[机经题库答案] 托福阅读生物类真题The Cambrian Explosion原文+题目汇总


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发表于 2017-5-9 17:49 |显示全部楼层
Title：The Cambrian Explosion
The earliest fossil evidence for eukaryotes complex organisms  whose cells contain a distinct  nucleus dates to only about 1.2 billion years ago. The fossil record suggests  that animal evolution progressed slowly, with relatively little change  seen between fossils from 1.2  billion years ago and those from a half-billion years later. But then  something quite dramatic happened  as can be judged by the many different animal groups that suddenly appear in the fossil record.  

Biologists classify animals according to their basic body plans.  For example, the basic body plan  shared by mammals and reptiles is fundamentally different from that of  insects. Animals are grouped by  body plan into what biologists call phyla. Mammals and reptiles both belong to the single phylum Chordata,  which includes animals with internal skeletons. Insects, crabs, and spiders belong to the phylum Arthropoda, which contains animals with body features such as jointed legs, an  external skeleton, and segmented bodies. Classifying animals into phyla is an ongoing project for biologists, but modern animals appear to  comprise about 30 different phyla,  each representing a different body plan.

Remarkably, nearly all of these different body plans, plus a few  others that have gone extinct,  make their first known appearance in the geological record during a period  spanning only about 40 million  years less than about 1 percent of Earth's history. This remarkable flowering of animal diversity appears  to have begun about 545 million years ago, which corresponds to the start  of the Cambrian period. Hence it is called the Cambrian explosion.  

The fact that the Cambrian explosion marks the only major  diversification of body plans in  the geological record presents us with two important and related questions: Why, so  long after the origin of eukaryotes, did the pace of evolution  suddenly accelerate dramatically at  the beginning of the Cambrian, and why hasn't there been another period of similarly explosive diversification  since then  

We can identify at least four  factors that might have contributed to the Cambrian explosion. First, the oxygen level in our atmosphere may have  remained well below its present level  until about the time of the Cambrian explosion. Thus, the rapid  diversification in animal life may  have occurred at least in part because oxygen reached a critical level for  the survival of larger and more  energy-intensive life forms.  

A second factor that may have been important was the evolution of  genetic complexity. As eukaryotes evolved, they developed  more and more genetic variation in their DNA. Some scientists believe that the Cambrian explosion marks the  point at which organisms developed  certain kinds of genes (homeobox genes) that control body form and that could  be combined in different ways, allowing the evolution of a great diversity  of forms over time.  

A third factor may have been climate change. Geological evidence  points to a series of episodes in  which Earth froze over before the Cambrian began. The extreme climate conditions of these episodes  eliminated many species, leaving a wide array of ecological niches available into which new species could  rapidly evolve when climate conditions eased  at the beginning of the Cambrian.

A fourth factor may have been the absence of efficient predators. Early predatory animals were probably  not very sophisticated, so some evolving animals that later might have been  eliminated by predation were given a chance to survive, making the beginning  of the Cambrian period a window of opportunity for many different adaptations  to establish themselves in the environment.
This last idea may partly explain why no similar explosion of  diversity has taken place since the  Cambrian: once predators were efficient and widespread, it may have been virtually impossible for animals with  entirely new body forms to find an environmental niche in which they could escape predation. Or  it may be that while more body plans may have been possible at some early point in evolution, it was not  possible to evolve into those other body  plans from the body plans that evolved in the Cambrian. Or perhaps the  various body forms that arose  during the Cambrian explosion represent the full range of forms possible  given the basic genetic resources  that characterize all Earth's organisms. In any case, no fundamentally new body forms have  emerged since the Cambrian explosion.  

1. Paragraph  1 implies which of the following about evolutionary change
A.       Eukaryotes  have a very slow rate of evolution.  
C.         Evolution has not always proceeded at the  same rate.  
2. The  word ongoing in the passage is closest in  meaning to
A.       uncertain  
B.         full-time  
C.         important  
D.       continuing  

3. In  paragraph 2, why does the author provide the information that Arthropoda represents animals with features like jointed legs, an external skeleton,  and segmented body parts.
A.        To indicate  basic physical differences among insects, crabs, and spiders  
B.         To illustrate  the types of physical characteristics considered when classifying animals  
C.         To show  the complexity of features that have evolved in organisms  
D.        To demonstrate  that some phyla include a wider range of body plans than others do  

4. The  phrase corresponds to in the passage is  closest in meaning to
A.       causes  
B.         occurs before  
C.         differs from  
D.       matches

5. According  to paragraphs 3 and 4, what was remarkable about the Cambrian explosion
A.        That the evolution of species occurred so  soon after the first eukaryotes appeared
B.         That most of the known animal types appeared  in a relatively short period in history  
C.         That many of the animal types that appeared  in the period have survived until today  
D.        That the pace of evolution slowed before it accelerated  

6. The  function of the two questions in paragraph 4 is to
A.        recognize two common questions that cannot be  addressed within the passage
B.         present the two different points of view  contrasted in the passage  
C.         provide important objections to the central  idea of the passage  
D.        indicate two important questions that will be  explored in the passage  

7. Paragraph  5 implies which of the following about oxygen
A.        It was not essential for the life forms that  appeared before the Cambrian period.  
B.         It has remained at relatively the same level  since the beginning of the fossil record.  
C.         Its changes in levels are associated with  animal extinctions.  
D.        Its levels before the Cambrian period were  too low for large animals to survive.  

8. The  word complexity in the passage is closest  in meaning to
A.       sophistication  
B.         adaptation  
C.         improvement  
D.       variation

9. The  word allowing in the passage is closest  in meaning to
A.       resulting  in  
B.         making possible  
C.         preceding
D.       spreading
10. According  to paragraph 7, all of the following occurred before the Cambrian began EXCEPT:
A.       Almost  all of Earth froze over.  
B.         New ecological niches were filled by new species.  
C.         A series of extreme climate episodes occurred.  
D.       Many  species became extinct.  

A.       Predators  at the beginning of the Cambrian period had to become more sophisticated in order to survive in environments with  newly adapted animals.  
B.         New adaptations had an opportunity to survive  at the beginning of the Cambrian period  because early predators were not yet sophisticated enough to eliminate the  animals with these adaptations.  
C.         Early predatory animals lacked sophisticated  adaptations because they did not develop them  during a window of opportunity in the early Cambrian period.  
D.       Early  predators had an opportunity at the beginning of the Cambrian period to  remove new adaptations before they  established themselves in the environment.  

12. Paragraph  9 suggests all of the following possible explanations for the uniqueness of the Cambrian explosion EXCEPT
A.        the inability of later animals to evolve body  plans different from those that appeared during  the Cambrian period  
B.         the post-Cambrian appearance of efficient  predators occupying nearly every environmental  niche  
C.         the decline in the number of habitats having  sufficient resources to support the rapid  evolution of new species  
D.       the limited  range of genetically possible body types  

After  all, evolution of body structure can act only on the structure that already exists. Where would the sentence best fit?
Click  on a square to add the sentence to the passage.
A.       Biologists  want to find out why the rapid diversification of body forms did not happen soon after the appearance of eukaryotes  around 1.2 billion years ago.  
B.         Biologists classify animals according to  their body plans into phyla such as Chordata,  animals with a backbone, and Arthropoda, animals with segmented exoskeletons.  
C.         The Cambrian explosion was a unique episode  in the history of evolution that produced  nearly all of the 30 or so animal body plans that have ever been seen.  
D.       The  Cambrian explosion may have been aided by genes that could yield a variety of body forms and the inability of early  predators to eliminate the new forms.  
E.         Once predators became efficient after the  Cambrian explosion, they were able to eliminate  any animals that began to evolve a new body  plan.  
F.         At the beginning of the Cambrian, an increase  in oxygen needed for animal growth and the  return of a hospitable climate may have contributed to the Cambrian explosion.
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