I The First Inhabitants

For a great many years, anthropologists have debated the origins of the first Americans. Today most agree that America's first settlers came from northeast Asia and entered the New World in the general area of Bering Strait（白令海峡，为丹麦航海家Bering〔1680—1741〕发现）, during the last Ice Age. During that period, which began about 70,000 B.C and ended about 10000BC, the weather in the northern half of the earth changed greatly. It grew colder. Much of the land was covered with huge glaciers. Sea water froze, and the water level in the sea dropped. There is a narrow strait between the Bering Sea（白令海，太平洋之最北部分）and Chukchi Sea（楚克其海，在西伯利亚东北端与阿拉斯加之间）. During the Ice Age this strait probably became very shallow. In some places it dried up completely and formed a land bridge from Asia to North America. At its widest points, the land bridge, known today as Beringia（an area covering easternmost Siberia and western Alaska）, was about 1600 kilometers across. The earliest Americans must have crossed the bridge from Siberia into Alaska as bands of hunters followed big animals such as giant buffalos and mammoths（猛犸象，已绝种）.

The scientists think that the people of Asia found this land bridge between 15,000 and 40,000 years ago. Group after group moved across the bridge to the unexplored continent of North America. They passed through what is now Alaska and western Canada. This crossing took place over hundreds of years. Because the people migrated in separate waves over such a long period of time, they were very different from one another in physical characteristics, skills, and talents. By the time the first Europeans came to shores of America, the early immigrants had spread throughout the Western hemisphere and developed cultures in keeping with the different terrains（地形）and climates in which they had settled.

Although the first explorers came to America by accident, they opened up two continents—North and South America—to European settlement. On the one hand, the settlers largely destroyed Native American cultures, but on the other hand, they laid the foundations for many new nations, including the United States.