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The Cambrian Explosion

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Directions: An introductory sentence for a brief summary of the passage is provided below. Complete the summary by selecting the THREE answer choices that express the most important ideas in the passage. Some sentences do not belong in the summary because they express ideas that are not presented in the passage or are minor ideas in the passage.

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At least four factors have been identified to explain the explosive diversification in Cambrian period.

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正确答案: C D F
  • 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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  • 原文
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  • 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.


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    1 感谢 不懂
    解析

    段落大意:

    第一段:最早化石证据表明动物进化缓慢(追溯到大约12亿年前)直到后来的化石记录中出现不同的动物群体--为后面讲述the Cambrian explosion背景做铺垫

    第二段:生物学家根据生物身体构造对动物进行划分,引出概念phyla

    第三段:Cambrian explosion的来源背景介绍

    第四段:提出两个问题:1)为什么在Cambrian时期进化速度加快2)为什么其它时期没有出现Cambrian explosion

    第五至八段:引起Cambrian explosion的因素:氧气含量;基因复杂化;气候变化;捕食者缺乏;并展开对这些因素进行阐述

    第九段:解释第四段提出的第二个问题(原因:1)后来的predators变得复杂起来,动物多样性进化受影响2)寒武纪进化出来的body plan很难再进化3)寒武纪时期包含地球上所有的生物基因,后期再没有出现新的基因)

    答案:C D F 

    题型:文章内容小结题

    解析:

    选项A:结合第一段及全文结构,本文主要在讨论为什么545million years ago开始迅速发展,而不是在讲为什么1.2million years ago没有迅速发生

    选项B:对应第二段内容,但属于背景信息,与文章主题关系弱;

    选项C:对应第二段末及第三段内容;

    选项D:对应第六、八段内容;

    选项E对应第九段首句,但与本题主题句信息不符

    选项F:对应第五段和第八段。

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