Thai Origins
The Tai-speaking peoples form part of a much larger ethno-linguistic group that spans a wide area of Southeast Asia. These groups are spread from northeastern India in the west to northern Vietnam in the east, and from southern China in the north to the Malay Peninsula in the south.
Linguistic and anthropology research lean toward a migration of Southwestern Tai-speaking tribes, in particular from Guangxi in China, that took place sometime between the 8th–10th centuries. Recent research also suggests that the Tai may have instead originated in northern Vietnam, around the area of Dien Bien Phu.
Tai Migration
The Tai migration into present day Thailand was a gradual process, with the Tai generally remaining near the mountainous areas in the region. The earliest Tai settlements in Thailand were in the river valleys in the northern reaches of the country, where they use their specialized agricultural knowledge relating to the use of mountain water for farming, especially rice production.
The Tai practiced nature worship, with beliefs that involved both malevolent and benevolent spirits. Appeasement of the spirits were through offerings and special ceremonies. From the 10th century onwards, the Tais gradually moved southward into the Chao Phraya valley. This brought them into contact with Mon-Khmer ethnic groups that were inhabiting the region. These were the lands of the Dvaravati Buddhist culture, and the Khmer Hindu civilization.
Tai Progression
The Mon were the first people in mainland Southeast Asia to adopt Buddhism. Between the 6th and 9th centuries, the Mon established several small Buddhist kingdoms that are collectively known as the Dvaravati across present day central Thailand. The Khmer civilization on the other hand, was heavily influenced by Hindu ideas and practices. Between the 9th and 13th centuries, Khmer rulers expanded their rule from Angkor in today's Cambodia, to over half of modern Thailand.
By the 11th century, the Tai had formed small independent principalities. Subsequent changes in political landscape gave rise these principalities, as the Mon and Khmer powers waned. This gave rise to regional kingdoms such as Lanna, Phayao, that also adopted Hindu – Buddhist culture. This was followed by national kingdoms of Sukhothai and Ayutthaya.
Eventually the Tai settled throughout the Chao Phraya basin to as far south on the Malay Peninsula. Over centuries, the Tai intermarried people of Mon, Khmer, Malay, and Chinese descent. This fusion of ethnicity has led to considerable diversity in the modern Thai people. It also gave rise to a population with its own unique beliefs, practices, language, food, clothing, and way of life that differ from other Tai ethnic groups around the region.