Rice is the main source of food for about half of the world's population.
It is cultivated in more than 100 countries on every continent except for Antarctica, although 91 percent of the world's rice is grown and consumed in Asia. The two main strains of O. sativa are japonica and indica. The differences between these two evolved both geographically and culturally over thousands of years as farming groups relocated to different ecosystems. Some groups moved to new land at higher elevations, while others established lowland rice fields in dry areas, seasonally flooded ground, or tidal swamp areas. Over the millennia, different types of rices evolved that were cultivated under different ecological conditions.