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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.
According to the second and third paragraph, which of the following is one way in which Earth’s early atmosphere differed from its current atmosphere?
选项 A、It had fewer greenhouse gases.
B、It was warmer.
C、It had lower levels of carbon dioxide.
D、It did not contain methane.
答案B
解析 本题提问早期大气与现代大气的区别。第2段第3句提到“地球最早的大气成分中，甲烷和二氧化碳的浓度比现在高很多”，而第4句和第5句指出这两种温室气体会使地球变暖，由此可推测出当时地球大气温度比现在高，因此B项正确。根据文意，早期大气层的温室气体，即甲烷和二氧化碳的含量高于现代大气层，因此A、C、D三项的说法均错误。
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