Japanese is clearly related to the language currently in use in Korea and much of Eastern Siberia. It's oldest vocabulary is substantially Turkic in origin. It is also a relatively recent language with its earliest literature suggesting it was BROUGHT TO Eastern Siberia, Korea and Japan by Western Chinese Turkic speaking conquerors about 1600 years ago.
An older theory has it arising in place among the existing Jomon and Ya Yoi cultures, but more recent work with ancient Sakha/Yakuts royal accounts/journals pins this language down to what is now the largest Russian province/republic in the Far East.
That population differs from those around it in that they herd cattle as well as horses, yaks, goats, sheep and reindeer! For all practical purposes they were part of the Chinese civilization to the East and were their technological equal. They traveled North and South from Siberia to Eastern India and appear to be identical to the Sakha mentioned in the Mahabarata.
These are the people who migrated from Siberia to Korea and Japan to escape the climate anomaly that brought the Dark Ages.