According to the Land bridge theory, Native Americans migrated

US History: Early America, Lesson One – The Land Bridge Theory
The Land Bridge Theory
Do you ever wonder where the first Americans came from? The Land
Bridge Theory is one idea of how people first appeared on this continent. It states
that Native Americans migrated from Asia to North America across the land
bridge during the Ice Age. During this time period, low temperature caused the
level of water in the ocean to drop. Because the frozen water caused the level of
the oceans to drop, the land that once was under water became exposed,
creating a long land bridge that connected North America to Asia. By following
herds of animals, hunter-gatherer people may have crossed this land bridge
from Asia into North America and then continued throughout many generations
across North America and even spreading into South America. It is believed that
the people we call “Native Americans” are the descendents of these first huntergatherers from Asia.