Thailand was earlier known as Siam. It was populated from the very beginning of civilization in Asia. There are many opinions and conflicts regarding the origins of the Thais. It is presumed that they originated about 4,500 years earlier. It is thought that the Thaïs first originated in northwestern Szechuan which is situated in China. Then, they migrated to Thailand which is situated in the southern part of China. They divided into two groups mainly. One group settled in the Northern Thailand who are claimed to be the first Thais, to settle and start a kingdom called "Lan Na" while the other group settled in far south, which was defeated afterwards by the Khmers, and it was under the kingdom of "Sukhothai".
Nobody really knows from where the Thais have originated but it is claimed by some that they have the same origin as the people of Mongolia.
But later on the opinions changed as the archaeological excavations start taking place in Udon Thani province in the northeast part of Thailand. There was evidence of bronze metallurgy which provided some sort of proof that the Thais originated in Thailand and later on they scattered to different places of Asia, including China.
There was no definite conclusion or consensus in the controversy regarding the origin of Thais. There are many theories which have come up with different ideas. Some go as far as to declare that the Thais originated from Austronesia and not from Mongolia. Though there is a lot of dispute regarding the origin Thais, there is consensus that they settled in southeast Asia by the 13th century.