TPO 10 Lecture 1　Marine Biology

　　Narrator: Listen to part of a lecture in a Marine Biology Class

　　Professor:
　　We know whales are mammals and that they evolved from land creatures. Sothe mystery is figuring out how they became ocean dwellers. Because untilrecently there was no fossil record of what we call “the missing link”- that isevidence of species that show the transition between land-dwelling mammalsand today’s whales. Fortunately, some recent fossil discoveries have made thepicture a little bit clearer. For example, a few years back in Pakistan, theyfound a skull of a wolf-like creature. It’s about 50 million years old. Scientistshad seen this wolf-like creature before, but this skull was different. The eararea of the skull had characteristics seen only in aquatic mammals, specificallywhales.Err, well, then also in Pakistan they found a fossil of another creature, whichwe call Ambulocetus natans That’s muffle lay. The name Ambulocetus natanscomes from Latin of course, and means “walking whale that swims”. It clearlyhad four limbs that couldn’t have been used for walking. It also had a long thintail, typical of mammals, something we don’t see in today’s whales. But, it alsohad a long skeletal structure. And that long skeletal structure suggests that itwas aquatic. And very recently in Egypt, they found a skeleton of Basilosaurus.Basilosaurus was a creature that we’ve already known about for over ahundred years. And it has been linked to modern whales because of its longwhale-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. Theyweren’t even connected to its Power San and couldn’t have supported itsweight. But it clearly shows Basilosaurus an evolution from land creature. Sothat’s a giant step in the right direction. Even better, it establishedAmbulocetus natans as a clear link between the wolf-like creature andBasilosaurus. Now these discoveries don’t completely solve the mystery. Imean, Ambulocetus natans is a mammal that shows a sort of bridge betweenwalking on land and swimming. But it also is very different from the whaleswho 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, DNAis the genetic code for any organism. And when the DNA from two differentspecies is similar, it suggests that those two species are related. And when wecompared some whale DNA with DNA from some other species, we got quite asurprise. 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 evolutionof the hippopotamus apparently contradicts the fossil record, which suggeststhat the hippopotamus is only a very distant relative of the whale, not anancestor. And of course as I mentioned, that whales are descendent not fromhippos but from that distant wolf-like creatures. So we have contradictoryevidences. And more research might just raise more questions and createmore controversies. At any rate, we have a choice. We can believe themolecular 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 spermwhales and killer whales were closely related to one another. And the same forthe toothless whales, like the blue whale and other baleen whales, weassumed that they be closely related. But recent DNA studies suggest thatthat’s not the case at all. The sperm whale was actually closely related to thebaleen whale, and it’s only distantly related to the toothed-whales. So that’sthe real surprise to all of us.