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.
In ancient times, Koreanic languages, then established in southern Manchuria and northern Korean peninsula, expanded southward to central and southern Korean peninsula, displacing the Japonic languages spoken there and possibly causing the Yayoi migrations. There is disagreement over the protohistorical or historical period during which this expansion occurs, ranging from the Korean Bronze Age period to the Three Kingdoms of Korea period. As there is disagreement among experts when the expansion of Koreanic languages started, there is room for interpretation on the proto-historical and historical extent of the Japonic language presence in the central and southern Korean peninsula. John Whitman and Miyamoto Kazuo believe Japonic speakers migrated from Manchuria to Korea and lasted there until Mumun pottery period in the Korean peninsula. After the Mumun pottery period and beginning with Korean Bronze Age, Koreanic speakers started expanding from Manchuria southward towards the Korean peninsula, displacing the Japonic speakers and causing the Yayoi migrations. On the other hand, Alexander Vovin believes southern Korea was Japonic until the southward migration of Koreanic speakers from Goguryeo during Three Kingdoms of Korea, thus establishing Baekje, Silla and Gaya.
The Japanese–Koguryoic proposal dates back to Shinmura Izuru's (1916) observation that the attested Goguryeo numerals—3, 5, 7, and 10—are very similar to Japanese. The hypothesis proposes that Japanese is a relative of the extinct languages spoken by the Buyeo-Goguryeo cultures of Korea, southern Manchuria, and Liaodong. The best attested of these is the language of Goguryeo, with the more poorly attested Buyeo languages of Baekje and Buyeo believed to also be related.