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整理的2012年12月英语六级阅读每日一练. The life story of the human species goes back a million years, and there is no doubt that man came only recently to the western hemisphere. None of the thousands of sites of aboriginal habitation uncovered in North and South America has antiquity comparable to that of old World sites. Mans occupation of the New World may date several tens of thousands of years, but no one rationally argues that he has been here even 100,000 years. Speculation as to how man found his way to America was lively at the outset, and the proposed routes boxed the compass. With one or two notable exceptions, however, students of American anthropology soon settled for the plausible idea that the first immigrants came b way of a land bridge that had connected the northeast comer of Asia to the northwest corner of North America across the Bering Strait. Mariners were able to supply the reassuring information that the strait is not only narrowit is 56 miles widebut also shallow, a lowering of the sea level there by 100 feet or so would transform the strait into an isthmus . With little eels in the way of evidence to sustain the Bering Strait land bridge, anthropologists embraced the idea that man walked dryshod from Asia to America. Toward the end of the last century, however, it became apparent that the Western Hemisphere was the New World not only for man but also for a host of animals and plants. Zoologists and botanists showed that numerous subjects of their respective kingdoms must have originated in Asia and spread to America. These findings were neither astonishing nor wholly unexpected. Such spread of populations is not to be envisioned as an exodus or mass migration, even in the case of animals. It is, rather, a spilling into new territory that accompanies increase in numbers, with movement in the direction of least population pressure and most favorable ecological conditions. But the immense traffic in plant and animals forms placed a heavy burden on the Bering Strait land bridge as the anthropologists ahead envisioned it. Whereas purposeful men could make their way across a narrow bridge, the slow diffusion of plant and animals would require an avenue as a continent and available for ages at a stretch. | 
