The cultivation of rice started in China around 12,000-15,000 years ago. It was spread through India and Sri Lanka, then to Greece and the Mediterranean (perhaps due to Alexander the Great's military campaigns in India). From there it spread through Europe and, much later, to the Americas.
A separate cultivation event seems to have happened in Africa around 3500 years ago, but that cultivar declined in favor of the Asian variety.
Just in case you don't know this about rice - I recently saw a report on Trust Me I'm a Doctor BBC TV programme that rice contains arsenic and should be washed thoroughly before being cooked. Brown rice has the highest levels and it's recommended by the medics that it is soaked overnight and then rinsed before being cooked.