2014年6月15日托福阅读真题+题目+答案：Early life-forms and Earth’s Atmosphere
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 burn 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 modern 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 bacterial oxygen release improved conditions for life in two ways. First, oxygen is essential for the metabolic process known as cell respiration that allows cells to efficiently harvest energy from organic food. Second, oxygen in the upper atmosphere reacts to form a protective shield of ozone. Earth is constantly bombarded by harmful ultraviolet (UV) radiation from the Sun. Today, Earth’s upper-atmosphere ozone shield absorbs enough UV to allow diverse forms of life to survive. But because early Earth lacked oxygen in its atmosphere, it also lacked a protective ozone barrier. As a result, early life on Earth was confined to the oceans, where the water absorbed the UV radiation. Only after oxygen released by ancient bacteria drifted up into the upper atmosphere and reacted with other oxygen molecules to form a protective layer of ozone could life flourish at the surface and on the land. The absence of an oxygen atmosphere on Mars and other planets in our solar system means that these planets also lack an ozone shield that would protect surface-dwelling life from UV radiation. The surface of Mars is bombarded with deadly radiation; if any life exists on Mars, it would almost certainly be subterranean.
1. The word ”altered” in the passage is closet in meaning to
C.made use of
2. Paragraph 1 supports which of the following inferences?
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 Sun. Paragraph 1 is marked with an arrow [→]
3. The word ”emitted” in the passage is closet in meaning to
4. The word ”retarding” in the passage is closet in meaning to
5. In paragraph 2, why does the author provide the information that methane and carbon dioxide kept the Earth warm during the Sun’s early history?
A.To explain how the early Earth and the early Sun were related
B. To support the claim that methane and carbon dioxide are greenhouse gases
C. To explain why the high levels of methane and carbon dioxide in Earth’s early atmosphere were favorable for early life
D. To suggest that these gases affect how brightly the Sun burns Paragraph 2 is marked with an arrow [→]
6. Paragraph 2 supports which of the following statements about methane and carbon dioxide present in Earth’s earliest atmosphere?
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. Paragraph 2 is marked with an arrow [→]
7. The word ”abundant” in the passage is closet in meaning to
8. According to paragraph 2 and 3, which of the following is one way in which Earth’s early atmosphere differed from its current atmosphere?
A.It had fewer greenhouse gases.
B. It was warmer.
C. It had lower levels of carbon dioxide.
D. It did not contain methane. Paragraphs2 and 3 are marked with arrows [→]
9. The word ”sustain” in the passage is closet in meaning to
10. According to paragraph 4, ancient bacteria changed the chemical composition of Earth’s atmosphere by performing all of the following activities EXCEPT
A.raising the temperature of the atmosphere
B. removing methane and carbon dioxide
C.creating organic carbon
D. producing oxygen Paragraph 4 is marked with an arrow [→]
11. It can be inferred from paragraph 5 that early life-forms on Earth were confined to the oceans because
A.the thick ozone layer at the time would have made it difficult for them to survive on land
B. water was the only available protection they had against ultraviolet radiation
C. land provided them with only limited amounts of water needed for survival
D. their metabolic systems were inefficient Paragraph 5 is marked with an arrow [→]
12. Which of the sentences below best expresses the essential information in the highlighted sentence in the passage? Incorrect choices change the meaning in important ways or leave out essential information.
A. Any life forms that may have existed on other planets probably were destroyed by UV radiation.
B. Other planets in our solar system lack the oxygen atmosphere that helps explain why life exists on Earth.
C. The absence of oxygen on other planets means that those planets lack an ozone shield to protect life forms against UV radiation.
D. Life forms cannot survive UV radiation without the protection of an ozone shield.
Paragraph 5 The bacterial oxygen release improved conditions for life in two ways. First, oxygen is essential for the metabolic process known as cell respiration that allows cells to efficiently harvest energy from organic food. Second, oxygen in the upper atmosphere reacts to form a protective shield of ozone. ■Earth is constantly bombarded by harmful ultraviolet (UV) radiation from the Sun. ■ Today, Earth’s upper-atmosphere ozone shield absorbs enough UV to allow diverse forms of life to survive. ■But because early Earth lacked oxygen in its atmosphere, it also lacked a protective ozone barrier. ■As a result, early life on Earth was confined to the oceans, where the water absorbed the UV radiation. Only after oxygen released by ancient bacteria drifted up into the upper atmosphere and reacted with other oxygen molecules to form a protective layer of ozone could life flourish at the surface and on the land. The absence of an oxygen atmosphere on Mars and other planets in our solar system means that these planets also lack an ozone shield that would protect surface-dwelling life from UV radiation. The surface of Mars is bombarded with deadly radiation; if any life exists on Mars, it would almost certainly be subterranean.
13. Look at the four squares [■] that indicate where the following sentence could be added to the passage. But protection against what?
Where would the sentence best fit? Click on a square [■] to add the sentence to the passage.
14. Directions: An introductory sentence for a brief summary of the passage is provided below. Complete the summary by selecting the THREE answer choices that express the most important ideas in the passage. Some answer choices do not belong in the summary because they express ideas that are not presented in the passage or are minor ideas in the passage. This question is worth 2 points. Drag your choices to the spaces where they belong. To review the passage, click on View Text.
Earth’s earliest atmosphere was transformed in ways that allowed the development of complex life forms.
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1.Volcanoes changed Earth’s earliest atmosphere in ways that allowed life to develop, and bacteria that used oxygen produced by photosynthesis further altered the atmosphere to what we find today.
2.When the sun became brighter, early bacteria removed methane and carbon dioxide from the atmosphere, preventing Earth from becoming too hot for life to survive.
3.Early bacteria provided the oxygen that was needed to support the metabolism of complex life forms and to form an ozone shield against deadly UV radiation.
4.Volcanic carbon dioxide in Earth’s early atmosphere kept Earth warm enough for life to begin during the time when the Sun was too dim to provide much warmth.
5.Earth has been able to support life because its position relative to the Sun provided it with enough heat, but not too much heat for early bacteria to evolve.
6.Complex life evolved on the Earth’s surface, but not on Mars or other planets in the solar system because on those planets, early surface life was killed by UV radiation.