托福official10听力lecture1 Whales原文解析+翻译音频

[00:00.00]]Narrator: Listen to part of a lecture in a marine biology class.
[00:10.50]FEMALE PROFESSOR: We know whales are mammals and that they evolved from land creatures. [00:15.54]So the mystery is figuring out how they became ocean dwellers. [00:20.45]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.[00:32.37]Fortunately, some recent fossil discoveries have made the picture a little bit clearer.
[00:38.16]For example, a few years back, in Pakistan, they found the skull of a wolflike creature; [00:44.70]it was about 50 million years old. [00:46.94]Scientists’d seen this wolflike creature before, but this skull was different; [00:52.49]the ear area of the skull had characteristics seen only in aquatic mammals, specifically whales. [01:00.00]Uh, then—also in Pakistan—they found the fossil of another creature, which we call Ambulocetus natans.
[01:10.14]That’s a mouthful, eh? [01:13.06]The name Ambulocetus natans comes from Latin, of course, and means “walking whale that swims”; [01:21.48]it clearly had four limbs that could have been used for walking. [01:25.84]It also had a long, thin tail typical of mammals, something we don’t see in today’s whales. [01:32.92]But: it also had a long skeletal structure, [01:36.82]and that long skeletal structure suggests that it was aquatic …
[01:41.69]And very recently, in Egypt, they found a skeleton of Basilosaurus. [01:48.68]Basilosaurus was a creature that we had already known about for over 100 years, [01:54.29]and it has been linked to modern whales because of its long whalelike body, [01:58.94]but this new fossil find showed a full set of leg bones, something we didn’t have before.
[02:05.62]The legs were too small to be useful—[02:07.92]they weren’t even connected to its pelvis and couldn’t have supported its weight—[02:12.21]but it clearly shows Basilosaurus’ evolution from land creatures, [02:16.52]so that’s a giant step in the right direction. [02:19.23]Even better, it establishes Ambulocetus as a clear link between the wolflike creature and Basilosaurus.
[02:26.81]Now, these discoveries don’t completely solve the mystery. [02:30.74]I mean, Ambulocetus is a mammal that shows a sort of bridge between walking on land and swimming, [02:37.20]but it also is very different from the whales we know today. [02:41.10]So really, we are working with just a few pieces of a big puzzle.
[02:47.01]Uh, a related debate involves some recent DNA studies. [02:53.38]Remember, DNA is the genetic code for any organism, [02:56.82]and when the DNA from two different species is similar, it suggests that those two species are related … [03:03.35]And when we compared some whale DNA with DNA from some other species, we got quite a surprise: [03:10.88]the DNA suggests that whales are descendants of the hippopotamus![03:15.84]Yes, the hippopotamus!
[03:17.56]Well, that came as a bit of a shock, [03:19.24]I mean, that a four-legged land and river dweller could be the evolutionary source of a completely aquatic creature up to 25 times its size? [03:28.69]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, [03:41.81]and, of course, as I mentioned, that whales are descended not from hippos but from that distant wolflike creature.
[03:48.77]So we have contradictory evidence. [03:51.63]And more research might just raise more questions and create more controversies. [03:56.70]At any rate, we have a choice: [03:58.72]we can believe the molecular data, the DNA, or we can believe the skeleton trail, but, unfortunately, probably not both …
[04:08.49]Uh, and there have been some other interesting findings from DNA research: [04:14.88]For a long time, we assumed that all whales that had teeth, including sperm whales and killer whales, were closely related to one another. [04:23.45]And the same for the toothless whales, like the blue whale and other baleen whales—we assumed that they’d be closely related.
[04:31.13]But recent DNA studies suggest that that’s not the case at all; [04:34.99]the sperm whale is actually closely related to the baleen whale, and it’s only distantly related to the toothed whales. [04:42.57]So that was a real surprise to all of us.