Later, Koreanic-speakers, already present in northern Korea, started to expand further south, replacing or assimilating Japonic-speakers and likely causing the Yayoi migration.[41][42] Whitman (2012) suggests that the proto-Koreans arrived in the southern part of the Korean Peninsula around 300 BCE and coexisted with the descendants of the Japonic Mumun cultivators (or assimilated them). Both had influence on each other and a later founder-effect diminished the internal variety of both language families.[43]