Alexander Vovin has recently argued that Japanese is a linguistic isolate. (Basque is the classic European example of an isolate: there are no known languages that seem to have sprung from the same prehistoric source as Basque.) Since, however, it is impossible in principle to prove defintively that two languages are not historically related, there is clearly a more than theoretical difference between saying that all linguistic relatives of language X became extinct before the historical period, and saying that, in the current state of knowledge, a genetic relationship between language X and some other language, past or present, has yet to be demonstrated. The former is a strong deontic claim; the latter is merely a temporary epistemic description.
In the case of Korean and Japanese, archaeological and historical facts currently known make it highly unlikely that most or all the similarities shared by the two languages are the result of contact-driven borrowing in Japan in the middle of the 1st millennium, as Vovin proposes. Thus, while it is certainly possible to criticize specific features of the reconstructions of proto-Korean-Japanese offered by Samuel E. Martin and John B. Whitman, the hypothesis of a proto-Korean-Japanese language is still the best basis we have for research into the linguistic prehistory of the region bounding the Sea of Japan. The deontic claim that Japanese is an isolate is excessive.
J. Marshall Unger is a Professor of Japanese at the Ohio State University, where he chaired the Department of East Asian Languages and Literatures from 1996 to 2004 having previously chaired similar departments at the University of Hawai’i and University of Maryland (1988–96). He has published on the history of Japanese, the teaching of Japanese as a second language, and the writing systems, script reforms, and impacts of computerization in Japan, China, and Korea. Though he continues to work in these areas, his latest project is a translation and commentary on twenty-six selected problems from Saijō-ryū sanpō kantsū jutsu (Inductive Calculation Techniques of the Saijō School) by the famous mathematician Aida Yasuaki (1747–1817).