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
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.
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。
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.
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。