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  3. Why has life flourished on Earth? This question has a two-part, answer. First, Earth has been a cradle for life because of its p

Why has life flourished on Earth? This question has a two-part, answer. First, Earth has been a cradle for life because of its p

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问题     Why has life flourished on Earth? This question has a two-part, answer. First, Earth has been a cradle for life because of its position relative to the Sun. Second, once life began on Earth, simple early life-forms (photosynthetic bacteria) slowly but inexorably altered the environment in a manner that not only maintained life but also paved the way for later, complex life-forms. These changes allowed later organisms to evolve and thrive. Humans and other higher organisms owe their life-supporting environment to these early life-forms.
    Earth’s earliest atmosphere contained several gases: hydrogen, water vapor, ammonia, nitrogen, methane, and carbon dioxide, but no oxygen. Gas mixtures emitted from present-day volcanoes resemble this early atmosphere, suggesting its origin from volcanic eruptions. In Earth’s earliest atmosphere, methane and carbon dioxide occurred at much higher levels than at present—a circumstance that was favorable for early life. Methane and carbon dioxide are greenhouse gases that warm atmospheres by retarding loss of heat to space. These two gases kept Earth warm during the Sun’s early history, when the Sun did not bum as brightly as it now does. (An early dim period, with later brightening, is normal for stars of our Sun’s type.)
    Earth’s modern atmosphere, which is 78 percent nitrogen gas, 21 percent oxygen, and about 1 percent argon, water vapor, ozone, and carbon dioxide, differs dramatically from the earliest atmosphere just described. The modern atmosphere supports many forms of complex life that would not have been able to exist in Earth’s first atmosphere because the oxygen level was too low. Also, if atmospheric methane and carbon dioxide were as abundant now as they were in Earth’s earliest atmosphere, the planet’s temperature would likely be too hot for most species living today. How and when did the atmosphere change?
    The answer to this riddle lies in the metabolic activity of early photosynthetic life-forms that slowly but surely transformed the chemical composition of Earth’s atmosphere. Some of these early organisms were photosynthetic relatives of modem cyanobacteria (blue-green bacteria). In the process of photosynthesis, carbon dioxide gas combined with water yields oxygen. In Earth’s early days, all over the planet countless photosynthetic bacteria performed photosynthesis. Together, these ancient bacteria removed massive amounts of carbon dioxide from Earth’s atmosphere by converting it to solid organic carbon. These ancient bacteria also released huge quantities of oxygen into the atmosphere. Other ancient bacteria consumed methane, greatly reducing its amount in the atmosphere. When our Sun later became hotter, the continued removal of atmospheric carbon dioxide and methane by early bacteria kept Earth’s climate from becoming too hot to sustain life. Modern cyanobacteria still provide these valuable services today.
The second paragraph supports which of the following statements about methane and carbon dioxide present in Earth’s earliest atmosphere?
选项 A、They slowed down the loss of heat from Earth’s atmosphere.
B、They caused the sunlight to be less bright than it currently is.
C、They occurred in smaller amounts than they currently do.
D、They prevented the development of early life-forms.
答案A
解析 题目问甲烷和二氧化碳有哪些特征。文中第2段第4句提到“甲烷和二氧化碳是温室气体，通过减缓热量丧失使大气变暖”，因此A项说法正确。B项“它们导致太阳不如现在明亮”，根据文意，地球早期较为昏暗是其本身特征，与甲烷和二氧化碳没有关系，故排除。C项与文意正好相反，文中说到早期甲烷和二氧化碳在大气中的含量比现在高，故也不选。D项同样与文意相悖，文中是说这两种气体为早期生命创造了适宜的生存环境，因此排除D项。
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