How did the Native Americans of the United States arrive in North America?
There are a number of theories for how the First Nations (or Native Americans) came to live in the Americas. The most generally accepted theory is that during the Ice Age which occurred about 12,000 years ago, so much water was locked up in ice that a bridge of land was exposed between what we now call Russia and Canada. It would have been possible for people to cross this land bridge, most likely motivated by the search for food, and spread into North and South America. There are some challenges to this theory, as human remains and stone tools have been found which date to earlier than 12,000 years. In fact, a human coprolite (fossilized poo) has been found in Oregon which dates to more than 14,000 years ago! Off the coast of Florida, human remains dating to 13,000 years ago have been found, which can challenge both the timing and direction of human migrations.
The second most popular theory is similarly based in geological and environmental processes, and suggests that people may have crossed the Pacific by "island hopping." With low enough water levels, people may have been able to swim, raft, wade, or even walk along a series of islands and shoals throughout the Pacific Ocean.