Classification of Japonic

The classification of the Japonic languages (Japanese and the Ryukyuan languages) is unclear. Linguists traditionally consider the Japonic languages to belong to an independent family; indeed, until the classification of Ryukyuan as separate languages within a Japonic family rather than as dialects of Japanese, Japanese was considered a language isolate. Among more distant connections, the possibility of a genetic relationship to the Goguryeo (Koguryŏ) languages, or perhaps to Kara (Gaya), has the most currency. Goguryeo itself may be related to Korean, and a Japonic–Korean grouping is considered plausible by some linguists. Independent of the question of a Japonic–Korean connection, both the Japonic languages and Korean were often included in the largely discredited Altaic family. The relevance for Old Korean and modern Korean is unknown. Recently the Austro-Tai theory is gaining acceptance by many linguists. In 2015 Alexander Vovin reconstructed parts of proto-Japanese and found many similarities in morphology with languages of Southeast Asia. This support the Austro-Tai theory.

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.

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