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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.
According to paragraph 2, what is a problem for the explanation that life began in the oceans?
选项 A、Some life-forms would have been killed by high concentrations of CH2O and HCN in the early Earth’s oceans.
B、If complex organic compounds formed in Earth’s oceans, they have been broken down by photochemical and thermal reaction.
C、The essential elements for complex organic compounds did not exist on the early Earth.
D、The creation of complex organic compounds on Earth’s oceans was influenced by the photochemical and thermal reactions.
答案B
解析 本题属于事实信息题，问根据第2段，关于生命起源于海洋的说法有什么问题。原文第2段倒数第2句提到了这个说法的一个问题：更复杂的有机化合物在海洋表面的环境中不会存在很长时间，因为光化学和热反应会将其破坏，对应B项“如果复杂有机化合物形成于地球海洋中，它们就会被光化学和热反应破坏”。A项“在早期的地球海洋中，一些生命形式可能已经被高度浓缩的甲醛和氰化氧杀死了”，第2段第2句提到，甲醛和氰化氢被高度浓缩的可能性很小，且这两个成分是组成生命的基本成分，不可能杀死一些生命形式，可排除A项。C项“构成复杂有机化合物的基本元素在早期地球上并不存在”，第2段第2句提到，甲醛和氰化氧这两个生命的基本组成部分可能存在。D项“地球海洋中复杂有机化合物的生成受到光化学和热反应的影响”不符合第2段倒数第2句的表述，该句提到有机化合物的生成会受到光化学和热反应的破坏，而非简单的影响，故排除D项。
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