文勇的新托福黄金&ccedil

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Unformatted text preview: 在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在 在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在 在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在 在在在在在在在在在在在在在在在在在在在在在在在在在在在 Heyerdahl 在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在在 Patrick Kirch 在在在在在在在在在在在在在在在在在在在在在在在在在在在在 在在在在在在在在在在在在 194 [email protected] 在在在在在在 www.liuwenyong.com 在在在在在在在在在在在在在在在,在在在在在在在在在在 THE CAMBRIAN EXPLOSION The geologic timescale is marked by significant geologic and biological events, including the origin of Earth about 4.6 billion years ago, the origin of life about 3.5 billion years ago, the origin of eukaryotic life­forms (living things that have cells with true nuclei) about 1.5 billion years ago, and the origin of animals about 0.6 billion years ago. The last event marks the beginning of the Cambrian period. Animals originated relatively late in the history of Earth – in only the last 10 percent of Earth’s history. During a geologically brief 100­million­year period, all modern animal groups (along with other animals that are now extinct) evolved. This rapid origin and diversification of animals is often referred to as “the Cambrian explosion.” Scientists have asked important questions about this explosion for more than a century. Why did it occur so late in the history of Earth? The origin of multicellular forms of life seems a relatively simple step compared to the origin of life itself. Why does the fossil record not document the series of evolutionary changes during the evolution of animals? Why did animal life evolve so quickly? Paleontologists continue to search the fossil record for answers to these questions. One interpretation regarding the absence of fossils during this important 100­million­year period is that early animals were soft bodied and simply did not fossilize. Fossilization of soft­bodied animals is less likely than fossilization of hard­bodied animals, but it does occur. Conditions that promote fossilization of soft­bodied animals include very rapid covering by sediments that create an environment that discourages decomposition. In fact, fossil beds containing soft­bodied animals have been known for many years. The Ediacara fossil formation, which contains the oldest known animal fossils, consists exclusively of soft­bodied forms. Although named after a site in Australia, the Ediac...
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This note was uploaded on 09/17/2013 for the course LANGUAGE 13DL208 taught by Professor Wang during the Fall '13 term at East China Normal University.

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