State, west-central U.S. Area: 77,353 sq mi (200,343 sq km). Pop. (2009 est.: 1,796,619). Capital: Lincoln. Nebraska is bordered by South Dakota, Iowa, Missouri, Kansas, Colorado, and Wyoming. The Missouri River is on its eastern boundary. The North Platte and South Platte rivers unite in southwest-central Nebraska to form the Platte River. Various prehistoric peoples inhabited the area as early as 8000 BCE. Native American tribes living in the area include the Pawnee, Oto, and Omaha in the east and centre, as well as the Oglala, Sioux, Arapaho, and Comanche in the west. The U.S. bought the territory from France as part of the Louisiana Purchase in 1803. In 1804 the Lewis and Clark Expedition visited the Nebraska side of the Missouri River. It became part of Nebraska Territory with the Kansas-Nebraska Act of 1854. Nebraska was admitted to the Union as the 37th state in 1867. Soon after, the population increased, and as Indian resistance on the frontier was broken, settlement extended to Nebraska's panhandle. At the turn of the 20th century, it experienced a short but influential Populist movement. In 1937 it established a unicameral legislature, the only one in the country. Most of the state is agricultural; its industries include food processing and machinery. Petroleum is the principal mineral resource. In addition to Lincoln, Omaha is the state's other cultural and industrial centre.