Human and animal skulls placed on the ground in a ceremonial offering by Mandan Indians. Birds circle overhead. Aquatint from 1839 by Karl Bodmer. The Mandan historically lived along the banks of the Missouri River and two of its tributaries in present-day North and South Dakota. Speakers of Mandan, a Siouan language, developed a settled, agrarian culture. Ethnologists and scholars studying the Mandan subscribe to the theory that, like other Siouan-speaking people, they originated in the area of the mid-Mississippi River and the Ohio River valleys in present-day Ohio. A migration is believed to have occurred possibly as early as the 7th century but probably between 1000 CE and the 13th century, after the cultivation of maize was adopted.