Mammals developed from land-based reptiles two hundred million years ago, and most of the mammals we see today are still land based. A notable exception are the cetaceans, which we all know as dolphins and whales. How did they move from land to sea?
两亿年前的哺乳动物由陆地爬行动物进化而来，如今我们所看到的大多数哺乳动物仍然是陆生生物。鲸类动物，正如我们知道的海豚和鲸鱼则是个显著的例外。它们是如何从陆地搬到海洋的呢？

The ancestor of modern whales moved into the water about fifty-two million years ago. One whale fossil, about forty million years old, provides an interesting snapshot from this gradual change. This early whale fossil had rudimentary legs growing from its sides.
现代鲸鱼的祖先大约在五千两百万年前就从陆地移民到海洋。一块四千万年前的鲸鱼化石为这个渐进的变化提供了有趣的线索。这个早期的鲸鱼化石两侧长有退化的腿。

These were puny compared to the bulk of the whale, so they were probably vestigial, in the slow process of disappearing. Moving from land to salt water requires more changes than simply trading in legs for fins, however. For example, the kidney of a modern whale allows it to drink salt water, but early whales must have been restricted to fresh water like other mammals.
与鲸鱼庞大的躯体相比这些腿显得微不足道，因此它们可能是漫长退化过程中正在消失的腿部。然而，鲸类想要从陆地迁往咸水区域需改变更多，而不仅仅只是把双腿换成鳍。例如，现代鲸鱼的肾脏可以承受咸水，但是早期鲸鱼则像其他哺乳动物一样只能依靠淡水生存。

Scientists have recently discovered that this important kidney change happened about forty eight million years ago to a whale called Indocetus. Kidneys don't leave behind fossils like bones do, so how can these scientists be sure?
科学家们最近发现有一类叫做 Indocetus 的鲸鱼，它们的肾脏于四千八百万年前发生重大改变。肾脏不能像骨头那样形成化石，因此科学家们是如何确定的呢？
The answer comes from the ancient whales' tooth enamel. Oxygen in an animal's tooth records information about the type of water they drank. The very earliest proto-whales had tooth enamel that indicated a fresh water diet. They probably lived in rivers, or at least returned to rivers to drink. Indocetus, however, had teeth that indicated a salt water diet. This means it had specialized kidneys, and was probably the first mammal to swim the open ocean. That's a lot of information from some old teeth!
从古鲸鱼的牙釉质里可以找到答案。动物牙齿内的氧记录了它们的饮水信息。最早的原始鲸鱼牙齿上有牙釉质，证明它具有淡水饮食习惯。它们可能生活在河里，或者至少从河里取水。然而，Indocetus 的牙齿表明它饮用咸水。这就意味着它的肾脏专门用来喝咸水，它可能就是第一个游向海洋的哺乳动物。古老的牙齿确实藏有大量的信息！

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