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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 first paragraph supports which of the following inferences?
选项 A、After complex life forms arose on Earth, bacteria quickly evolved in order to maintain favorable conditions for life.
B、Early life forms on Earth arose from the process of photosynthesis.
C、The development of complex life on Earth depended on the presence of bacteria that could be consumed by larger organisms.
D、Complex life forms on Earth may not have evolved if Earth had been farther from or closer to the Sim.
答案D
解析 本题为细节理解题。由第1段第3句可知，地球与太阳的距离合适，为生命的起源和进化提供了一个适合的环境，也就是说，若地球离太阳过近或过远，这一切都不可能，因此D项说法正确。根据文意，复杂生物的出现晚于细菌，且细菌进化过程是缓慢的，因此A项错误。文中第1段没有提及早期的生命形式是如何诞生的，故B项也排除。文中提到远古细菌改变环境，为高等生物提供适宜的生存环境，而并不是细菌为高等生物提供食物，所以C项也错误。
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