Day 54

Passage 54

Origin of Life

①

There are many different theories concerning where the origin of life occurred. Some of these theories are more credible than others, yet all provide an interesting explanation for life's beginnings.

②

One early explanation, known as the Materialistic Theory or Physico-chemical Theory, was proposed independently by two scientists—A.I.Oparin, a Russian scientist in 1923 and J.B.S Haldane, an English scientist in 1928. It states that the origin of life on earth is the result of a slow and gradual process of chemical evolution that probably occurred about 3.8 billion years ago. This theory was the precursor of another theory called abiogenesis, also known as spontaneous generation, which studies how life on Earth could have arisen from inanimate matter. It suggests that life began in water as a result of the combination of chemicals from the atmosphere and some form of energy to make amino acids, the building blocks of proteins, which would then evolve into all the species.

③

Essential to this spontaneous origin of life was the availability of organic molecules as building blocks. Already in 1922 Oparin had proposed that the early Earth had a reducing atmosphere comprised mainly of ammonia, methane, and large amounts of hydrogen. Later in 1953, Stanley Miller successfully carried out the famous Miller-Urey experiment which showed that amino acids could be produced under a condition where a mixture of methane, hydrogen, ammonia and water is reacted in a spark discharge apparatus, the condition similar to those on the primitive Earth. The idea of spontaneous generation was popular almost till seventeenth century, supported by many scientists like Descartes, Galileo and Helmont.

④

Toward the end of the 19th century and the beginning of the 20th century, as scientists were learning about the genetic and biochemical complexity of the cell, confusion about the origin of life grew. One way around the problem was to say that life had never emerged, but that it had always been an inherent part of the universe. A Swedish chemist named Svente Arrhenius developed the panspermia theory which simply states that life did not originate on the Earth, but originated elsewhere in the universe. He believed that cellular life reached the Earth hiding inside a meteor which hit the Earth long ago.

⑤

Newly uncovered evidence suggests that this theory might be possible, since an organism inside a meteor would be safe from the high levels of radiation in space, and would be kept at a relatively low temperature. The odds of an organism surviving inside a meteor for thousands of years, however, are not high. It is even less likely that organisms would be able to withstand the high energy impacts of bolides into the Earth or other planetary objects. Most scientists today do not look at this hypothesis as a very likely origin of life on earth. However, it is considered possible, at least for now, and is still a candidate for an explanation of life's origin on earth.

⑥

Currently many scientists believe that life originated deep beneath the surface of the ocean at deep sea hydrothermal vents. Previously scientists were sure that life could not exist deep beneath the surface of the ocean. After the discovery of hydrothermal vents, however, they found ecosystems thriving in the depths of the ocean. These ecosystems contained various types of fish, worms, crabs, bacteria and other organisms which found a way to survive in a cold, hostile environment without energy from sunlight. These discoveries made many scientists begin to ask questions as to whether or not this was where life may have originated on the earth.

⑦

On the molecular level, the chances of life originating at deep sea thermal vents are very meager. It has been estimated that life could not have arisen in the ocean unless the temperature was less than 25oC, or 77oF, because organic molecules are unstable at high temperatures and are destroyed as quickly as they are produced. On the contrary, the hot gaseous substances released by these vents are at temperatures in excess of 572oF. Supporters of this theory, however, claim that the organic molecules at the thermal vents are not formed in 300oC temperatures, but rather in a gradient formed between the hydrothermal vent water and the extremely cold water, 4oC (39.2oF), which surrounds the vent at the bottom of the ocean.

——2011年7月28日北美考试机经

The word “meager” in Paragraph 7 is closest in meaning to____.

A. small

B. enormous

C. meander

D. high

核心词汇：

词汇练习：

阅读下列句子，用所给单词的正确形式填空。

evolve primitive originate cellular odds withstand hostile availability unstable independently release molecular credible confusion thriving meager

1. Researcher Charles Atkin found that children believe that the characters used to advertise breakfast cereals are knowledgeable about cereals, and children accept such characters as_____sources of nutritional information.(TPO-14:Children and Advertising)

2. It is still unclear whether domesticated cattle were tamed _____ in northern Africa or in-troduced to the continent from southwest Asia.(TPO-28:Early Saharan Pastoralists)

3. The physical structure of species competing for resources in the same ecological niche tends to gradually _____ in ways that allow them to occupy different niches. (TPO-29:Competition)

4. The _____ of steam power and the demands for new machines facilitated the transformation of the iron industry.(TPO-26:Energy and the Industrial Revolution)

5. Among plants, the K-T boundary saw a sharp but brief rise in the abundance of ____vas-cular plants such as ferns, club mosses, horsetails, and conifers and other gymno-sperms.(TPO-33:Extinction Episodes of the Past)

6. Whether the box appeared in front of the right wall or the left wall, they showed no signs of ____. On overcast days, however, the birds were disoriented and had trouble locating their food box.(TPO-11:Orientation and Navigation)

7. The plumes apparently _____ at great depths, perhaps as deep as the boundary be-tween the core and the mantle, and many have been active for a very long time. (TPO-11:The Formation of Volcanic Islands)

8. Leatherbacks apparently do not generate internal heat the way we do, or the way birds do,as a by-product of_____metabolism.(TPO-15:A Warm-Blooded Turtle)

9. Attack by scavengers and bacteria, chemical decay, and destruction by erosion and other geologic agencies make the_____against preservation very high.(TPO-19:Fossil Preservation)

10. Ancient fresh water green algae must have evolved features that enable them to ____extremes of temperature and periods of dryness. (TPO-19: The Evolutionary Origin of Plants)

11. The pool of excess cash rapidly stimulated a _____ economy outside fort gates. (TPO-19:The Roman Army's Impact on Britain)

12. As oil becomes increasingly difficult to find, the search for it is extended into more _____environments.(TPO-4:Petroleum Resources)

13. Also, because the _____ motion of a gas depends on temperature, at the low temperatures of the Jovian planets even the lightest gases are unlikely to acquire the speed needed to escape.(TPO-16:Planets in Our Solar Systems)

14. By comparison, the terrestrial planets have _____ atmospheres at best. (TPO-16:Planets in Our Solar Systems)

15. Although aragonite has the same composition as the more familiar mineral known as calcite, it has a different crystal form, is relatively _____ , and in time changes to the more stable calcite.(TPO-20:Fossil Preservation)

16. During the day, heat from the Sun can be conducted into these materials and stored—to be_____at night.(TPO-20:Urban Climates)

参考答案：

1. credible 2. independently 3. evolve 4. availability 5. primitive 6. confusion 7. originate 8. cellular 9. odds 10. withstand 11. thriving 12. hostile 13. molecular 14. meager 15. unstable 16. released