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 flora, 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 the korean peninsula and the island of tsushima, and then from tsushima to the major island of kyūshū.