migrations into Japan is that the original inhabitants, the Jomon people, were met about 2,500 years ago by a separate group coming mainly from the Korean Peninsula, the Yayoi people. Archaeologists have identified fewer Jomon sites from the Late Jomon Period, the era immediately before the Yayoi arrival. Global temperatures and sea levels dropped during that period, which could have made life more difficult for the hunter-gatherer Jomon people. When the Yayoi people arrived, they brought wet rice farming to Japan, which would have led to a more stable food supply for the remaining Jomon people living with the new Yayoi migrants. The lesser amount of archaeological remains from the Late Jomon Period could be evidence of an actual population decline, or just that the archaeological dig sites have not yet been found.
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