附近学电脑的 2017年12syllabus月宝贝翻译成英文16日托福阅读考试预测题目
Title：The Cambrian Explosion
僻静 The earliest fossil evidence for eukaryotes complex organisms who cells contain a distinct nucleus dates to only about 1.2 billion years ago. The fossil record suggests that animal evolution progresd slowly, with relatively little change en between fossils from 1.2 billion years ago and tho 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 incts.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. Incts, crabs, and spiders belong to the phylum Arthropoda, which contains animals with body features such as jointed legs, an external skeleton, and gmented bodi
es. Classifying animals into phyla is an ongoing project for biologists, but modern animals appear to compri about 30 different phyla, each reprenting a different body plan.
Remarkably, nearly all of the 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 prents 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 atmosph
ere may have remained well below its prent level until about the time of the Cambrian explosion. Thus, the rapid diversification in animal life may have occurred at least in part becau oxygen reached a critical level for the survival of larger and more energy-intensive life forms.
A cond 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 ries of episodes in which Earth froze over before the Cambrian began. The extreme climate conditions of the episodes eliminated many species, leaving a wide array of ecological niches available into which new species could rapidly evolve when climate conditions ead at the beginning of the Cambrian.
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A fourth factor may have been the abnce 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 themlves 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 tho other body plans from the body plans that evolved in the Cambrian. Or perhaps the various body forms that aro during the Cambrian explosion reprent the full range of forms possible given the basic genetic resources that characterize all Earth's organisms. In any ca, 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.
经典英语故事
2. The word ongoing in the passage is clost in meaning to
A. uncertain
B. full-time
C. important
linked in
杜特蒂D. continuing
three rivers 3. In paragraph 2, why does the author provide the information that Arthropoda reprents animals with features like jointed legs, an external skeleton, and gmented body parts.
A. To indicate basic physical differences among incts, crabs, and spiders
