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我们都是鱼的后代

2008-05-23
鱼的后代,你体内的鱼,Your Inner Fish
我们都是鱼的后代?
作者:英国《金融时报》专栏作家艾伦·凯恩(Alan Cane)
2008年5月23日 星期五《你体内的鱼:35亿年的人体历程》(Your Inner Fish:A Journey Into the 3.5 Billion-Year History of the Human Body)
作者:尼尔•舒宾(Neil Shubin)
Allen Lane出版,20英镑,240页
FT书店价:16英镑

如果你想要了解人类以及其它动物的演化史,并且今年还没翻过其他读本的话,那就读一读这本杰出的专著吧。如果你赞成“造物说”,你也可一读,同时会后悔自己原先怎么会认同那些可怜的解释方式。

在240页极其引人入胜的文字中,身为化石搜寻者、比较解剖学家和发展动物学家的尼尔•舒宾(Neil Shubin)解释了每一个生命体构成部分的渊源。舒宾的探寻显示出了生命的艰难历程:简单、自在的细胞生命如何在原古的黏土中奋力求生,直至演化为科学所知的最为复杂的动物:人类。

在描述这个过程的时候,舒宾以一种能让人迅速理解的方式阐释了演化的机制,并揭露了那些宣称生命过于复杂、因此不会偶然产生的人,证明他们所说的不过是谎言。舒宾还对以下事实做出了解释:为什么醉酒者的眼睛会固执地转向右侧(这被称作眼球震颤(nystagmus));为什么凌晨两点时他会在陈列有25具尸体的公司实验室里感到恐慌,并在飞身逃离的过程中大意地将家门钥匙落在办公室里。

也许对于一本如此权威的著作来说,《你体内的鱼》是一个比较陈腐的标题。这个标题意指舒宾的论点——其实这也是多数进化生物学家的论点:生命来自于海洋,并在数十亿年的时间里演化出鱼形的动物。它们当中的一些物种登上了陆地,只为避开同期的掠食类动物;另一些物种则在硬骨演化成形之前变生出坚硬的牙齿,以便更好地撕咬和分解猎物。舒宾辩称,所有的哺乳类器官都可以溯源到这些鱼类的类似器官。

舒宾的基本观点囊括在他所称的生命体“万物法则”中,就是说,这个星球上的任何生命体都有各自的父母。这种乏善可陈、老生常谈的陈述掩盖了一种微言大义:生命世界中的任何结构都不是无中生有的。不仅每一种生灵如此,就连每一个肢体、每一种器官和每一类组织都是从早先的形式中生发出来的。而且,它们在这一过程中经历的变形,会让其中的一类与另一类的联系变得难以理解。

舒宾写道,哺乳类中耳骨起源的发现即是一例。中耳骨是由锤骨、砧骨和镫骨这三块独立的骨骼组成,这种结构与其他任何纲目的动物都有所不同:爬行动物与两栖动物都只有单个耳骨,而鱼类则没有。因此舒宾问道:我们的中耳骨究竟来自何方?答案由德国解剖学家卡尔•赖歇特(Karl Reichert)在1837年揭晓。赖歇特曾一直关注腮弓的发展,这是哺乳类与爬行类动物胚胎头部底端形成的一种膨大。让赖歇特感到吃惊的是,他发现哺乳类的两块耳骨与爬行类颚部的一些骨片相吻合。舒宾写道:“结论不可避免:构成爬行类颚部一部分的鳃弓,同样也构成了哺乳类的耳骨。赖歇特提供了一种连他自己都难以想象的观念,那就是:哺乳类耳朵的某一部分与爬行类的颚部是同一回事。”

作为一名化石搜寻者,舒宾的杰出成就是在2004年带领了一次北极探险。在这次探险中,他们发现了一种半鳍半肢的动物——一条有腕的鱼化石。这种鱼被命为“提克塔利克(Tiktaalik,因纽特语中意为“一种大型浅水鱼”)。这是第一块显示出半陆地半海洋中间状态特征的化石。在它生存的年代当中,这种鱼很可能会通过那种怪异的肢状鳍来支撑身体,或是贴着溪流和池塘的底部向前推进,或是沿着河岸的泥滩来导航。

我们都认为化石的发现纯属意外。登山者们会发现山坡上露出的恐龙骨骼,小学生们会在海滩上捡到甲胄鱼(ostracoderm)的骨骼。这类巧合确实经常发生。但舒宾解释道,专业人士正在尽一切可能来获取主动。如今的野外考察就好比军事战役一样计划周密。搜寻者细心地选择地貌,在年代适宜、适合保存化石、以及暴露在地表的岩层中认真查找。但即使是舒宾本人也坦承: 偶然因素在成功的搜寻中起到了极大的作用。

舒宾著作中所罗列的证据集中支持了以下论点:生命史中任何新颖的事物,或是明显与众不同的事物,“其实都是循环使用、重新组合、再赋新意、或是再度改装以备新用的老东西。”这算不上是什么新观念,只是这种观念从未以如此清楚、幽默的方式阐述。

艾伦•凯恩(Alan Cane)是英国《金融时报》的科学专栏记者
译者/李晖
阅读本文章英文,请点击 (BOOK REVIEW) OUR FAMILIES AND OTHER FISH
国际 > 特稿

(BOOK REVIEW) OUR FAMILIES AND OTHER FISH
Review by Alan Cane
Friday, May 23, 2008
Your Inner Fish: A Journey Into the 3.5 Billion-Year History of the Human Body

By Neil Shubin
Allen Lane £20, 240 pages
FT bookshop price: £16

If you want to understand the evolutionary history of man and other animals, and read no other account this year, read this splendid monograph. And if you subscribe to “creationist” tendencies, read it also and repent your sorry ways.

In 240 profoundly fascinating pages, Neil Shubin, fossil-hunter, comparative anatomist and developmental zoologist, explains the origins of the constituent parts of every living body. He traces the tortuous path from the simple, free living cells which fought for survival in the primeval slime to the most sophisticated animal known to science: man.

On the way, he explains the mechanism of evolution in a manner which makes it instantly comprehensible and gives the lie to those who would claim that life is too complex to have arisen by chance. He also explains why the eyes of the inebriated slide inexorably to the right (it's called nystagmus) and how he panicked at two in the morning in a laboratory with 25 dead bodies for company, inadvertently locking his house keys in the lab as he fled.

Your Inner Fish is, perhaps, a rather trite title for such a magisterial work. It refers to Shubin's contention – and indeed the contention of most evolutionary biologists – that life began in the sea, evolving over billions of years to create fish-like creatures. Some of which took to the land to avoid their predatory contemporaries; hard teeth, all the better to bite and dismember the prey, evolved before hard skeletons. All mammalian organs, he argues, can be traced back to their fishy equivalents.

Shubin's basic proposition is encapsulated in what he calls the biological “law of everything”: that every living thing on the planet has parents. This innocuous, quite banal statement conceals a great profundity: that no structure in the living world arises de novo. And not only every creature but also every limb, every organ and every tissue is derived from an earlier form – and in the process, may go through a transformation which renders the relationship between one and the other hard to understand.

He describes, for example, the discovery of the origin of the bones of the mammalian middle ear. Comprising three separate bones, the malleus, the incus and the stapes, this structure is unlike that of any other class of animal: reptiles and amphibians have one bone while fish have none. So where, Shubin asks, did our middle-ear bones come from? The answer was provided in 1837 by the German anatomist Karl Reichert. He had been following the development of gill arches, swellings around the base of the embryonic head, in mammals and reptiles. He was astonished to find that two of the ear bones in mammals corresponded to pieces of the jaw in reptiles. Shubin writes: “The conclusion was inescapable: the same gill arch that formed part of the jaw of a reptile formed ear bones in mammals. Reichert proposed a notion that even he could barely believe – that parts of the ears of mammals are the same thing as the jaws of reptiles”.

As a fossil hunter, Shubin distinguished himself by leading an expedition to the Arctic in 2004 which uncovered the remains of a fish with a wrist, a creature with part fin, part limb. Named Tiktaalik (Inuktitut for “large freshwater fish”), it was the first fossil to show characteristics which placed it midway between land and sea; in its time, Tiktaalik probably propelled itself along the bottom of streams and ponds or navigated mudflats along the riverbank, supporting its body on its strange, limb-like fins.

We all suppose that fossils are found by accident. Hill walkers find dinosaur bones sticking out of hillsides: schoolchildren pick up ostracoderm skeletons on the beach. And such discoveries do happen. But Shubin explains that the professionals do all they can to weigh the odds in their favour. Expeditions are planned in as much detail as a military campaign. The hunters choose their terrain carefully, looking for rocks of the right age, the right type to preserve fossils and rocks that are exposed at the surface. But even he admits that serendipity plays a big part in a successful hunt.

Shubin's book is packed with the evidence to support his contention that everything innovative or apparently unique in the history of life “is really just old stuff that has been recycled, recombined, repurposed or otherwise modified for new uses”. It's not a new notion, but rarely has it been expressed so clearly and with such good humour.

Alan Cane is the FT's science correspondent

本栏目主要介绍生命探索方面,包括生命起源、生命科学、生命现象、人类起源、我们都是鱼的后代 等。特别关注有关人与生命的意义与生命的价值方面的研究。

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