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  3. THREE THEORIES ABOUT ORIGIN OF LIFE (1) Oxygen and nitrogen are major components of our current atmosphere. But the kinds of

THREE THEORIES ABOUT ORIGIN OF LIFE (1) Oxygen and nitrogen are major components of our current atmosphere. But the kinds of

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问题                                                 THREE THEORIES ABOUT ORIGIN OF LIFE
    (1) Oxygen and nitrogen are major components of our current atmosphere. But the kinds of hydrogen reactions with other gases that are required to transform simple organic molecules into complex ones are interrupted by oxygen, which combines with hydrogen atoms from other compounds. Therefore, life on Earth must have originated when there was very little oxygen in Earth’s atmosphere. The modern scientific theory of life’s origin was first formulated in the 1920s by Russian scientist Aleksandr Oparin and independently by British scientist J. B. S. Haldane. The assumption that life sprang up from chemical reactions that were initiated in the early atmosphere (oxygen-poor/hydrogen-rich) and came to completion in the early oceans was posited by the Oparin-Haldane hypothesis, as it came to be called. Oparin and Haldane suggested that the hydrogen-containing gases were caused to react with each other to form organic compounds by energy sources such as sunlight and lightning.
    (2) With regard to the view that these complex organic compounds could have begun to shape in Earth’s oceans, some researchers remain skeptical. The probability that the fundamental building blocks of life, formaldehyde (CH2O) and hydrogen cyanide (HCN), even though they were probably available, would have been concentrated sufficiently to allow further reactions to occur was likely small. And the more complex organic compounds that might have formed in this way would not have lasted long in the surface of the ocean environment, because photochemical and thermal reactions would have destroyed them. Therefore, researchers have sought alternative explanations for how complex organic compounds formed.
    (3) There is one possibility that the relevant organic compounds were created in space, and asteroids or comets brought them to Earth, probably as tiny dust particles. Recovered from the stratosphere (an upper region of Earth’s atmosphere), interplanetary dust particles (IDPs) are tiny particles that are known to be extraterrestrial origin. From research, we know that organic compounds, including amino acids, exist in IDPs as well as in some meteorites. It is believed that they form from reactions between charged particles and neutral molecules. Those atoms appear in interstellar dust clouds at very low temperatures—on the order of 200 or more degrees below zero on the Celsius scale. It may seem surprising that organic chemistry could occur in the interstellar environment, but it. is precisely the extremely low temperatures involved that allow complex organic molecules to exist because temperatures are too cold to allow them to decompose.
    (4) [A] It is thought that some of the molecules created in the interstellar environment have survived the collapse of the gas and dust cloud that formed our solar nebula and Sun. [BJ They would have been incorporated into solid materials that condensed out of the nebula and formed asteroids and comets. [C] Such materials might have been delivered to Earth in great quantities during the heavy bombardment period of solar system history, between 4.5 and 3.8 billion years ago. [D]
    (5) The hypothesis that life took place in or around hydrothermal vents (hot springs), where new seafloor is being created along mid-ocean ridges (underwater mountain chains) at the ocean’s bottom is the third theory of life’s origin. Seawater that flows a kilometer or more down through crevices in the rock, is heated, and then rises rapidly back to the surface, and the ridges are cooled. During the process, the water gathers substances such as hydrogen, hydrogen sulfide, and dissolved ferrous iron. When it meets the cold water, the extremely hot (350 degree centigrade) vent water generates a dark plume comprised mostly of iron sulfide, a compound produced by the reaction between ferrous iron and hydrogen sulfide.
    (6)Still, the explanation that submarine hydrothermal vents are a likely place for life to have originated is controversial. In vent systems, there are various types of materials from which organic molecules can be synthesized. However, complex organic molecules are not stable at the high temperatures observed in vents positioned directly on the axis of a ridge. If life did originate at the mid-ocean ridges, it probably did so in cooler, off-axis vents. Some researchers claim that the perfect place for life to have begun would be in some near-freezing surface environment because even the off-axis vents are too warm. The dispute as to whether life originated in a hot or cold environment is unlikely to stop.
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 sentences 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.
The origin of life on Earth has been the subject of scientific study, and several theories developed to clarify it.
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Answer Choices
(A) The Oparin-Haldane hypothesis about the origin of life explained the relationship between early atmosphere and chemical reactions but it has been abandoned by modern scientists.
(B) All of the hypotheses about the origin of life have been disproved, and it seems impossible to devise better hypotheses.
(C) Organic compounds can occur and remain stable in interstellar space and these chemicals may have led to the origin of life after they reached Earth.
(D) One theory assumed that organic molecules formed at hydrothermal vents are responsible for the origin of life, and the high temperatures in some vents make such molecules unstable.
(E) One theory argues that molecules in the cold interstellar environment reacted in the heat of hydrothermal vents, creating the stable organic molecules which are essential for life.
(F) Modern hypothesis suggested life may come from chemical reactions in Earth’s ocean, but some scientists still doubt that the chemical products would have concentrated or long-lasting enough.
选项
答案C,D,F
解析 本题属于文章总结题。C项“有机化合物可以在星际空间中形成并保持稳定，这些化学物质在到达地球后可能导致了生命起源”即第二个生命起源理论，是对文章第3段和第4段的总结概括。D项“一种理论认为，在热液喷口形成的有机分子是生命起源的原因，而一些喷口的高温状态使得这些分子不稳定”对第5、6段的概括，即第三个生命起源理论及其不合理之处。F项“现代假设认为生命可能来自地球海洋中的化学反应，但一些科学家仍然怀疑这些化学产物是否足够浓缩或持久”是对文章第1、2段的总结概括，即第一个生命起源理论及其不合理之处。A项“关于生命起源的奥巴林.霍尔丹假说解释了早期大气和化学反应之间的关系，但它已被现代科学家所抛弃”错误，第1段只说到科学家对这个假说持怀疑态度(remain skeptical)，没有说将其抛弃了(abandoned)。B项“所有关于生命起源的假说都被推翻了，似乎不可能再提出更好的假说”没有依据。再者，关于第二个假说，文章没有指出其错误之处。E项“一种理论认为，在寒冷的星际环境中的有机分子在热液喷口的高温下发生反应，产生了组成生命必要的稳定有机分子”是把第二个生命起源理论和第三个生命起源理论糅合在一起，歪曲了事实。
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