The history of Muay Thai is the history of the Thai people - both though are difficult to discover.
When the Burmese army sacked and razed Ayuddhaya to the ground, the archives of Thai history were lost. With them, much of the early history of Muay Thai also went.
The little we do know, comes from the writings of the Burmese, Cambodian, early European visitors and some of the chronicles of the Lanna Kingdom - Chiangmai.
What all sources agree on, is that Muay Thai began as a close combat battlefield fighting skill. More deadly than the weapons it replaced.

As to where Muay Thai came from, its evolution, the sources aren't clear and often contradict each other. But there are two main theories.
One says that the art developed as the Thai people moved down from China; honed in the struggle for land. The other theory says that the Thai people were already here and that Muay Thai developed to defend the land and people from constant invasion threats.
The second, while controversial, has considerable academic backing and archaeological evidence. The first is, however, possible as the area opened up to the early pioneers.What is known is that Muay Thai was an essential part of Thai culture right from its dawn. And in Thailand, it's the sport of kings.
In olden days, national issues were decided by Muay Thai contests.
The first great upsurge of interest in Muay Thai as a sport, as well as a battlefield skill, was under King Naresuan in 1584, a time known as the Ayuddhaya period. During this period, every soldier trained in Muay Thai and could use it, as the King himself did. Slowly Muay Thai moved away from its root in the 'Chupasart' and new fighting techniques were evolving.
The change in the art was to continue under another fighting King - Prachao Sua - the Tiger King. He loved Muay Thai so much that he often fought incognito in village contests, beating the local champions. During the reign of the Tiger King the nation was at peace. The King, to keep the army busy, ordered it to train in Muay Thai. The interest in the sport was already high but now it took off yet again.
Thai Boxing became the favourite sport and pastime of the people, the army and the King. Historical sources show that people from all walks of life flocked to training camps. Rich, poor, young and old all wanted some of the action. Every village staged its prize fights and had its champions. Every bout became a betting contest as well as a contest of local pride. The betting tradition has remained with the sport and today large sums are wagered on the outcome of fights.
Thai boxing has always been popular but like most sports, there have been times when it was more in fashion. In the reign of King Rama V, many Muay Thai matches were Royal Command fights. These boxers were rewarded with military titles from the King. Today the titles, like Muen Muay Mee Chue from Chaiya or Muen Muay Man Mudh from Lopburi are virtually untranslatable. They mean something comparable to Major of Boxing. At the time they were much prized and respected titles.
The Rama V period was another golden age for Muay Thai. Boxing camps were set up, talent scouts - at Royal Command - recruited potential boxers from up country. Match makers began to make the great matches which were fought for big prizes and honour. This thrilled the people then as much as the main bouts do today at the Bangkok boxing stadiums.
The matches then were not fought in a ring as we know it today - for Muay Thai that is a recent
innovation. Any available space of the right size was used, a courtyard, a village clearing.