The human species evolved in Africa and began migrating from there to other parts of the world more than 70 thousand years ago. Some of them reached east Asia more than 40 thousand years ago and some kept on going across the Bering strait to the Americas, perhaps along the coast by boat. The first ancestors of the native Americans reached North America at least 20 thousand years ago and possibly much earlier. Others kept coming in waves from Asia, the most recent arriving perhaps 3 or 4 thousand years ago. Some other humans from Africa reached Europe about 40 thousand years and their descendants began coming across the Atlantic ocean to the Americas about 500 years ago. All of this was long before the United States was formed. There are many books about this story. One interesting one is 1491:New Revelations of the Americas Before Columbus published in 2005 by Charles C. Mann.