托福考試之托福聽力背景之鯨的起源

　　據美國兩個科學家小組日前公布的科學報告稱，他們在考古中發現的新化石更進一步證明了鯨祖先與生活在陸地上的食草動物有關，如牛和河馬，而不是即將滅絕的食肉類動物。

　　科學家們已經知道，鯨是幾百萬年前由生活在陸地上的四肢動物演變而來的，然而，鯨究竟是從動物王國中哪一支演變而來的一直是引起爭議的問題。

　　20世紀50年代進行的免疫試驗和最近進行的DNA試驗都表明鯨的進化與食草的偶蹄哺乳動物有關，如豬、牛和河馬。此前，這些試驗發現的結果一直沒有得到化石考古方面的支持，以前發現的化石則更表明鯨與食肉類動物有關。然而，現在兩項分別進行的科學研究報告的作者稱，他們分別在巴基斯坦不同地區發現的化石，使他們相信這些試驗的結果是正確的。

　　美國俄亥俄州東北醫學院的漢斯·斯威森說，在發現了這些化石后，他們意識到，顯然以前他們在有關鯨起源問題上的主張是錯誤的.。斯威森在巴基斯坦東北部地區旁遮普發現了兩種5000萬年以前可能是鯨祖先的四肢動物化石。以斯威森主筆，多人合寫的有關論文發表在本月20日出版的英國《自然》雜志上。

　　另外一名科學家菲利普·金格里奇是美國密歇根大學地質學和古生物學教授。他說，他們一個考古小組在巴基斯坦西南部地區俾路支海岸發現了另外兩種大約4700萬年以前可能是鯨祖先的化石，其中一架骨骼基本上是完整的。金格里奇說，他們由此發現，進行DNA試驗的同行發現的結果可能是正確的，鯨的起源與河馬有關。以金格里奇主筆，多人合寫的有關論文即將發表在21日出版的一期美國《科學》雜志上。但他這篇論文公布的日期與《自然》雜志上的那篇論文公布的時間相差無幾。

　　這兩篇論文的主題都是，他們發現的這種動物化石特殊的耳朵形狀與鯨很相似，很可能是鯨的祖先，同時這些動物化石都有四肢和明顯的足踝結構，與其它的食草偶蹄哺乳動物相似。

　　假如鯨祖先與食草動物有關，那么現代鯨的飲食習慣如何進化而來還是一個謎。一些有牙的鯨如虎鯨可以吃魚和其它的海中哺乳動物;而另外一些鯨則有被稱為鯨須的嘴部結構，可以從海水中吸濾浮游生物。金格里奇則說，他已讀到一些有關現代河馬殺死和吃掉水窩中太靠近它們的瞪羚，也許人們有點夸大了這些偶蹄動物的食草性，盡管他們以食草為主。

　　Whales most likely evolved from four-legged land animals that foraged for food or hunted for fish along ocean shorelines. The ancestors of whales gradually became more dependent on the ocean for food, passing through an amphibian stage before evolving into fully aquatic animals. Recent work with whale fossils indicates that the anklebones of whale ancestors resembled those of artiodactyls, the group of even-toed hoofed mammals that includes pigs, deer, antelopes, and hippopotamuses. Molecular studies comparing the genetic makeup of modern whales to that of other animals indicates that the hippopotamus is the whale’s closest genetic match. Based on these studies, scientists now generally agree that whales are most closely related to artiodactyls.

　　The fossil record of whales spans over 50 million years to ancestors such as Pakicetus that lived around the mouths of rivers in what is now Pakistan. Pakicetus was a four-legged, land animal that measured about 2.5 m (about 8 ft) in length. The whale fossil Rodhocetus was discovered in 1993. Dating from about 46.5 million years ago, this fossil came from fully marine sediments. Its legs were smaller than those of Pakicetus and its hind feet were turned into webbed paddles. Rodhocetus was clumsy on land, perhaps like modern sea lions. It probably swam using an up-and-down flexing of the body to power its finlike feet in the manner of otters. Unlike modern whales, it had nostrils instead of a blowhole. Rodhocetus also had a powerful tail, although scientists do not know whether this early whale had begun to evolve tail flukes.

　　A fossil whale known as Basilosaurus dates from 42 million years ago and represents the stage of whale evolution in which the hind legs are very small but still visible. The adult Basilosaurus reached lengths of 15 m (50 ft), but its hind limbs resembled short sticks only 0.5 m (1.6 ft) long. Although all of the pelvic bones, leg bones, kneecaps, feet, and toe bones were present, these legs could not have been used to support such a large animal on land. Modern whales, which first appeared in the fossil record 5 million to 10 million years ago, have no visible hind limbs, although some whale species still have tiny pelvic and leg bones embedded in muscle near the spine.

　　綜上所述：古生物學者paleontologist研究化石，Molecular biologist 研究基因和DNA，分子生物學家從基因分析認為鯨的近親是河馬hippopotamus，古生物學家以前認為鯨的近親是一種食肉動物。

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