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 widely considered plausible. 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.
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Koguryoic hypothesis
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.
A monograph by Christopher Beckwith (2004) has established about 140 lexical items in the Goguryeo corpus. They mostly occur in place-name collocations, many of which may include grammatical morphemes (including cognates of the Japanese genitive marker no and the Japanese adjective-attributive morpheme -sa) and a few of which may show syntactical relationships. He postulates that the majority of the identified Goguryeo corpus, which includes all of the grammatical morphemes, is related to Japanese.
Japanese-Korean hypothesis
Japanese and Korean languages share a substantial number of similarities, such as an agglutinative morphology including verbs being preffixes, a subject–object–verb (SOV) typology, important systems of honorifics (see Japanese honorifics and Korean honorifics), besides some few lexical resemblances. Factors like these led some historical linguists to suggest a genetic relationship between the two languages.
William George Aston suggested in 1879 in the Journal of the Royal Asiatic Society that Japanese is related to Korean. A relationship between Japanese and Korean was endorsed by the Japanese scholar Shōsaburō Kanazawa in 1910. Other scholars took this position in the twentieth century (Poppe 1965:137). Substantial arguments in favor of a Japanese–Korean relationship were presented by Samuel Martin, a leading specialist in Japanese and Korean, in 1966 and in subsequent publications (e.g. Martin 1990). Linguists who advocate this position include John Whitman (1985) and Barbara E. Riley (2004), and Sergei Starostin with his lexicostatistical research, The Altaic Problem and the Origins of the Japanese Language (Moscow, 1991). A Japanese–Korean connection does not necessarily exclude a Japanese–Koguryo or an Altaic relationship.
The possible lexical relationship between Korean and Japanese can be briefly exemplified by such basic vocabulary items as are found in the tables below.
The same possible cognates are often observed in other members of the potential Altaic family, especially among the Tungusic languages. Compare, for instance, Nanai muke "water"; giagda- "to walk on foot"; anaa, anna "not" (from Starostin's database).
Some critics of this hypothesis (such as Alexander Vovin) claim that there are difficulties in establishing exact phonological laws and that Japanese and Korean have few shared innovations. There are also drastic differences between the native Korean and Japanese number systems.
The idea of a Japanese–Korean relationship overlaps the extended form of the Altaic hypothesis (see below), but not all scholars who argue for one also argue for the other. For example, Samuel Martin, who was a major advocate of a Japanese–Korean relationship, only provided cautious support to the inclusion of these languages in Altaic, and Talat Tekin, an Altaicist, includes Korean, but not Japanese, in Altaic (Georg et al. 1999:72, 74).
Altaic hypothesis
According to its proponents, Altaic is a language family comprising at least Turkic, Mongolic, and Tungusic. G.J. Ramstedt's Einführung in die altaische Sprachwissenschaft ('Introduction to Altaic Linguistics') in 1952–1957 included Korean in Altaic. Roy Andrew Miller's Japanese and the Other Altaic Languages (1971) included Japanese in Altaic, as well. The most important recent work in favor of this expanded Altaic family is An Etymological Dictionary of the Altaic Languages (3 volumes) by Sergei Starostin, Anna V. Dybo, and Oleg A. Mudrak (2003).
The Altaic family is by no means generally accepted, either in its core form of Turkic, Mongolic, and Tungusic or its expanded form that includes Korean and/or Japanese. The best-known critiques are those by Gerard Clauson (1956) and Gerhard Doerfer (1963, 1988). Current critics include Stefan Georg and Alexander Vovin. However, even critics agree to some degree of pre-historic areal contact having occurred between even the languages of the expanded group (e.g. between Turkic and Japonic), as similarities of common origin appear to exist in the languages.
Evidence for this grouping mostly lies in claimed correspondences in vocabulary, as shown in the following table, although attempts have been made to reconstruct a number of suffixes.
These examples come from Starostin's database, which contains a comprehensive list of comparisons and hypothetical Altaic etymologies.
Austronesian
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 an Altaic superstratum and an Austronesian substratum.
Dravidian
A more rarely encountered hypothesis is that Japanese is related to the Dravidian languages. The possibility that Japanese might be related to Dravidian was raised by Robert Caldwell (cf. Caldwell 1875:413) and more recently by Susumu Shiba, Akira Fujiwara, and Susumu Ōno (n.d., 2000).
Eurasiatic
Joseph Greenberg (2000–2002) argued for the inclusion of Japanese in his proposed Eurasiatic language family. In contrast to Sergei Starostin, he rejected the inclusion of Korean in Altaic. According to Greenberg, Japanese–Ryukyuan, Korean, and Ainu form a separate subgroup within Eurasiatic.
In contrast to Greenberg, many historical linguists remain convinced that systematic phonological reconstruction is necessary to establish genetic relationships among languages and reject his "mass comparison" methodology and, consequently, the Eurasiatic hypothesis.
Nostratic
Suggestions of connections among Japanese, Altaic, and Dravidian were made by Hermann Jacobi in 1897 (Compositum und Nebensatz, pp. 106–131), who further noted structural similarities to Proto-Indo-European.