In the 1960s, Tokutaro YASUDA stated in his book "Manyo no Nazo" (Mystery of the Manyoshu) that the parent language of Japanese is Rong from the northern part of India, and that the Manyoshu can be read by the Rong.
1960年代には安田徳太郎が『万葉集の謎』に於いて日本語の祖語はインド北部レプチャ語であるとし、万葉集はレプチャ語で読めると主張していた。