托福 托福 61 - Conditions on Earth When Life Began
题目
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10.According to paragraph 3, what is the significance of the fact that the rocks of Earth's crust now contain an enormous amount of calcium carbonate?
  • A.It explains why Oparin and Miller believed that Earth's early atmosphere was methane rich.
  • B.It supports the idea that the atmosphere of early Earth was rich in carbon dioxide.
  • C.It helps explain where the carbon dioxide in Earth' s atmosphere came from
  • D.It provides evidence about the likely composition of the crust of early Earth.
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    In the 1920s, Aleksandr Oparin, a Russian biochemist, proposed and developed the idea that life originated in the warm, watery environment of early Earth' s surface, under an atmosphere mostly composed of methane. The early seas were believed by Oparin to be rich in simple organic molecules, which reacted to form more complex molecules, eventually leading to proteins and life. Then, almost 30 years after Oparin published his ideas, Stanley Miller demonstrated that amino acids, the building blocks of the proteins necessary for life, could form under conditions thought to prevail on early Earth Miller' s experiment was elegant. He passed electric discharges through a mixture of methane, hydrogen, ammonia, and steam, and when he analyzed the results, found that he had made amino acids. The discharges were a proxy for lightning, the gas mixture an educated guess about what the early atmosphere may have been like. Amino acids cannot replicate themselves, and are not themselves alive. Nevertheless,, this experiment has long been recognized as a landmark for understanding a process that must have been one of the important steps in the evolution of life on Earth, the natural synthesis of amino acids However, it now seems likely that Miller' s experiments may not be directly applicable to the events of the early Archean (that is, early in the geologic eon that lasted from Earth' s formation until about 2.5 billion years ago).

    One of the problems hindering understanding of the origin of life is that environmental cond巾ons on early Earth are not known with any certainty. It is possible to make only reasoned estimates. For example, for some fairly long period of time after formation, perhaps as much as several hundred million years, the surface must have been much hotter than it is today. Continued impacts of meteorites, large and small, would have added further heat energy, and in the earliest part of Earth history the larger impacting bodies may have broken through the cooling crust to expose underlying molten material Large quantities of volcanic gases would have been released into the atmosphere as lavas erupted onto the surface, producing a greenhouse effect far more severe than anything likely to result from human activity. It is quite possible that the early atmosphere was many times as dense as today' s, and that the seas and oceans were hot. Some have even suggested that because of the high atmospheric pressure, the oceans could have been hotter than the bo巾ng point of water today However, life as we know it is quite sensitive to temperature, and no modern organisms are known to survive much above 100°C It is unlikely that life became established until surface temperatures had decreased to this level, or lower.

    Although we do not know the precise composition of the early atmosphere, there has been enough progress made on this subject in recent years that it is possible to say with some certainty that the methane-rich composition envisioned by Oparin, and the methane-ammonia-hydrogen mixture used by Miller in his experiments, are probably not very realistic. Based on studies of our closest neighbor planets, Mars and Venus, and also considering evidence from Earth' s sedimentary rocks, it seems probable that Earth' s early atmosphere was rich in carbon dioxide rather than methane. On both Mars and Venus, carbon dioxide is by far the most abundant gas in the atmosphere. On Earth it is a minor constituent. But there is an enormous amount of this compound buried in the sedimentary rocks of Earth' s crust, enough so that, if it were all released, our atmosphere would be much more like those of our neighboring planets. How did carbon dioxide gas end up in the crust? The answer lies in what geologists refer to as the carbon cycle Through a series of chemical reactions, carbon dioxide from the atmosphere finds itself, in dissolved form, in the oceans. In seawater it combines with calcium to precipitate as calcium carbonate, the main constituent of limestone Over geologic time so much carbon dioxide from the atmosphere has been converted to limestone in this fashion that there is more than 100,000 times as much stored as limestone as there is in the atmosphere.

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    最新提问
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    • wx_5576
      这道题C为什么对,E为什么不对?
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      B为什么不能选啊?
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      这道题不选E是因为太细节了吗
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      小雨淅沥哗啦的下 去解答 去解答
      B为啥不对
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      鑫哥,想知道E错在哪里?
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      B为什么不选
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      鑫哥,这个哪里看不use了。BD是修饰错,C是无中生有,怎么能直接选出A?
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      为什么选b?
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    • 我是啦啦啦
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      鑫哥,D选项里的19世纪出现了很多假设,原文中并没有提到啊
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    • wx_8122
      D为什么不选
    • wx_1655
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    • wx_6697
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    • wx_6697
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    • wx_6697
      鑫哥,这题的C是怎么得到的?B也没有找到啊?难道不是应该选B
    • wx_6697
      鑫哥,还有这个,好多人选A答案是不是错了
    • wx_6697
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    • 此楠楠
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