The essence of my model is as follows. I contend that
the Jōmon culture (c. 10,000–300 BCE) on the Japanese
archipelago was the product of Ainu and Malayo-Polynesian
people, while the Yayoi culture (c. 300 BCE-300 CE) was the
product of Kaya (Karak) people from the southern Korean
peninsula together with Ainu and Malayo-Polynesian
aborigines. The proto-Japanese people, speaking proto-
Japanese language, were formed during the Yayoi period. I
contend that the Kaya dialect of the Korean language provided
the basic structure of the proto-Japanese language although
lexically (in loan words) and phonologically (in sound), the
influence of Ainu and Malayo-Polynesian languages was
substantial. I also regard the early tomb culture (c. 300-375 CE)
as an extension of the Yayoi culture.
The late tomb culture (c. 375-675 CE) was, however,
brought about by the Yamato kingdom, the first unified state
on the Japanese islands that was newly established at the end
of the fourth century by the Paekche people from the Korean
peninsula. Syntactically (in patterns of word arrangement) and
morphologically (in systems of word formation), the similarity
between the Korean and Japanese languages was very much
strengthened. However, the lexical and phonological influence
of the Ainu and Malayo-Polynesian languages cast a long
shadow on the subsequent evolution of the Japanese language.
Therefore, by the early ninth century at the latest, due to ever
increasing lexical, semantic (in meaning) and phonological
differences, the people of the Korean peninsula and the people
of the Japanese islands could no longer directly communicate
with each other without interpreters.