Well, Sean Lee and Toshikazu Hasegama released a study using phylogenetics (stemming from evolutionary genetics) demonstrating that there was a common ancestor no more than 2200 years ago:
Some scholars argue the main settlement of the archipelago occurred 12,000 to 30,000 years ago, and that modern Japanese -- both the language and the people -- descend directly from this stone-age culture, which had some agriculture but was based mainly on hunting and gathering.Lee and Hasegama fall into the latter camp. Their research methodology is interesting:
According the this theory, the migration of other peoples from mainland Asia around 200 B.C. brought metal tools, rice and new farming techniques but had scant impact on linguistic development.
Other researchers counter that this influx from the Korean Peninsula had a far deeper influence, largely replacing or displacing both the indigenous inhabitants and their spoken tongues.
Recent archaeological and DNA evidence support this theory, but researchers at The University of Tokyo wondered if additional clues might be found by tracing dozens of distinct dialects back through time to their earliest common ancestor.
"Accumulating empirical evidence suggests that languages have, astonishingly, gene-like properties, and they also evolve by a process of descent," he said.This kind of thing is controversial, but it's not as if the possibility that modern Japanese are mostly descendants of several waves of Korean migrants is ignored in Japan. In fact, back in the 1990s, I recall visiting a national museum in Tokyo's Ueno Park, where a display described the non-mythical origins of the Japanese people. It read in English, "Koreans exerted great genetic influence on Japan," a roundabout way of saying that same thing.
Lee and Hasegama created a list of 210 key vocabulary words -- body parts, basic verbs, numbers and pronouns -- and duplicated that list across 59 different dialects.
The researchers chose words unlikely to be borrowed across dialects and "resistant to change," much in the same way biologists seek out so-called "highly-conserved" genes that remain unaltered for thousands of generations.
Computer modelling showed that all of these "Japonic" languages descended from a common ancestor some 2,182 years ago -- coinciding with the major wave of migration from the Korean Peninsula.
The exact timing of the farmers' arrival may go back a little further, Lee said by email, but the core conclusion seems inescapable: "the first farmers of Japan had a profound impact on the origins of both people and languages."