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.

　　B. The fossil record of evolutionary change is incomplete for the first half-billion years of animal evolution.

　　C. Evolution has not always proceeded at the same rate.

　　D. Evolutionary rates of change in animals were slowing down considerably before a dramatic reversal happened 1.2 billion years ago.

　　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 accelered

　　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.

　　11. 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.

　　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

　　D. the limited range of genetically possible body types

　　13. Look at the four squares that indicate where the following sentence could be added to the passage.

　　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