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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 6, what evidence supports the theory that undersea hydrothermal vents are a possible location for the origin of life?
选项 A、Undersea hydrothermal vents probably were much colder during the period of the early Earth than they are observed to be today.
B、The fact that materials with the kind of chemical composition from which organic molecules can be produced are in undersea hydrothermal vents.
C、Located on the ridge axis, complex organic materials are more stable.
D、They are located directly on the ridge axis, near-freezing surface environments.
答案B
解析 本题属于事实信息题，问根据第6段，哪项证据可支持海底热液喷口可能是生命起源地的这一理论。第6段第2句提到，在喷口系统中，有多种物质可以用来合成有机分子，对应B项“海底热液喷口中含有有机分子的化学成分这一事实”。A项“海底热液喷口在早期的地球时期可能比今天所观察到的要冷得多”没有依据。C项“位于海脊轴上的复杂有机物质更稳定”，第6段第3句提到，复杂有机分子在海脊轴线上喷口的高温状态下并 “不”稳定。D项“喷口直接位于海脊轴上，接近冰冻的地表环境”，第6段第4句提到，如果生命确实起源于大洋中脊，那么它很可能起源于较冷的、“远离”海脊轴线的喷口，这说明直接位于海脊轴上的喷口温度不会是较冷的。
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