Prehistory
Japan seems to have been connected to the Asian landmass during the last Ice Age until around 20,000 BCE, both because of glaciation of sea water and the concomitant lowering of sea level by about 80 to 100 meters. This allowed for the transmission of fauna and flaura, including the establishment of the Jōmon culture. After that period however, Japan became an isolated island territory, depending entirely on sporadic naval activity for its interactions with the mainland. The shortest seapath to the mainland (besides the inhospitable northern path from Hokkaidō to Sakhalin) then involved two stretches of open water about 50 kilometers wide, between Korean peninsula and the island of Tsushima, and then from Tsushima to the major island of Kyūshū.
Various influences have also been suggested from the direction of the Pacific Ocean, as various cultural and even genetic traits seem to point to partial Pacific origins, possibly in relation with the Austronesian expansion.
Read more about this topic: Naval History Of Japan