The Mandan were relatives of the Sioux people. Around 500 AD, they probably lived in the Midwest of North America, in what is now Ohio. Like their Sioux relatives, and like the Mississippians to their south, the Crow to their west and the Shawnee to their east, the Mandan probably built earth mounds and farmed corn and beans and sunflowers.
When the Sioux moved north about 1000 AD, in response to a global warming period, they moved into the Mandans' territory in Ohio. Probably the Mandan were forced out; they moved further north and west, pushing the Crow before them, to much less fertile land in what is now North and South Dakota. Mandan historians recalled that the Mandan had moved from an eastern location near a lake.
In the Dakotas, the Mandan built nine villages on either side of the Heart River. Mandan women built and owned all of the Mandan houses, which were rectangular, with four wooden pillars whose crossbeams held up the roof. Each house held several families - thirty or forty people. Villages often had wooden fences around them for defense. In the Dakotas, the Mandan continued to farm corn and beans, though they also began to hunt buffalo.