The Origins of Life on Earth
As early as the 1920s, some scientists recognized that Earth’s early atmosphere should have been oxygen-free(oxygen is a product of life on Earth); they also hypothesized that, in this oxygen- free environment, sunlight-fueled chemical reactions could have led to the spontaneous creation of organic molecules, molecules like those found in living systems. This hypothesis was tested in the 1950s in a famous experiment known as the Miller-Urey experiment.
The original Miller-Urey experiment used small glass flasks to simulate chemical conditions they thought represented those on early Earth. One flask was partially filled with water to represent the sea and heated to produce water vapor(water in the form of gas). Gaseous methane and ammonia were added and mixed with the water vapor to represent the atmosphere. These gases flowed into a second flask, where electric sparks provided energy for chemical reactions. Below this flask, the gas was cooled so that it could condense to represent rain and then was cycled back into the water flask. The water soon began to turn a murky brown, and a chemical analysis(performed after letting the experiment run for a week) showed that it contained many amino acids, the building blocks of life, and other organic molecules. Thus, the experiment appeared to support the idea that organic compounds would have formed spontaneously and in vast quantities on early Earth, covering our planet and filling at least the top layers of the oceans with a rich “organic soup” containing all the building blocks of life.
However, other scientists soon realized that the methane and ammonia gas mixture used by Miller and Urey probably was not representative of Earth’s early atmosphere and the significance of their results was called into question. That the relevance of the Miller- Urey experiment is an even hotter scientific controversy today demonstrates the way that science often progresses unsteadily.
The major issue concerns exactly what the early atmosphere was made of. When analyzed in more detail, the Miller-Urey experiment shows that hydrogen is a key ingredient needed to form large quantities of organic compounds. In contrast, oxygen in any form, even when tied up in carbon dioxide, tends to suppress the formation of organic compounds. Today, we assume that Earth’s early atmosphere was produced by volcanic outgassing(release of trapped gas). Outgassing may have occurred at a somewhat higher rate in the distant past than it does today, but we expect the gases to have been similar: largely water, carbon dioxide, nitrogen, and methane, along with sulfur-bearing gases and hydrogen. The water condensed to form the oceans, leaving these other gases as potential ingredients for the early atmosphere. If there was a lot of hydrogen relative to carbon dioxide, then the Miller-Urey results might still be valid, suggesting that our young planet did indeed have a rich organic soup. However, if there was little hydrogen, then the oxygen in carbon dioxide would have kept the amount of organic material far lower than the Miller-Urey experiment suggests.
Prior to 2005,it was generally assumed that hydrogen would have escaped from the early atmosphere at the same rate that it escapes today, which is rapid enough that little hydrogen would have been present at any time. However, new calculations, still subject to verification, suggest that conditions in Earth’s early upper atmosphere could have slowed the escape considerably, allowing hydrogen to make up as much as thirty percent of Earth’s early atmosphere. Until this issue is settled, we will not know whether spontaneous chemical reactions would have turned Earth’s surface and oceans into a rich organic laboratory or left them with only small amounts of locally produced organic compounds.
We know of at least two other potential sources of organic molecules on early Earth. The first of these potential sources is chemical reactions near deep-sea vents. As these undersea volcanoes heat the surrounding water, a variety of chemical reactions can occur between the water and the minerals released from the vents. These chemical reactions would have occurred spontaneously in the conditions thought to have prevailed in the early oceans, and they should have resulted in the production of the same types of organic molecules thought to have been necessary for the origin of life. The other additional source of organic molecules may have been material from space. Analysis of meteorites shows that they often contain organic molecules, including complex molecules such as amino acids.
1
As early as the 1920s, some scientists recognized that Earth’s early atmosphere should have been oxygen-free(oxygen is a product of life on Earth); they also hypothesized that, in this oxygen- free environment, sunlight-fueled chemical reactions could have led to the spontaneous creation of organic molecules, molecules like those found in living systems. This hypothesis was tested in the 1950s in a famous experiment known as the Miller-Urey experiment.
According to paragraph 1, the purpose of the Miller-Urey experiment was to determine whether
Factual Information Questions事实信息题
Asunlight reaching early Earth’s atmosphere was as strong as it is today
Borganic molecules could have formed on early Earth as a result of chemical reactions powered by sunlight
CEarth’s early atmosphere was completely oxygen-free
DEarth’s early environment could have acquired oxygen spontaneously
2
The original Miller-Urey experiment used small glass flasks to simulate chemical conditions they thought represented those on early Earth. One flask was partially filled with water to represent the sea and heated to produce water vapor(water in the form of gas). Gaseous methane and ammonia were added and mixed with the water vapor to represent the atmosphere. These gases flowed into a second flask, where electric sparks provided energy for chemical reactions. Below this flask, the gas was cooled so that it could condense to represent rain and then was cycled back into the water flask. The water soon began to turn a murky brown, and a chemical analysis(performed after letting the experiment run for a week) showed that it contained many amino acids, the building blocks of life, and other organic molecules. Thus, the experiment appeared to support the idea that organic compounds would have formed spontaneously and in vast quantities on early Earth, covering our planet and filling at least the top layers of the oceans with a rich “organic soup” containing all the building blocks of life.
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.
Factual Information Questions事实信息题
AThe experiment appeared to support the idea that organic compounds containing the building blocks of life would have formed spontaneously and in vast quantities on early Earth.
BThe experiment appeared to support the idea that only the top layers of the oceans on early Earth would have contained large quantities of the building blocks of life.
CThe experiment appeared to support the idea that all of the building blocks of life covered the planet before becoming contained in the top layers of early Earth’s oceans
DThe experiment appeared to support the idea that the”organic soup”of the building blocks of life caused organic compounds to form in the oceans.
3
The original Miller-Urey experiment used small glass flasks to simulate chemical conditions they thought represented those on early Earth. One flask was partially filled with water to represent the sea and heated to produce water vapor(water in the form of gas). Gaseous methane and ammonia were added and mixed with the water vapor to represent the atmosphere. These gases flowed into a second flask, where electric sparks provided energy for chemical reactions. Below this flask, the gas was cooled so that it could condense to represent rain and then was cycled back into the water flask. The water soon began to turn a murky brown, and a chemical analysis(performed after letting the experiment run for a week) showed that it contained many amino acids, the building blocks of life, and other organic molecules. Thus, the experiment appeared to support the idea that organic compounds would have formed spontaneously and in vast quantities on early Earth, covering our planet and filling at least the top layers of the oceans with a rich “organic soup” containing all the building blocks of life.
Which of the following can be inferred from paragraph 2 about the sparks in the Miller-Urey experiment?
Factual Information Questions事实信息题
AThey were used to cause gases to flow throughout the flask.
BThey were needed to keep water vapor in the second flask from becoming liquid water.
CThey reduced the number of chemical reactions that could have affected the outcome of the experiment.
DThey provided energy similar to the energy that would have been available on early Earth.
4
However, other scientists soon realized that the methane and ammonia gas mixture used by Miller and Urey probably was not representative of Earth’s early atmosphere and the significance of their results was called into question. That the relevance of the Miller- Urey experiment is an even hotter scientific controversy today demonstrates the way that science often progresses unsteadily.
According to paragraph 3, which of the following is true about the acceptance by scientists of the Miller-Urey experiment?
Factual Information Questions事实信息题
AThe experiment’s results were considered highly significant at the time it was conducted but have been called into question only recently.
BThe experiment’s results were questioned by other scientists at the time but have now gained wide acceptance.
CThe experiment’s results were questioned by some scientists at the time, and the experiment’s importance is still debated today.
DThe experiment’s results were once considered highly significant, and continuous scientific progress has confirmed and refined them.
5
The major issue concerns exactly what the early atmosphere was made of. When analyzed in more detail, the Miller-Urey experiment shows that hydrogen is a key ingredient needed to form large quantities of organic compounds. In contrast, oxygen in any form, even when tied up in carbon dioxide, tends to suppress the formation of organic compounds. Today, we assume that Earth’s early atmosphere was produced by volcanic outgassing(release of trapped gas). Outgassing may have occurred at a somewhat higher rate in the distant past than it does today, but we expect the gases to have been similar: largely water, carbon dioxide, nitrogen, and methane, along with sulfur-bearing gases and hydrogen. The water condensed to form the oceans, leaving these other gases as potential ingredients for the early atmosphere. If there was a lot of hydrogen relative to carbon dioxide, then the Miller-Urey results might still be valid, suggesting that our young planet did indeed have a rich organic soup. However, if there was little hydrogen, then the oxygen in carbon dioxide would have kept the amount of organic material far lower than the Miller-Urey experiment suggests.
According to paragraph 4, scientists today largely believe all of the following about out gassing EXCEPT:
Negative Factual Information Questions否定事实信息题
AThere may be less outgassing now than there was on early Earth.
BOutgassing on early Earth probably released gases very different from those released by outgassing today.
COceans on early Earth were probably formed by the condensation of water vapor from outgassing.
DOutgassing was the source of both hydrogen and carbon dioxide on early Earth.
6
Prior to 2005,it was generally assumed that hydrogen would have escaped from the early atmosphere at the same rate that it escapes today, which is rapid enough that little hydrogen would have been present at any time. However, new calculations, still subject to verification, suggest that conditions in Earth’s early upper atmosphere could have slowed the escape considerably, allowing hydrogen to make up as much as thirty percent of Earth’s early atmosphere. Until this issue is settled, we will not know whether spontaneous chemical reactions would have turned Earth’s surface and oceans into a rich organic laboratory or left them with only small amounts of locally produced organic compounds.
The word “verification” in the passage is closest in meaning to
Vocabulary Questions词汇题
Adebate
Bexplanation
Ccompletion
Dconfirmation
7
Prior to 2005,it was generally assumed that hydrogen would have escaped from the early atmosphere at the same rate that it escapes today, which is rapid enough that little hydrogen would have been present at any time. However, new calculations, still subject to verification, suggest that conditions in Earth’s early upper atmosphere could have slowed the escape considerably, allowing hydrogen to make up as much as thirty percent of Earth’s early atmosphere. Until this issue is settled, we will not know whether spontaneous chemical reactions would have turned Earth’s surface and oceans into a rich organic laboratory or left them with only small amounts of locally produced organic compounds.
According to paragraph 5, the issue about Earth’s atmosphere that remains to be settled is
Factual Information Questions事实信息题
Awhether the rate at which hydrogen escapes the atmosphere has slowed since 2005 when chemical reactions were measured
Bwhether the new calculations concerning the rate at which hydrogen would have escaped from Earth’s early upper atmosphere are correct
Cwhether very small amounts of locally produced organic compounds could have reproduced and spread across Earth’s atmosphere, surface, and oceans
Dwhether an atmosphere consisting of 30 percent hydrogen would have provided suitable conditions for the spontaneous formation of organic compounds
8
We know of at least two other potential sources of organic molecules on early Earth. The first of these potential sources is chemical reactions near deep-sea vents. As these undersea volcanoes heat the surrounding water, a variety of chemical reactions can occur between the water and the minerals released from the vents. These chemical reactions would have occurred spontaneously in the conditions thought to have prevailed in the early oceans, and they should have resulted in the production of the same types of organic molecules thought to have been necessary for the origin of life. The other additional source of organic molecules may have been material from space. Analysis of meteorites shows that they often contain organic molecules, including complex molecules such as amino acids.
In paragraph 6, the author discusses deep-sea vents and meteorites in the larger discussion of early Earth and the Miller-Urey experiment in order to
Rhetorical Purpose Questions修辞目的题
Aexplain why scientists have rejected the original theory of the sources of organic molecules and focused on other theories
Bestablish the sequence of events in Earth’s early history that allowed organic molecules to form
Csuggest that there may be other plausible explanations for the origins of life on Earth than the one scientists long considered
Demphasize the lack of agreement among scientists about the origins of life on Earth
9
The major issue concerns exactly what the early atmosphere was made of. When analyzed in more detail, the Miller-Urey experiment shows that hydrogen is a key ingredient needed to form large quantities of organic compounds. In contrast, oxygen in any form, even when tied up in carbon dioxide, tends to suppress the formation of organic compounds. Today, we assume that Earth’s early atmosphere was produced by volcanic outgassing(release of trapped gas). [■]Outgassing may have occurred at a somewhat higher rate in the distant past than it does today, but we expect the gases to have been similar: largely water, carbon dioxide, nitrogen, and methane, along with sulfur-bearing gases and hydrogen. [■]The water condensed to form the oceans, leaving these other gases as potential ingredients for the early atmosphere. If there was a lot of hydrogen relative to carbon dioxide, then the Miller-Urey results might still be valid, suggesting that our young planet did indeed have a rich organic soup. [■]However, if there was little hydrogen, then the oxygen in carbon dioxide would have kept the amount of organic material far lower than the Miller-Urey experiment suggests.[■]
Look at the four squaresthat indicate where the following sentence could be added to the passage
The key question, therefore, is what the ratio of hydrogen to carbon dioxide would have been.
Insert Text Questions句子插入题
Where would the sentence best fit？Click on a square sentence to the passage.
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In the 1950s, the Miller-Urey experiment attempted to explain how organic molecules could have been created on early Earth.
Prose Summary Questions概要小结题
Select 3 answers
AThe Miller-Urey experiment employed two connected flasks, one with water and one filled with various gases, which produced chemical reactions along with electric sparks.
BAlthough the Miller-Urey experiment may not have accurately represented Earth’s early atmosphere it did show the key role of hydrogen in forming organic compounds.
CIt is not yet known if early Earth’s upper atmosphere would have kept hydrogen concentrations high enough for spontaneous chemical reactions to have resulted in vast quantities of organic material.
DThe results of the Miller-Urey experiment are even more controversial today because of the growing amount of evidence suggesting that the results are not valid.
EA secondary goal of the Miller-Urey experiment was to learn the impact that oxygen in carbon dioxide would have had on the amount of organic matter produced on early Earth.
FOrganic molecules may have arrived on material from space and could also have resulted from chemical reactions that are likely to have occurred near deep-sea vents in the oceans of early Earth.
