纠错
置顶

The Cambrian Explosion

纠错

Paragraph 9 suggests all of the following possible explanations for the uniqueness of the Cambrian explosion EXCEPT

Click on an oval to select your answer. To choose a different answer,

click one different oval.

  • 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
显示答案
正确答案: C

我的笔记 编辑笔记

  • 原文
  • 译文
  • 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.


  • -
  • 官方解析
  • 网友贡献解析
  • 标签
    0 感谢 不懂
    解析

    答案:C

    题型:否定事实信息题

    解析:

    根据题干信息“possible explanations for the uniqueness of the Cambrian explosion”,选项A对应本段倒数第三句“ it was not possible to evolve into those other body plans from the body plans that evolved in the Cambrian.”

    选项B对应本段冒号后的句子“once predators were efficient and widespread, it may have been virtually impossible for animals with entirely new body forms to find an environmental niche...”;

    选项C“the number of habitats”信息未提及;

    选项D对应本段倒数第二句“Or perhaps the various body forms that arose during the Cambrian explosion represent the full range of forms possible given the basic genetic resources...”

题目讨论

如果对题目有疑问,欢迎来提出你的问题,热心的小伙伴会帮你解答。

最新提问