From Wikipedia

America's Stone Age Explorers

Scientists have several theories as to the origins of the early human population of North America. The indigenous peoples of North America themselves have many creation myths, by which they assert that they have been present on the land since its creation.

Before contact with Europeans, the natives of North America were divided into many different polities, from small bands of a few families to large empires. They lived in several "culture areas", which roughly correspond to geographic and biological zones and give a good indication of the main lifeway or occupation of the people who lived there (e.g. the Bison hunters of the Great Plains, or the farmers of Mesoamerica). Native groups can also be classified by their language family (e.g. Athapascan or Uto-Aztecan). It is important to note that peoples with similar languages did not always share the same material culture, nor were they always allies.

Scientists believe that the Inuit people of the high Arctic came to North America much later than other native groups, as evidenced by the disappearance of Dorset culture artifacts from the archaeological record, and their replacement by the Thule people.

During the thousands of years of native inhabitation on the continent, cultures changed and shifted. Archaeologists often name different cultural groups they discover after the site where they are first found. One of the oldest cultures yet found is the Clovis culture of modern New Mexico. A more recent example is the group of related cultures called the Mound builders (e.g. the Fort Walton Culture), found in the Mississippi river valley. They flourished from 300 BC to the 150s AD.