变道手势新托福TPO5阅读原文(三):The Cambrian Explosion
TPO-5-3：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 life-forms (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 100-million-year 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 ems a relatively simple step compared to the origin of life itlf. Why does the fossil recor金牛座和狮子座
d not document the ries of evolutionary changes during the evolution of animals? Why did animal life evolve so quickly? Paleontologists continue to arch the fossil record for answers to the questions.
One interpretation regarding the abnce of fossils during this important 100-million-year period is that early animals were soft bodied and simply did not fossilize. Fossilization of soft-bodied animals is less likely than fossilization of hard-bodied animals, but it does occur. Conditions that promote fossilization of soft-bodied animals include very rapid covering by diments that create an environment that discourages decomposition. In fact, fossil beds containing soft-bodied animals have been known for many years.
党内最高处分下载mv The Ediacara fossil formation, which contains the oldest known animal fossils, consists exclusively of soft-bodied forms. Although named after a site in Australia, the Ediacara formation is worldwide in distribution and dates to Precambrian times. This 700-million-year-old formation gives few clues to the origins of modern animals, however, becau paleontologists believe it reprents an evolutionary experiment that failed. It contains no ancestors of modern animal groups.数学之神
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段位漂移 A slightly younger fossil formation containing animal remains is the Tommotian formation, named after a locale in Russia. It dates to the very early Cambrian period, and it also contains only soft-bodied forms. At one time, the animals prent in the fossil beds were assigned to various modern animal groups, but most paleontologists now agree that all Tommotian fossils reprent unique body forms that aro in the early Cambrian period and disappeared before the end of the period, leaving no descendants in modern animal groups.
A third fossil formation containing both soft-bodied and hard-bodied animals provides evidence of the result of the Cambrian explosion. This fossil formation, called the Burgess Shale, is in Yoho National Park in the Canadian Rocky Mountains of British Columbia. Shortly after the Cambrian explosion, mud slides rapidly buried thousands of marine animals under conditions that favored fossilization. The fossil beds provide evidence of about 32 modern animal groups, plus about 20 other animal body forms that are so different from any modern animals that they cannot be assigned to any one of the modern groups. The unassignable animals include a large swimming predator called A
nomalocaris and a soft-bodied animal called Wiwaxia, which ate detritus or algae. The Burgess Shale formation also has fossils of many extinct reprentatives of modern animal groups. For example, a well-known Burgess Shale animal called Sidneyia is a reprentative of a previously unknown group of arthropods (a category of animals that includes incts, spiders, mites, and crabs).
