The predecessor of Muay Thai style called Muay Boran. It can be traced to XVI century Siam, modern Thailand, where it was developed as a military martial art that could be used on a battlefield.
Muay Boran wasn't limited by any rules and the sole purpose of that fighting style was to kill or maim the enemy soldiers using any technique available, including headbutting and other brutal strikes.
It was an aggressive and very effective style and a lot of Muay Boran fighting techniques can still be observed in modern Muay Thai.
There is a legend about an experienced Siam fighter Nai Khanomtom which describes how effective Muay Boran was. He was captured and kept imprisoned during the Burmese-Siam war in the second half of the XVII century.
Since he was an expert fighter and a famous warrior he was offered a chance to get his freedom back if he defeats a Burmese champion.
Nai Khanomton has knocked down the champion and the angered king has sent another 9 warriors one after another to beat him down but they failed to do so. Soon after the famous warrior got released by his captors and returned to Thailand. The day when he returned to his homeland is celebrated as The National Muay Thai Day.
Whether you believe in this story or not the fact is that Muay Boran was a lethal weapon in times of hand-to-hand combat and it granted Thais an advantage on the battlefield over the Burmese invaders.
Modernization of Muay Thai has started at the beginning of the XX century:
History of Muay Thai is rich with interesting traditions and events that shaped Thai boxing into a tough and competitive martial art it is now. The challenge lies in staying true to its traditions and be able to avoid stagnation at the same time.