Proto-Japanese speaking Mumun migrants (Whitman, 2011) coming into North Kyushu interacted with Jomon peoples there, creating an initial admixed Yayoi population which then spread throughout the Japanese islands as they occupied new lands for farming.
Whitman, J. Northeast Asian Linguistic Ecology and the Advent of Rice Agriculture in Korea and Japan. (2011) doi: https://doi.org/10.1007/s12284-011-9080-0