Answer:
The most commonly held is the Bering Land Bridge Theory. This has been at the forefront for over 50 years. Basically, the idea is that during the last ice age, about 20,000 years ago (or a little less), lower water levels created a frozen bridge of land. The first settlers of the Americas are believed to have come across this land bridge called Beringia. This theory has dominated for a long time, but there are other ideas discussing what may have brought the people who developed into native cultures.
The first theory is the Atlantic Theory. The name kind of tells you that this is going to assume that people had to cross the Atlantic Ocean. Archeologists have found these specific, early spear points, originally found near Clovis, New Mexico. Being deemed Clovis Points, for a long time they were offered as evidence for the Land Bridge theory because similar points have been discovered in the area the around Beringia. It turns out, though, these spear points are very similar to points found in Europe, and the oldest example to be found in the Americas have actually been found in the Eastern U.S. This points to migration from Europe to the east of the Americas. This migration pattern would mean that the people who made these earliest spear points had to cross the Atlantic.