時差N小時:鯨是怎么演變成水手的?

Mammals developed from land-based reptiles twohundred million years ago, and most of themammals we see today are still land based. Anotable exception are the cetaceans, which we allknow as dolphins and whales. How did they movefrom land to sea?

兩億年前的哺乳動物由陸地爬行動物進化而來，如今我們所看到的大多數哺乳動物仍然是陸生生物。鯨類動物，正如我們知道的海豚和鯨魚則是個顯著的例外。它們是如何從陸地搬到海洋的呢?

The ancestor of modern whales moved into the water about fifty-two million years ago. Onewhale fossil, about forty million years old, provides an interesting snapshot from this gradualchange. This early whale fossil had rudimentary legs growing from its sides.

現(xiàn)代鯨魚的祖先大約在五千兩百萬年前就從陸地移民到海洋。一塊四千萬年前的鯨魚化石為這個漸進的變化提供了有趣的線索。這個早期的鯨魚化石兩側長有退化的腿。

These were puny compared to the bulk of the whale, so they were probably vestigial, in theslow process of disappearing. Moving from land to salt water requires more changes than simplytrading in legs for fins, however. For example, the kidney of a modern whale allows it to drinksalt water, but early whales must have been restricted to fresh water like other mammals.

與鯨魚龐大的軀體相比這些腿顯得微不足道，因此它們可能是漫長退化過程中正在消失的腿部。然而，鯨類想要從陸地遷往咸水區(qū)域需改變更多，而不僅僅只是把雙腿換成鰭。例如，現(xiàn)代鯨魚的腎臟可以承受咸水，但是早期鯨魚則像其他哺乳動物一樣只能依靠淡水生存。

Scientists have recently discovered that this important kidney change happened about fortyeight million years ago to a whale called Indocetus. Kidneys don't leave behind fossils likebones do, so how can these scientists be sure?

科學家們最近發(fā)現(xiàn)有一類叫做 Indocetus 的鯨魚，它們的腎臟于四千八百萬年前發(fā)生重大改變。腎臟不能像骨頭那樣形成化石，因此科學家們是如何確定的呢?

The answer comes from the ancient whales' tooth enamel. Oxygen in an animal's tooth recordsinformation about the type of water they drank. The very earliest proto-whales had toothenamel that indicated a fresh water diet. They probably lived in rivers, or at least returned torivers to drink. Indocetus, however, had teeth that indicated a salt water diet. This means ithad specialized kidneys, and was probably the first mammal to swim the open ocean. That's alot of information from some old teeth!

從古鯨魚的牙釉質里可以找到答案。動物牙齒內的氧記錄了它們的飲水信息。最早的原始鯨魚牙齒上有牙釉質，證明它具有淡水飲食習慣。它們可能生活在河里，或者至少從河里取水。然而，Indocetus 的牙齒表明它飲用咸水。這就意味著它的腎臟專門用來喝咸水，它可能就是第一個游向海洋的哺乳動物。古老的牙齒確實藏有大量的信息!