Shuichi Chiba thinks Japanese came from Mongolian and Sanskrit.
Shuichi Chiba is a famous creator of Japanese advertising. Chiba is enthusiastic and interested in the relationships of different linguistics around the world. In recent years, he has been studying the origin of the Japanese language.
While studying Mongolian and Sanskrit, he discovered some common features
- Japanese) watashi no namae wa Takai desu.
- Mongolian) Намайг ….. гэдэг
The two are very similar in grammar and pronunciation.
Like many Japanese scholars, he views Japanese as an Altaic language.
Three language groups are widely recognized as Altaic:
The phylogenetic relationships among them have been demonstrated, but not determined.
Following are the Altaic Languages:
Turkic (Altaic, Turkish, Uighur, Uzbek, Kazakh, Kyrgyz, Tuba, etc.)
Mongolian language family (Mongolian, Oirati, Buryat, etc.)
Tungusic (Evenki, Manchuria, etc.)
These language groups share several important common features.
Some of the common features are stated below:
- harmonize vowels
- be a colloquial term
- In principle, use SOV word order 
- dislike having an R at the beginning of a word and have few words in one’s proper language beginning with the R
Furthermore, many Japanese words came from Sanskrit.
When Indian Sanskrit came to Japan with Buddhism, many words were introduced.
Also, along with the introduction of Buddhism from India, the words were transmitted from Sanskrit to Mongolia. Chiba emphasizes this point and postulates that the words came from Mongolia to Japan.