时差N小时 鲸是怎么演变成水手的?

Mammals developed from land-based reptiles¹ 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 puny² 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 fins³, 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 enamel⁴. 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 specialized⁵ 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！

1 reptiles   

n.爬行动物，爬虫( reptile的名词复数 )

参考例句：

• Snakes and crocodiles are both reptiles. 蛇和鳄鱼都是爬行动物。 来自《简明英汉词典》
• Birds, reptiles and insects come from eggs. 鸟类、爬虫及昆虫是卵生的。 来自《现代汉英综合大词典》

2 puny   

adj.微不足道的，弱小的

参考例句：

• The resources at the central banks' disposal are simply too puny.中央银行掌握的资金实在太少了。
• Antonio was a puny lad,and not strong enough to work.安东尼奥是个瘦小的小家伙，身体还不壮，还不能干活。

3 fins   

[医]散热片；鱼鳍；飞边；鸭掌

参考例句：

• The level of TNF-α positively correlated with BMI,FPG,HbA1C,TG,FINS and IRI,but not with SBP and DBP. TNF-α水平与BMI、FPG、HbA1C、TG、FINS和IRI呈显著正相关，与SBP、DBP无相关。 来自互联网
• Fins are a feature specific to fish. 鱼鳍是鱼类特有的特征。 来自辞典例句

4 enamel   

n.珐琅，搪瓷，瓷釉；(牙齿的)珐琅质

参考例句：

• I chipped the enamel on my front tooth when I fell over.我跌倒时门牙的珐琅质碰碎了。
• He collected coloured enamel bowls from Yugoslavia.他藏有来自南斯拉夫的彩色搪瓷碗。

5 specialized   

adj.专门的，专业化的

参考例句：

• There are many specialized agencies in the United Nations.联合国有许多专门机构。
• These tools are very specialized.这些是专用工具。