Proto-Donghainese

Proto-Donghainese is the theorized ancestor of the Donghainese branch of the Austronesian language family. Its descendants include Proto-Tsushiman, the direct ancestor to the modern Tsushiman language, as well as the now long-extinct Nansei language and Zhejiang language.

Theories of Phylogeny
Early theories stated that Proto-Tsushiman descended directly from Proto-Austronesian during the Austronesian colonization of Tsushima. This would quickly be challenged due to inconsistencies between Proto-Tsushiman words and their expected reflexes based on sound changes from Proto-Austronesian. As more Austronesian artifacts were discovered in the Ryukyu Islands and Kyushu, the theory of Austronesian colonization was revised; most relevantly, a new theory was developed positing close contact between Japono-Austronesians in Tsushima and Japan proper. The new branch was named "Donghainese" after the Chinese word for the East Chinese Sea, 东海 (Dōng Hǎi). The theoretical language spoken by Japono-Austronesians was named the Nansei language after an alternative name for the Ryukyu Islands, though this is slightly inaccurate as the language is now theorized to have been spoken in Kyushu and possibly parts of Honshu.

Sound Changes from Proto-Austronesian
Proto-Donghainese is considered to have the following sound changes, which are strikingly similar (but not at all identical) to those of northwest Formosan languages. Older texts still showing Proto-Tsushiman as having directly descended from Proto-Austronesian contain similar sound changes.

In addition to the following sound changes, the sounds /i/ and /u/ enter into complementary distribution with /e/ and /o/: the latter sounds only appear word-finally, excluding diphthongs.

Grammar
Proto-Donghainese's grammar is considered to be identical to that of Proto-Tsushiman.