The Adena began constructing large earthwork mounds around 600 BCE. They are the earliest known people to have been Mound Builders, however, there are mounds in the United States that predate this culture. Watson Brake is an 11-mound complex in Louisiana that dates to 3,500 BCE, and nearby Poverty Point, built by the Poverty Point culture, is an earthwork complex that dates to 1,700 BCE. These mounds likely served a religious purpose.

The Adenans were absorbed into the Hopewell tradition, a powerful people who traded tools and goods across a wide territory. They continued the Adena tradition of mound-building, with remnants of several thousand still in existence across the core of their former territory in southern Ohio. The Hopewell pioneered a trading system called the Hopewell Exchange System, which at its greatest extent ran from the present-day Southeast up to the Canadian side of Lake Ontario.^[8] By 500 CE, the Hopewellians had too disappeared, absorbed into the larger Mississippian culture.

The Mississippians were a broad group of tribes. Their most important city was Cahokia, near modern-day St. Louis, Missouri. At its peak in the 12th century, the city had an estimated population of 20,000, larger than the population of London at the time. The entire city was centered around a mound that stood 100 feet (30 m) tall. Cahokia, like many other cities and villages of the time, depended on hunting, foraging, trading, and agriculture, and developed a class system with slaves and human sacrifice that was influenced by societies to the south, like the Mayans.^[9]