The civilization of ancient China first developed in the Yellow River region of northern China, in the 3rd and 2nd millennia BC. A large part of this area is covered by loess soil. This very fine earth has blown in from the highlands of central Asia over thousands of years, and makes one of the most fertile soils in the world. In ancient times, the main crop in northern China was millet, a highly nutritious food still grown in many parts of the world as a major crop.