Most anthropologists today believe that the ancestors of all American Indians immigrated from northwestern Asia across the Bering land bridge during the Ice Age, between 12,000 and 60,000 years ago. Known as the Bering Strait theory, this idea is supported by archaeological, biological, and geological evidence.
———As water became locked up in the polar ice caps, sea levels dropped as much as 300 feet. The Bering Sea between Siberia and Alaska is no more than 180 feet deep and would have been dry land at those times. The land bridge, called Beringia, as opened several times in the last sixty thousand years: 50,000 to 60,000 years ago, 40,000 to 45,000 years ago, 28,000 to 33,000 years ago, and 13,000 to 23,000 years ago. During each of these periods, central and northwestern Alaska were ice free, and for 80 percent of the period from 10,000 to 30,000 years ago there was an ice-free corridor between the eastern (Laurentide) and western (Cordillerran) North American ice sheets that linked Alaska to the American Great Plains.
———Biologically, American Indians are quite similar to the peoples of eastern Asia. They share a number of physical similarities — in teeth, skeletal features, coloration, hair type, and lack of body hair, as well as less visible characteristics — that set them apart from other humans in Europe and Africa.
———Some scholars have suggested prehistoric migrations to North America from northern Europe, the Middle East, Africa, or Polynesia. Little or no evidence supports these theories, however. Many American Indians, in contrast, believe their people were divinely created in the Americas.

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