Official 39 Passage 2

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The Extinction of Moa

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Paragraph 1 supports which of the following inferences about mammal evolution?

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  • A
    Bats and seals most probably originated in New Zealand and then migrated to other parts of the world.
  • B
    Most mammal species were already present in Gondwanaland before it broke up, between 80 to 85 million years ago.
  • C
    No mammals that evolved after New Zealand separated from Gondwanaland were able to migrate to New Zealand.
  • D
    No mammals evolved from the animals that originally inhabited New Zealand.
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正确答案: D

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  • Between 80 and 85 million years ago, Gondwanaland, a giant continent made up of what today is Africa, Antarctica, Australia, and South America, broke up, thus causing what is now New Zealand to become separated from the larger landmass. After the separation, any creature unable to cross a considerable distance of ocean could not migrate to New Zealand. Snakes and most mammals evolved after the separation. Thus there are no New Zealand snakes, and bats, which flew there, and seals, which swam there, were the only mammals on New Zealand when Polynesian settlers (the Maori) arrived there about a thousand years ago.



    When the Maori arrived in New Zealand, they encountered birds that had been evolving for 80 million years without the presence of mammalian predators. The most striking of these animals must have been moa. Now extinct, moa were gigantic wingless birds that stood as much as 10 feet (3 meters) tall and weighed as much as 550 pounds (250 kilograms). They are known from a diverse array of remains including eggshells, eggs, a few mummified carcasses, vast numbers of bones, and some older fossilized bone. The species of moa that are currently recognized occupied ecological niches customarily filled elsewhere by large mammalian browsing herbivores. They may have had relatively low reproductive rates; apparently, they usually laid only one egg at a time.



    It seems possible that when Captain James Cook first visited New Zealand in 1769, moa (or at least one of the moa species) may have still survived in the remote areas in the western part of New Zealand's South Island. If so, these individuals would have been the last of their kind. Climatic conditions in New Zealand appear to have been relatively stable over the period during which moa became extinct. Different factors could have worked in concert to account for their abrupt disappearance.



    Vegetation was considerably altered by the Maori occupation of New Zealand, a change not easily explained by climate variation or other possible factors. Forest and shrubland burning appears to have reduced the prime habitat of many moa species. However, the main forest burning started around 700 years ago, after what current archaeological evidence indicates was the most intensive stage of moa hunting. While there appears to have been extensive burning on the east side of New Zealand's South Island, large forest tracts remained in the most southern part of the island. Because major habitat destruction seems to have occurred after moa populations already were depleted, and because some habitat that could have sheltered moa populations remained, it would seem that other factors were also at work in the extinction of these birds.



    For South Island, human predation appears to have been a significant factor in the depletion of the population of moa. At one excavated Maori site, moa remains filled six railway cars. The density of Maori settlements and artifacts increased substantially at the time of the most intensive moa hunting (900 to 600 years ago). This period was followed by a time of decline in the Maori population and a societal transition to smaller, less numerous settlements. The apparent decline fits the pattern expected as a consequence of the Maori's overexploitation of moa.



    Finally, the Maori introduced the Polynesian rat and the dog to New Zealand. The actions of these potential nest predators could have reduced moa populations without leaving much direct evidence. The Maori may have also inadvertently brought pests and disease organisms in fowls, which could have crossed over to eradicate moa populations. The possibility of analyzing ancient DNA to identify past diseases of extinct animals is being explored. However, evidence of such diseases is difficult to determine directly from paleoecological or archaeological remains. For these reasons, it is hard to determine the likelihood that introduced disease organisms were a cause of the decline of moa, but they are potentially significant.



    While the last of these possible causes remains speculative, definite clues exist for the action of the first two causes. The story of moa species and their demise raises ecological issues on the vulnerability of species to human-caused changes-including altered vegetative cover of the landscape, change in the physical environment, and modification of the flora and fauna of a region by eliminating some species and introducing others.


  • 在八千万年至八千五百万年前,冈瓦纳大陆——一个由现如今的非洲、南极洲、大洋洲和南美洲组成的巨大大陆——解体了,导致现在的新西兰从大陆上分离出去。 在此之后,任何不能游过中间这片海洋的动物都没法移居到新西兰。 蛇和大多数哺乳动物都是在这次分离之后演化的。 因此新西兰没有蛇,飞过去的蝙蝠和游过去的海豹是新西兰上仅有的哺乳动物,这种状况一直持续到波利尼西亚人(毛利人)一千年前抵达那里之前。

    当毛利人到达新西兰的时候,他们遇到了那些进化了八千万年而从没遇到过哺乳动物捕食者的鸟。 这其中最引人注意的非恐鸟莫属。 恐鸟是一种非常高大、没有翅膀的鸟,站立起来可以达到10英尺(3米)高,体重可以达到550磅(250千克),它们现在已经灭绝了。 它们因为各种各样的遗迹而被世人知晓,这些遗迹包括蛋壳、蛋、一些木乃伊化了的尸体、大量的骨头和一些古老的化石化了的骨头。 恐鸟在新西兰生态圈的位置等同于其他地方的大型哺乳类食草动物。 它们繁殖率相对较低,很显然,它们通常一次只生一个蛋。

    当库克船长1769年第一次抵达新西兰的时候,恐鸟(或者至少是和恐鸟一个类别的鸟)可能仍然存活于新西兰南岛的西部边远地区。 如果真的是这样的话,这些鸟已经是这类鸟的最后一批了。 在恐鸟灭绝期间,新西兰的天气状况似乎是比较平稳的。 不同的因素共同导致了恐鸟的突然消失。

    在毛利人占领了新西兰之后,植被就被大幅度地改变了,这种变化是不能简单地用气候变化或其他可能的因素来解释的。 对恐鸟类的动物而言,森林和灌木丛的燃烧使得它们的主要栖息地范围缩小了。 然而,主要的森林大火开始于700年前,在此之前是当代考古证据表明的捕猎恐鸟最为密集的时期。 新西兰的南岛的东部地区发生了大范围的燃烧,但岛屿的南部仍保留有大面积的森林。 因为主要栖息地的破坏是在恐鸟数量已经大量减少之后才发生的,并且仍有一些栖息地保存了下来,导致这些恐鸟灭绝的原因似乎还有其他因素在起作用。

    在南岛,人类的捕食是恐鸟数量大量减少的重要因素。 在一个发掘出土的毛利人遗址,恐鸟遗骸装满了六节火车车厢。 毛利人口密度和人工制品的大量增长恰好是在大量猎杀恐鸟的时期(900至600年前)。 这段时间之后,毛利人口数量减少,聚居区变得更小,人数也更少了。 这一变化正是毛利人过量捕杀恐鸟的后果。

    最后,毛利人将波利尼西亚鼠和狗引进了新西兰。 这些潜在的巣掠食者可能导致了恐鸟数量的减少而没有留下什么直接的证据。 毛利人也可能不经意间将害虫和致病有机体带给禽类,这也可能相互传染从而导致了恐鸟的灭绝。 通过研究灭绝了的动物的DNA来判断它曾经得过的疾病的办法已经被探讨过了。 但是,这些疾病的证据很难直接从古生物学和考古学遗迹上得到。 因为这些原因,很难判定致病有机体的引入是导致恐鸟数量下降的原因,但是这的确是个潜在的重要因素。

    尽管这些原因中的最后一个仍然是个推断,前两个原因似乎是有些道理的。 恐鸟灭绝的故事表明,人类活动造成的生态环境的改变使得物种变得非常脆弱,这些改变包括自然环境的改变、通过消灭一些物种并引入其他物种而造成的一个地区动植物的改变。
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    题型分类:事实信息题

    题干分析:关键词:mammal evolution

    原文定位:定位:第一段倒数第二句

    选项分析:根据定位信息Snakes and most mammals evolved after the separation.”说到蛇类和哺乳动物都是自从新西兰分裂以后才开始进化的。根据定位句前后两句的内容,大意说蛇类和哺乳动物都是自从新西兰分裂以后才开始进化的。所以在一千年前毛利人到来的时候,新西兰的哺乳动物只有后来迁移过去的蝙蝠和海豹。所以新西兰的哺乳动物没有本土的,选项D符合题意。

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