It is amazing how long it took for rice to spread and take over. Guessing it depended on advances in irrigation systems.
Growing it in flooded padis, to control weeds, came surprisingly late, like 4000 ya. Putting fish in the padis to eat mosquito larvae and provide protein started only ~1800 ya.
From Ricepedia: "Recent genetic evidence show that all forms of Asian rice, both indica and japonica, come from a single domestication event that occurred 8,200–13,500 years ago in the Pearl River valley region of China."