Genetic mapping studies by Luigi Luca Cavalli-Sforza have shown a pattern of genetic expansion from the area of the Sea of Japan towards the rest of eastern Asia.
This appears as the third principal component of genetic variation in Eurasia (after the "Great expansion" from the African continent, and a second expansion from the area of Northern Siberia), which suggests geographical expansion during the early Jōmon period.
(The History and Geography of Human Genes p249, Cavalli-Sforza ISBN 0-691-08750-4.)
These studies also suggest that the Jōmon demographic expansion may have reached America along a path following the Pacific coast
