For millennia, microorganisms, fungi, plants, and animals have struggled within their physical environment to produce balanced, climax grasslands. Within a mere 50 years or so of first trekking across seemingly endless oceans of prairie (520,000 km 2 or 200,000 sq. mi.), American settlers converted most of these fertile, native grasslands into farmland. This became our nation's "breadbasket" and cereal grains, which are grass seeds, became one of the most important foods grown on the deep, rich soils of the Midwest. Wheat, corn, oats, barley, rice, sugar cane, sorghum, and millet are all used for human or livestock consumption. Grains became important foods to people perhaps as early as 9,000 years ago. Different cultures around the world developed various grains, such as wheat in the Middle East and corn in Mexico.