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  3. Over a century ago, Alfred Russell Wallace wrote that " we live in a zoologically impoverished world, from which all the hugest,

Over a century ago, Alfred Russell Wallace wrote that " we live in a zoologically impoverished world, from which all the hugest,

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问题     Over a century ago, Alfred Russell Wallace wrote that " we live in a zoologically impoverished world, from which all the hugest, and fiercest, and strangest forms have recently disappeared...". Researchers seeking to explain this "marvelous fact", as Wallace called it, fall into two camps, one invoking global climatic change and the other human hunting as the cause.
    Over the past few decades, the debate has become deadlocked, in part because most researchers have focused their attention on the America and northern Eurasia, where the extinction of the huge, fierce, and strange creatures, such as mammoths, and giant sloths, occurred between 12,500 and about 11,000 years ago. This was a time of rapid climatic change, but it was also when humans first arrived in these regions, making it difficult to discern causality.
    Australia provides the only separate, continent sized natural laboratory in which dramatic Quaternary extinctions occurred. It is thus of exceptional importance as a testing ground for extinction theories, but until now problems with dating have limited its potential. Miller et al. have now documented the extinction of the gigantic Australian bird Genyornis and so have broken new ground in dating megafaunal(大型动物的)extinction in Australia. At the same time, these authors have broken the current deadlock in the great megafaunal extinction debate.
    It has long been appreciated that the intensity of Quaternary extinctions varied greatly around the world. In the oceans, Africa, and southeast Asia, they were nonexistent or mild. Europe experienced moderate extinction rates, whereas the Americas, Australia, Madagascar, and many Oceanic islands suffered dramatic extinctions. North America lost 73% of all genera weighing more than 44 kg, but Australia suffered the most severely of all the continents, losing every terrestrial vertebrate species larger than a human, as well as many smaller mammals, reptiles, and flightless birds, the latter down to about a kilogram in weight. In all, about 60 vertebrate species were lost, including bizarre marsupials that resembled giant sloths, carnivorous kangaroos, and a terrestrial horned tortoise that approached the size of a Volkswagen Beetle car.
    Establishing just when this bizarre array of creatures last trod Australia’s outback has been a tortuous business, with many false leads and sites that are difficult to interpret.
    For decades, it was believed that the megafauna survived until close to the time of the glacial maximum, some 20,000 years ago, when temperatures were up to 9°C cooler than at present and the continent was extremely arid. Conditions were so extreme that trees virtually disappeared from the inland, and 40% of Australia was transformed into a vast active dune field.
In America and northern Eurasia, the direct reason for the megafaunal extinction is______.
选项 A、human’s hunting
B、climate’s change
C、impoverished land
D、in dispute
答案D
解析 由题干关键词megafaunal extinction定位到第一段第二句可知，引起美洲及北欧亚大陆大型动物灭绝的原因可能是全球气候的变化，也可能是人类的狩猎活动。故选D)“有争议的”。
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