托福TPO9阅读原文翻译及答案:Part1

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托福TPO9阅读原文翻译及答案：Part1

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托福TPO9阅读原文：Part1

It has long been accepted that the Americas were colonized by a migration of

peoples from Asia, slowly traveling across a land bridge called Beringia (now the

Bering Strait between northeastern Asia and Alaska) during the last Ice Age. The

first water craft theory about this migration was that around 11,000-12,000 years

ago there was an ice-free corridor stretching from eastern Beringia to the areas of

North America south of the great northern glaciers. It was this midcontinental

corridor between two massive ice sheets-the Laurentide to the east and the

Cordilleran to the west-that enabled the southward migration. But belief in this ice-

free corridor began to crumble when paleoecologist Glen MacDonald

demonstrated that some of the most important radiocarbon dates used to support

the existence of an ice-free corridor were incorrect. He persuasively argued that

such an ice-free corridor did not exist until much later, when the continental ice

began its final retreat.

Support is growing for the alternative theory that people using watercraft,

possibly skin boats, moved southward from Beringia along the Gulf of Alaska and

then southward along the Northwest coast of North America possibly as early as

16,000 years ago. This route would have enabled humans to enter southern areas

of the Americas prior to the melting of the continental glaciers. Until the early

1970s,most archaeologists did not consider the coast a possible migration route

into the Americas because geologists originally believed that during the last Ice

Age the entire Northwest Coast was covered by glacial ice. It had been assumed

that the ice extended westward from the Alaskan/Canadian mountains to the very

edge of the continental shelf, the flat, submerged part of the continent that extends

into the ocean. This would have created a barrier of ice extending from the Alaska

Peninsula, through the Gulf of Alaska and southward along the Northwest Coast of

north America to what is today the state of Washington.

The most influential proponent of the coastal migration route has been

Canadian archaeologist Knut Fladmark. He theorized that with the use of watercraft,

people gradually colonized unglaciated refuges and areas along the continental

shelf exposed by the lower sea level. Fladmark's hypothesis received additional

support form from the fact that the greatest diversity in native American languages

occurs along the west coast of the Americas, suggesting that this region has been

settled the longest.

More recent geologic studies documented deglaciation and the existence of

ice-free areas throughout major coastal areas of British Columbia, Canada, by

13,000 years ago. Research now indicates that sizable areas of southeastern Alaska

along the inner continental shelf were not covered by ice toward the end of the

last Ice Age. One study suggests that except for a 250-mile coastal area between

southwestern British Columbia and Washington State, the Northwest Coast of

North America was largely free of ice by approximately 16,000 years ago. Vast areas

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