It's not simple, that's for sure. Although Tai people emigrated from China down into Thailand some 1,500 years ago (in different waves), the bulk of the population was Mon and easily assimilated the recent immigrants quickly. Making this more difficult to understand, most of South Asia East of India, and including southern China, was populated by peoples similar to Mon people, up to and including today. The difference in southern China is the larger number of immigrants from northern China, who are noticeably different.

 

There seem to be three different groups of East Asians - the southern group that includes Thais, a northeastern group that includes northern Chinese, Koreans, Japanese, Manchurians, Mongols, etc, groups that reside in northeastern China. Their appearance is more "derived" than the southerners. A third group, in Siberia and western China, look like Thais, but are cold adapted, which also means shorter and stocky. These are related to American Indians.

 

Of course, there has been large amounts of genetic interchange as mong these groups, both ancient and recent.

 

What is striking is that scientists have discovered three different "clades" of Denisovan hominids, which correspond to these three East Asian groups. This Denisovan heritage may be the driver for the emergence of the three East Asian groups. With more discoveries, more will be known about the Denisovan impact.

 

Note that the "southern" clade of Denisovans influenced Indochina, Malaysia, Indonesia, and ultimately spread over the Pacific Islands. Some scientists believe that the last refuge of Denisovans was in the Pacific Islands in relatively recent times.

 

In describing the genetic makeup of Thais, I have omitted some components, such as incursions from India and Myanmar, which have lesser Mon elements.

Monkeys

My wife along with most Thai friends refuse to accept being descendants of Chinese and Indian and are appalled by thought of it. 

14 minutes ago, novacova said:

My wife along with most Thai friends refuse to accept being descendants of Chinese and Indian and are appalled by thought of it. 

Indian heritage is lightly sprinkled among Thais.

 

But all Thais lived in China Back in the Day. They were just another barbarian Chinese tribe, and a minority fled south into Laos, Vietnam and Thailand. It happened that Thais entering Thailand were able to linguistically replace the local Mon language, just as Anglo-Saxon replaced the local languages in early Medieval England. Otherwise, Thailand is basically Mon culture, not too much of their early heritage survived the migration into Thailand.

 

There was a significant Han Chinese migration into Thailand in the 19th century, just as Chinese emigrated all over the world at that time. Many Chinese emigrants were ancestors of today's HiSo Thais.

The closest I've seen to the real Isaan are Nepalese, in appearance anyway.

Could this be something, or am I imagining things?