Official 10 Set 2

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Whales

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What is the lecture mainly about?
  • A. Recent fossil evidence connecting whales and the hippopotamus

  • B. Difficulties in the determining the evolutionary history of whales

  • C. Similarities among ancient ancestors of whales

  • D. Similarities between whales and other modern-day animals

显示答案 正确答案: B

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    NARRATOR:Listen to part of a lecture in a Marine Biology Class.

    FEMALE PROFESSOR:We know whales are mammals and that they evolved from land creatures.So the mystery is figuring out how they became ocean dwellers.Because until recently there was no fossil record of what we call "the missing link", that is evidence of species that show the transition between land-dwelling mammals and today's whales.Fortunately, some recent fossil discoveries have made the picture a little bit clearer.

    For example, a few years back in Pakistan, they found a skull of a wolf-like creature.It's about fifty million years old.Scientists had seen this wolf-like creature before, but this skull was different.The ear area of the skull had characteristics seen only in aquatic mammals, specifically whales.

    Err, well, then also in Pakistan they found a fossil of another creature, which we call Ambulocetus natans.That's a mouthful,eh?The name Ambulocetus natans comes from Latin of course, and means "walking whale that swims".It clearly had four limbs that could have been used for walking.It also had a long thin tail, typical of mammals, something we don't see in today's whales.But it also had a long skeletal structure, and that long skeletal structure suggests that it was aquatic.

    And very recently, in Egypt, they found skeleton of Basilosaurus.Basilosaurus was a creature that we've already known about for over a hundred years.And it has been linked to modern whales because of its long whale-like body.But this new fossil find showed a full set of leg bones, something we didn't have before.The legs were too small to be useful.They weren't even connected to its pelvis and couldn't have supported its weight.But it clearly shows Basilosaurus’s evolution from land creatures.So that's a giant step in the right direction.Even better, it establishes Ambulocetus as a clear link between the wolf-like creature and Basilosaurus.

    Now these discoveries don't completely solve the mystery.I mean, Ambulocetus is a mammal that shows a sort of bridge between walking on land and swimming.But it also is very different from the whales we know today.So, really we are working with just a few pieces of a big puzzle.

    Emm, a related debate involved some recent DNA studies.Remember, DNA is the genetic code for any organism, and when the DNA from two different species is similar, it suggests that those two species are related.And when we compared some whale's DNA with DNA from some other species, we got quite a surprise.The DNA suggests that whales are descendants of the hippopotamus.Yes the hippopotamus! Well, that came as a bit of shock.I mean that a four-legged land and river dweller could be the evolutionary source of a completely aquatic creature up to twenty five times its size.Unfortunately, this revelation about the hippopotamus apparently contradicts the fossil record, which suggests that the hippopotamus is only a very distant relative of the whale, not an ancestor.And of course as I mentioned that the whales are descendant not from hippos but from that distant wolf-like creature.

    So we have contradictory evidence.And more research might just raise more questions and create more controversies.At any rate, we have a choice.We can believe the molecular data, the DNA, or we can believe the skeleton trail.But unfortunately, probably not both.

    Um, and there have been some other interesting findings from DNA research.For a long time, we assumed that all whales that had teeth, including sperm whales and killer whales were closely related to one another, and the same for the toothless whales, like the bule whale and other baleen whales.We assumed that they be closely related.But recent DNA studies suggest that's not the case at all.The sperm whale is actually closely related to the baleen whale, and it's only distantly related to the toothed-whales.So that was the real surprise to all of us.

  • 旁白:听一段关于海洋生物学的讲座。

    教授:我们知道鲸鱼是哺乳动物,而且他们从陆地生物进化而来。所以需要解开的谜团是他们怎样变成海洋生物的。因为至今为止,我们还没有发现所谓关于二者联系消失的化石证据。这是很重要的证据来证明陆上生物到鲸鱼的进化。幸运的是最近一些化石证据能使这个“进化图”稍显清晰了。

    比如几年前在巴基斯坦,发现了一具像狼一样的头盖骨距今五千万年了。科学家以为这和以前的化石一样,但事实上,它有所不同。在它耳朵区域的头骨是有水声哺乳动物的特点,特别像鲸。

    而且也在巴基斯坦,我们找到了另一具生物化石,称之为陆行鲸。那很~~~~。陆行鲸这个名字来自于拉丁语,意为能行走的游泳鲸。它有四肢可以行走也有一个和哺乳动物一样的长细尾巴,尽管现代鲸没有了。而且还有一副修长骨架结构,而且这个骨架结构表明它是水生生物。

    而且最近在埃及,他们发现了一具骨架,称之为龙王鲸。龙王鲸是我们已经知道百年的生物了而且他们也与鲸有关系,因为他们有着修长的鲸一样的身体。但这具新化石有4 个腿骨,这是之前所没有过的。它们腿骨太小而无用而且没有链接到骨盆,也不能支撑那么大的躯体重量。但能很清楚的证明龙王鲸是由陆地生物进化而来。所以这是在正确研究方向上的重大进步。更有甚者,这指明了陆行鲸这样的狼型生物与龙王鲸之间的关系。

    但这发现不能完全将谜团解开。我的意思是,尽管陆行鲸已经指明一个路上行走动物和水中游泳动物的清晰联系但陆行鲸与今天的鲸鱼仍有很大不同。所以我们在几个大困惑上兜圈子。

    有一个包涵最近关于DNA研究的话题。记住,DNA是有关任何生物的基因密码,当两种不同种类的生物DNA相似是,这说明二者是有一定关联的。当我们拿鲸的DNA与其他生物比对时,我们惊奇的发现鲸鱼竟然是河马的后代!对,就是河马,这一定让你们大吃一惊了。我是说,一个在河里生活有四条腿的陆生生物能成为比自己大25倍的水生生物的进化源么?很不幸,河马进化理论与化石是有冲突的。我们只能说,河马是鲸鱼的一个远亲,并不是祖先。当然我提到了,鲸并不是河马的后代,而是由那个久远的狼型化石的后代。

    所以我们有了个相矛盾的证据随着更多的研究衍生出更多的问题,继而得到更多的辩驳总之,我们可以选择我们可以相信分子数据—DNA 理论,也可以相信骨架结构踪迹理论但事实上,都不是。

    有一些DNA 研究发现以前我们认为,有牙齿的鲸鱼,比如巨头鲸和虎鲸,他们是有一定联系的,没有牙齿的鲸鱼,比如蓝鲸和须鲸他们是有一定联系的但DNA 研究表明,根本就没这回事儿。现在DNA 表明,巨头鲸与须鲸有关联,而且仅仅与有齿鲸有遥远的关系所以这让我们都很惊奇。

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  • 本题对应音频:
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    音频1
    解析

    题型分类:主旨题
    原文定位:We know whales are mammals and that they evolved from land creatures. So the mystery is figuring out how they became ocean dwellers.
    选项分析:
    主旨题的答案一般在 lecture 的第一段就有提示。教授一开始就说 We know whales are mammals and that they evolved from land creatures. So the mystery is figuring out how they became ocean dwellers.而接下来整篇lecture 就讲述了对于鲸鱼从陆生动物变成海洋动物的过程的研究及遇到的问题,印证了鲸鱼的发展历史在目前很难得到准确结论这个主旨,B选项正确。

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