新托福考试必备：新托福TPO（1-24）听力原文文本TPO10

　　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 50 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 muffle lay. The name Ambulocetus natans
　　comes from Latin of course, and means “walking whale that swims”. It clearly
　　had four limbs that couldn’t 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 a 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 Power San and couldn’t have supported its
　　weight. But it clearly shows Basilosaurus an evolution from land creature. So
　　that’s a giant step in the right direction. Even better, it established
　　Ambulocetus natans as a clear link between the wolf-like creature and
　　Basilosaurus. Now these discoveries don’t completely solve the mystery. I
　　mean, Ambulocetus natans is a mammal that shows a sort of bridge between
　　walking on land and swimming. But it also is very different from the whales
　　who know today. So really we are working 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 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, it came as a bit of a shock. 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. Unfortunately this evolution
　　of 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 whales are descendent not from
　　hippos but from that distant wolf-like creatures. So we have contradictory
　　evidences. 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,
　　not both.
　　Err… 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 blue whale and other baleen whales, we
　　assumed that they be closely related. But recent DNA studies suggest that
　　that’s not the case at all. The sperm whale was actually closely related to the
　　baleen whale, and it’s only distantly related to the toothed-whales. So that’s
　　the real surprise to all of us.