Anasazi, Adena, Hopewell, Mississippians
Had plants that they used for food and when added maize and beans had an increase of population.
They needed more room for all of the new people so they needed more land.
An increase in land caused numerous cities with up to 10,000 people.
One of the biggest was Cahokia, near present day St. Louis.
A large mound was found in the middle of Cahokia that's bigger than the pyramids in Egypt surrounded by smaller mounds.