The origins of the first Americans have been debated for years. Nowadays most people agreed that the first settlers came from northeast Asia and entered the New World during the last Ice Age. As ocean water was frozen and the sea level was dropped, a land bridge, Beringia（白令陆桥）, from Siberia（西伯利亚） to present-day Alaska was formed. The earliest Americans crossed the bridge and then spread southward throughout the America.

    This migration may have begun as early as 30,000 years ago and continued until10,000 ago, when the land bridge was covered by the rising sea level. Because these immigrants came in separate waves over such a long period of time, they were divided into hundreds of tribes, enormously varied in physical appearance,language, skills and civilization. The Native Americans and the rest of the world knew little about each other until the New World was discovered in 1492.