The first proposal of a genealogical relationship was that of
Paul Benedict in 1942, which he expanded upon through 1990. This took the form of an expansion of
Wilhelm Schmidt's
Austric phylum, and posited that Kra–Dai and Austronesian had a sister relationship within Austric, which Benedict then accepted. Benedict later abandoned Austric but maintained his Austro-Tai proposal, adding the
Japonic languages to the proposal as well. The proposal remained controversial among linguists, especially after the publication of Benedict (1975) whose
methods of reconstruction were idiosyncratic and considered unreliable. For example, Thurgood (1994) examined Benedict's claims and concluded that since the
sound correspondences and
tonal developments were irregular, there was no evidence of a genealogical relationship, and the numerous cognates must be chalked up to early language contact. However, the fact that many of the Austro-Tai cognates are found in core vocabulary, which is generally more resistant to borrowing, continued to intrigue scholars. There were later several advances over Benedict's approach: Abandoning the larger Austric proposal; focusing on lexical reconstruction and regular sound correspondences; including data from additional branches of Kra–Dai, Hlai and Kra; using better reconstructions of Kra–Dai; and reconsidering the nature of the relationship, with Kra–Dai possibly being a branch (daughter) of Austronesian. Sagart (2005a) cited a core of regular sound correspondences relating words belonging to the basic vocabulary in Benedict's work. He pointed out the lack of a substantial body of shared cultural words. He took these facts as indications that Benedict's Austro-Tai cannot be explained as a contact phenomenon. He further listed a number of specifically Malayo-Polynesian features in the vocabulary shared by Tai-Kadai and Austronesian, concluding that Tai-Kadai is a subgroup within Austronesian, rather than a sister group to it. Ostapirat (2000) reconstructed
proto-Kra, one of the least-well attested branches of Kra–Dai. Ostapirat (2005) later presented fifty core vocabulary items found in all five branches of Kra–Dai, and demonstrated that half of them—words such as
child, eat, eye, fire, hand, head, I, you, louse, moon, tooth, water, this, etc.—can be related to
proto-Austronesian by regular sound correspondences, a connection which Reid (2006) finds convincing. Austronesian is characterized by disyllabic roots, whereas Kra–Dai is predominantly monosyllabic. It appears that in Kra–Dai, the first vowel
reduced and then dropped out, leaving a
consonant cluster which frequently reduced further to a single consonant. For example, the proto-Austronesian root * "live, raw" corresponds to proto-Kra '
and its reflex ' in Laha, as well as Tai '''', all with the same meaning (the *-D- consonant is Ostapirat's voiced plosive of undetermined quality, probably alveolar as opposed to dental articulation). In
proto-Kra–Dai, there appear to have been three tones in words ending in a
sonorant (vowel or nasal consonant), labeled simply A, B, C, plus words ending in a
stop consonant, D, which did not have tone. In general, Austronesian words ending in a sonorant correspond to A, and words ending in a stop correspond to D. This accounts for most of the words. There are also a few cognates with B and C tone. From Indic borrowings it appears that tone B was originally a final
h in Kra–Dai, and some of the corresponding Austronesian roots also end in
h, such as AN * "chaff", Kam–Sui
paa-B (Mulam
kwaa-B), though there are few examples to go on. Tone C seems to have originally been
creaky voice or a final glottal stop. It may correspond to *H, a
laryngeal consonant of uncertain manner, in proto-Austronesian (AN * "head", Thai
klau-C), but again the number of cognates is too low to draw firm conclusions. Sagart (2004) presented data from a newly described Kra language,
Buyang, which—like many other
Kra languages—retains the disyllabic roots characteristic of Austronesian. Some examples are: Ostapirat (2013) lists the following potential cognates between
Proto-Kra-Dai and
Proto-Austronesian. •
Sonorant-final Austronesian terms corresponded with tone A in Kra-Dai. • Proto-Austronesian uvular fricative finals corresponded with tone B. • Proto-Austronesian final sibilants and corresponded with tone C.
Ostapirat (2005) Austro-Tai
sound correspondences and
cognate sets listed by Ostapirat (2005) are as follows.
Core vocabulary Kra-Dai core vocabulary and
Proto-Austronesian cognates:
Final consonants Summary of Austro-Tai final
sound correspondences: Cognates with final consonant correspondences: Contrast between *-C and *-t in both Kra-Dai and Austronesian: Proto-Austronesian final *-q and Proto-Kra-Dai *-k/-C: Proto-Austronesian final *-s and Proto-Kra-Dai *-c: Proto-Austronesian final *-R and *-N and Proto-Kra-Dai *-l/-n: Special Proto-Kra-Dai development corresponding to Proto-Austronesian *-R: Proto-
Atayal voiced stop endings corresponding to Kra-Dai final voiced glides: Proto-Austronesian final *-l corresponding to Kra-Dai final glides (possible development):
Medial consonants Medial correspondences between
Proto-Austronesian and Proto-Kra-Dai, assuming that Proto-Kra-Dai was polysyllabic: Proto-Kra–Dai *d- corresponds to both Proto-Austronesian *d- and *j- according to Ostapirat (2023). For example: Proto-Kra–Dai *b-l- corresponds to Proto-Austronesian *bVl- according to Ostapirat (2023): Additional supporting data is also published in Smith (2022). Lexical correspondences between
Proto-Austronesian and
Proto-Tai proposed by Smith (2021) are: Other lexical correspondences (basic vocabulary) between Proto-Austronesian and Proto-Tai from Smith (2021) are: Lexical correspondences between
Proto-Malayo-Polynesian (PMP) and
Proto-Tai proposed by Smith (2021) are: Smith deduces that Proto-Austronesian final-syllable
*a regularly corresponds to Proto-Tai
*ɯ(ə) if penultimate Proto-Austronesian syllable contained a high vowel, like
*i or
*u. On the other hand, if that penultimate syllable had a low vowel instead, Proto-Austronesian
*a would instead correspond to Proto-Tai
*aː. Lexical correspondences between
Proto-Austronesian and
Proto-Hlai, as well as
Proto-Kra: ==Relationship==