Most important, scientists are beginning to realize that the climatic changes have had a major impact on the evolution of the human species. New research now suggests that climate shifts have played a key role in nearly every significant turning point in human evolution: from the dawn of primatessome 65 million years ago to human ancestors rising up to walk on two legs, from the huge expansion of the human brain to the rise of agriculture. Indeed, the human history has not been merely touched by global climate change, some scientists argue, it has in some instances been driven by it.

　　The new research has profound implications for the environmental summit in Rio. Among other things, the findings demonstrate that dramatic climate change is nothing new for planet Earth. The benignglobal environment that has existed over the past 10,000 yearsduring which agriculture, writing, cities and most other features of civilization appearedis a mere bright spot in a much larger pattern of widely varying climate over the ages. In fact, the pattern or climate change in the past reveals that Earths climate will almost certainly go through dramatic changes in the futureeven without the influence of human activity.

　　21. Farming emerged as a survival strategy because man had been obliged .

　　A) to give up his former way of life

　　B) to leave the coastal areas

　　C) to follow the ever-shifting vegetation

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