At the same time DNA evidence demonstrates that 40% of the genetic background of the modern Japanese derives from the original inhabitants known as Jomon (and today as Ainu).
Old Billybob out there in Oregon ~ Kenniwick Man ~ is pretty much identical to a Jomon.
The Japanese living in a land of islands and having boat technology could probably have gone anywhere they wished ~ I think there's this find in Chile that suggests Jomon made it there.
A later arrival from China are the Hakka people. Outside of the Koreans, they provide that other 60% of the DNA to the Japanese. They absorbed boat technology on their arrival in Manchuria before traveling on to japan. I think they are also called the Ya Yoi people when studying their settlement of Japan.
Well since Japan is nothing but islands, they would have to have boat technology.