I would like to reply to some answers above:

> Jonathan P. Evans:

Thank you for the article by Weera Ostapirat. To be honest, I am not quite convinced by his examples, which are all taken from secondary sources. Most of the Kra-Dai (KD) examples are two-syllable words. If I collected the KD words, I would try to collect monosyllabic variants for those words. As I speak both Central Thai and Northern Thai, looking on those KD examples, I can quickly think of some monosyllabic variants to replace those words. Ancient Thai and ancient Chinese were monosyllabic languages.

> Jonathan P. Evans and Matthew Yu:

Both of you mentioned "morphology". The Chinese and Thai morphologies are alike in that they are different from the English morphology. San Duanmu argues that in Chinese, " there is no way to study derivation or inflection" (http://www-personal.umich.edu/~duanmu/RevPackard02.pdf ). This situation is exactly the same for the Thai language. 

> David J Lohman:

I love your answer, "the evolutionary history of a human group's written language, spoken language, and genetic lineage don't always derive from the same ancestors." And I have just noticed that the term "genetic" I used in my question above seems to have confused some readers. The term used in my question is completely about the "linguistic relationship", and not about the "biological relationship". 

The evidence of linguistic relationship is found in observable shared characteristics that are not attributed to contact or borrowing (Qoted from Wiki). One of the most basic methods in establishing genetic relations between languages is through morphology, i.e. on the basis of judging whether words in a language and those in another one are related to each other, or decended from a common ancestor (in this case I doubt that Thai and Sino-Tibetan languages, with Chinese as a representative, are actually related).  The Tai–Kadai languages were formerly considered to be part of the Sino-Tibetan family (the reason may be that they are related to some languages varieties in Southern China), but outside China they are now classified as an independent family. So what you mentioned in you post is reasonable to some sense, the grouping of languages into language families are subject to change, and the standards are not purely linguistic. 

The evidence required to consider the Tai-Kadai languages as a branch of Sino-Tibetan (or the two groups as branches of a macrofamily) would be to show that the similarities between the two groups are due to inheritance from a shared ancestor and not due to borrowing or chance. That is usually done by showing that they follow a path of inheritance that can be demonstrated to follow application of successive systematic sound changes. So, someone who wanted to show that Tai-Kadai and Sino-Tibetan languages form a single group should compare the reconstructed vocabularies of proto-Sino-Tibetan and proto-Tai-Kadai and demonstrate that the similarities exhibit regular sound correspondences. 

สวัสดีคับคุณPongprapunt -

I'm an evolutionary biologist/molecular systematist who works mostly on insects in SE Asia. My understanding is that the evolutionary history of a human group's written language, spoken language, and genetic lineage don't always derive from the same ancestors.  Modern Vietnamese script, for example, is derived from the Roman alphabet, while the people and much of the spoken language can be traced to China.  Similarly, written Thai stems from southern Indian scripts, whereas the people and spoken language can trace their provenance to southern China.  There is no reason to presume, a priori, that a molecular phylogeny of Asian ethnic groups will mirror the phenetic history of their written or spoken languages.  I think it's safe to assume that there would also be recurrent gene flow (and its linguistic equivalent) among these human genetic and linguistic lineages.

No, of course the genetic and ethno-linguistic origin of a group are different - if probably related - questions, since populations can and do often shift languages over time. But the question of the classification about the classification of the Thai and by extension Tai-Kadai languages is one about the genealogy of the languages not about the genetic origins of Thai speaking populations.

Dear Pongprapunt

I am stimulated to write this by the note written by David. I am an Oncologist turned Linguist (Biolinguist) and Thai or Sino-Tibetan languages is not my field of study.

I was in Thailand for two days long back on the way to China for a Oncology meeting.

What struck me very much was the similarities in the meanings of Thai words and their 'phonation' as well as overall shape of the letters with Malayalam, a South Indian Language of Kerala. Now David also notes that written Thai is derived from South Indian.

I think there is more to it.

The word Thai in Malayalam means Mother. Like in Thailand, the Elephant is almost a pet like animal in Kerala, with many homes having their own elephants, with strong bonding between the Animal and the people (see the Images from Kerala and Thailand). Of course What is an Elephant for Linguistics :-))?

Kerala has a rich cultural heritage and connections with many countries since millennia, by the sea route.

Narayanan

- Thank you everyone for your interest and useful answers.

- I noticed that all of us have no disagreement that the orthography has nothing to do with the genetic relationship of languages.

- I agree with Hansen that the language origin and the ethnic origin should be considered as separate issues. Thailand (like almost every modern nation in the world) is composed of people from different ethnic backgrounds. As the national language, Standard Thai is now spoken as a mother tongue by numerous Thai nationals who inherit none of "Tai" ethnic background. 

- Numerous scholars in China who argue that the Thai language belong to the Sino-Tibetan family believe that the ancestors of the Thais who spoke Thai, Northern Thai, Lao and other Tai-Kadai languages or their proto-language might have migrated from a part of Yunnan in the 13th century. And much earlier than that, this group of people is assumed to have migrated from the central part of China. If this were true, it would be very easy to guess why there have been (political) attempts to forget or even negate this connection.

- There have been studies which attempt to reconstruct the proto-Thai-Chinese vocabulary. Even though the loanwords introduced by the Chinese immigrants were excluded, they still found significant similarities between Thai and the Southern Chinese varieties (like what Matthew suggested above).

- In fact, tn Chinese dialectal studies, the scholars commonly agree that the Southern Chinese varieties have preserved most original features from the ancient Chinese language. If the proto-Thai-Chinese language really existed, then most remaining evidence would be found in these Chinese varieties.

- Hope that we could also hear from the historical linguists of Sino-Tibetan or Tai-Kadai.

The migration from southern China does not seem to explain the Ban Chiang village in northeastern Thailand. See https://en.wikipedia.org/wiki/Ban_Chiang

I would encourage you to look at the writings of Weera Ostapirat on the different theories of relatedness of Tai-Kadai to other language families in the area.  Here is one sample: http://www.jseals.org/seals23/ostapirat2013austro-taih.pdf.

I notice that one of the writers above said: "the standards are not purely linguistic. "  That may be true for some people, but if we are asking a linguistic question - the relatedness of two or more languages -- then it needs a linguistic answer.

The SHORT answer is that we are looking for similarities that are more likely to be inhereited than borrowed.  That is why morphology was mentioned.  However, Sino-Tibetan does not have agreed-upon reconstructed morphology. People usually look at sound laws among core vocabulary. If, after reading Ostapirat, the argumentation doesn't make sense, I'd suggest picking up a good text introducing historical linguistics.

I would like to reply to some answers above:

> Jonathan P. Evans:

Thank you for the article by Weera Ostapirat. To be honest, I am not quite convinced by his examples, which are all taken from secondary sources. Most of the Kra-Dai (KD) examples are two-syllable words. If I collected the KD words, I would try to collect monosyllabic variants for those words. As I speak both Central Thai and Northern Thai, looking on those KD examples, I can quickly think of some monosyllabic variants to replace those words. Ancient Thai and ancient Chinese were monosyllabic languages.

> Jonathan P. Evans and Matthew Yu:

Both of you mentioned "morphology". The Chinese and Thai morphologies are alike in that they are different from the English morphology. San Duanmu argues that in Chinese, " there is no way to study derivation or inflection" (http://www-personal.umich.edu/~duanmu/RevPackard02.pdf ). This situation is exactly the same for the Thai language. 

> David J Lohman:

I love your answer, "the evolutionary history of a human group's written language, spoken language, and genetic lineage don't always derive from the same ancestors." And I have just noticed that the term "genetic" I used in my question above seems to have confused some readers. The term used in my question is completely about the "linguistic relationship", and not about the "biological relationship". 

The comparisons given by Weera Ostapirat for Austronesian and KD are so few that they could lie within the realm of chance, or in some cases might result from early borrowing. A correspondence *s = h is perfectly sensible, but is immediately contradicted by another example *h  =  h. If two-syllable forms can be found that are not, say, recent compounds, they are certainly preferable for historical comparisons, since they are more likely to reflect earlier forms. An example would be comparing English head and Spanish cabeza, which seem to have little in common, but the modern English form descends from heafoth (th = thorn) and the Spanish comes from Latin caput (c = k), showing regular correspondences h : k, f : p, th : t, i.e., Grimm's Law. English much and Spanish mucho, however, result from coincidental evolution from different sources, mycele and multus, respectively (though the m-may go back to Indo-European). 

    Morris Swadesh long ago argued for looking at pair-wise comparisons of adjacent languages or language groups in terms of a dialect chain, rather than forcing everything into a unitary Ur-sprache. Given the dispersion of ethnic and linguistic groups in SE Asia, such a perspective might produce more insightful and useful results than assuming a single Stammbaum. Contiguous dialects are more likely to share retained forms than distantly separated dialects, and apparent competing proto-forms might simply be the result of earlier regional differences. 

  Rudy Troike

  University of Arizona

Weera Ostapirat's work should maybe be approached through his book-length study: Proto-Kra, in Linguistics of the Tibeto-Burman Area, 23(1), spring 2000.

http://sealang.net/sala/archives/pdf8/weera2000proto.pdf

Therein six Kra languages are analysed, and Proto-Kra is reconstructedfor over 300 etyma arranged in ten semantic areas.

Found also a review of Ostapirat 2000, which lists Chinese loanwords in Ostapirat’s proto-Kra lexicon:

Jacques Guillaume 2001 Weera Ostapirat: Proto-Kra, in: Cahiers de linguistique - Asie orientale 30(1): 101-107.

www.persee.fr/doc/clao_0153-3320_2001_num_30_1_1595

More data to answer your question can be found in this:

Pittayaporn Pittayawat 2009 The phonology of Proto-Tai.

Ph.D. diss., Cornell University. [441p.; AAT 3376648.]

URI: http://hdl.handle.net/1813/13855 

[Proto-Tai is the ancestor of the Tai languages of Mainland Southeast Asia. Modern Tai languages share many structural similarities and phonological innovations, but reconstructing the phonology requires a thorough understanding of the convergent trends of the Southeast Asian linguistic area, as well as a theoretical foundation in order to distinguish inherited traits from universal tendencies, chance, diffusion, or parallel development. / This dissertation presents a new reconstruction of Proto-Tai phonology, based on a systematic application of the Comparative Method and an appreciation of the force of contact. It also incorporates a large amount of dialect data that have become available only recently. In contrast to the generally accepted assumption that Proto-Tai was monosyllabic, this thesis claims that Proto-Tai was a sesquisyllabic language that allowed both sesquisyllabic and monosyllabic prosodic words. / In the proposed reconstruction, it is argued that Proto-Tai had three contrastive phonation types and six places of articulation. It had plain voiceless, implosive, and voiced stops, but lacked the aspirated stop series (central to previous reconstructions). As for place of articulation, Proto-Tai had a distinctive uvular series, in addition to the labial, alveolar, palatal, velar, and glottal series typically reconstructed. In the onset, these consonants can combine to form tautosyllabic clusters or sequisyllabic structures. Regarding the rime, PT had seven vowel qualities that contrasted in height, backness, and rounding. A vowel length contrast also existed for each quality. Palatal and lateral consonants also occurred in the coda in addition to the final consonants generally assumed. Furthermore, Proto-Tai was a tonal language whose four tonal categories *ABCD contrasted both in terms of pitch and voice quality. / Many of these Proto-Tai traits are not attested in modern Tai languages. The current reconstruction of Proto-Tai phonology is thus a demonstration of the power of the Comparative Method as well as the role that phonological theory can play in reconstruction. This thesis presents a picture of the history of Tai languages as characterized by divergent changes overridden by waves of convergent trends that transformed Proto-Tai into a network of typologically homogenous dialects that differ markedly from their parent. This analysis offers a comprehensive account of the transformation of the Proto-Tai phonology into modern systems.]

A recent overview with most relevant references is:

Weera OSTAPIRAT, “Sino-Tai Hypothesis”, in: Encyclopedia of Chinese Language and Linguistics, General Editor Rint Sybesma.

http://dx.doi.org/10.1163/2210-7363_ecll_COM_000251

First published online: 2015, (3,578 words).