The Japanese linguist Ryumine Katayama found many similar basic words between Ainu and Japanese. Because of a great amount of similar vocabulary, phonology, similar grammar, and geographical and cultural connections, he and Takeshi Umehara suggested that Japanese was closely related to the Ainu languages, and was influenced by other languages, especially Chinese and Korean.
The Endangered Languages Project surmises a relationship between Ainu, isolate languages from the Indian subcontinent (such as Kusunda and Nihali) and the Andamanese languages, as part of Joseph Greenberg's Indo-Pacific hypothesis.
Several linguists have proposed that Japanese may be a relative of the Austronesian family. Some linguists think it is more plausible that Japanese might have instead been influenced by Austronesian languages, perhaps by an Austronesian substratum. Those who propose this scenario suggest that the Austronesian family once covered the islands to the north as well as to the south. Alexander Vovin calls Proto-Japanese suggestive of an Southeast-Asian origin. The phonological similarities of Japanese to the Austronesian languages and the geographical proximity of Japan to Formosa and the Malay Archipelago have led to the theory that Japanese may be a kind of very early creole language, with a Korean superstratum and an Austronesian substratum. Alexander Vovin reconstructed the morphology of Proto-Japanese, and found many similarities between Proto-Japanese and several Southeast-Asian languages.
It is also possible that the similarities between Japanese and Austronesian are due to an ancient Ainu substratum dating from the Jōmon period. The eminent Japanese linguist Shichirō Murayama tried to link Ainu to the Austronesian languages, which include the languages of the Philippines, Taiwan, and Indonesia, through both vocabulary and cultural comparisons. Newer theories are the Austro-Tai languages, where Ainu is grouped together with Japanese as para-Austronesian. Several Japanese linguists classify Japanese as Para-Austronesian. Others classify Japanese as member of the bigger Austric languages.