<human>:  What is the history of sumo wrestling?
<bot>: Sumo wrestling is an ancient form of Japanese martial art and was developed over a thousand years ago in Japan. The earliest known reference to the sport dates back to the year 236 at the Nihon Ryouiki, a Japanese chronicle. From the 8th century, this martial art was used by Samurai warriors as a form of training. During the Edo period (1603–1868), sumo wrestling became an established event at festivals, spanning the country. It has since been recognized as one of Japan's traditional sports and has become an integral part of the country's culture and history.