【经济学人】一步一步解开黑暗时代的面纱|2016.12.10|总第762期

ONE of the most important steps on the journey to Homo sapiens was that made by the first fish to crawl onto dry land. It was both a metaphorical and a literal step, but knowing exactly when it happened is tricky. It depends, for one thing, on the definition of “dry land”. Scrambling over the mud from one pool to another, assisted by fins that had evolved to walk along the seabed in the way modern coelacanths do, was probably going on by 385m years ago. 

鱼类首次爬上陆地无疑是生物向“智人”进化阶段中的众多重要步骤中的一步。这是既有字面又有隐喻意义的一步，但是要想弄清楚具体发生时间却很难。这取决于“陆地”一次的定义。可能在大约3亿8千5百万年前，鱼类就可以借助鱼鳍从一个泥潭爬到另一个泥潭了。就像现代腔棘鱼那样，它们的鱼鳍已经进化到可以在海床上行走了。

远古鱼类化石三维复原图

By 375m years ago, the descendants of these first-footers had evolved four limbs clearly recognisable as legs. They were no longer fish, but “tetrapods”. Their legs, though, could have as many as eight digits each, and do not look capable of supporting an animal properly when it was out of the water. Some might thus argue that even by this stage, the step onto dry land had not been truly made.

大约3亿7千5百万年前，第一批鱼类“登陆者”的后代进化出了四肢（可以清晰的看出来是腿）。它们已经不再属于鱼类，而是“四足动物”。尽管它们每个腿的脚趾最多可以达到八个，但是看起来并不能够支撑它们在陆地行走。因此有人认为甚至直到这一时期，鱼类还没有真正地从海洋爬上陆地。

All of these events occurred during a period called the Devonian when, though the oceans teamed with organisms no less varied than today’s, life on the continents was just getting going. Vascular plants (those bigger than mosses and liverworts) had evolved only recently. Insects were evolving fast, too. But there were no large land animals. Occupying the new habitat thus looked like an evolutionary open goal for the tetrapods. But then, 359m years ago, in a mass extinction as big as that which did for the dinosaurs, the Devonian came crashing to an end. （读者试译） For 25m years after this the tetrapods more or less disappear from the fossil record. When they re-emerge, in what is called the Lower Carboniferous period, they do, indeed, live up to their potential. They are now proper terrestrial animals, possessing five-digit limbs powerful enough to support them without the assistance of water’s buoyancy. But how they got there has been a mystery.

这一切都发生在泥盆纪时期，虽然当时的海洋生物多种多样，种类一点也不比现在少，但陆地上的生命才刚刚开始。在这一时期，维管植物（比苔藓和苔类要大）才刚刚出现。昆虫类生物也在快速进化。但尚未有大型陆地生物。（期待您的翻译，明天会有针对这句话的读者试译详解哟~）2千5百万年后，四足动物差不多已从化石记录里消失了。当它们再次出现时，已经到了始石炭纪时期。它们确实充分发挥了自身的潜力。现在它们已经是标准的陆生生物了，四肢都有五趾，无需借助海水的浮力即可支撑身体。但是四足动物如何进化成今天的样子仍是个迷。

• Buoyancy: n. 浮力

Walking and eggshells

行走和蛋壳

This 25m-year dark age is known as Romer’s gap, after Alfred Romer, an American paleontologist of the 20th century, who was the first to notice it. But it is dark no longer. A team of fossil hunters led by Jennifer Clack of Cambridge University has been collecting and analysing material from Lower Carboniferous outcrops in Scotland. As they report in Nature Ecology and Evolution, Dr Clack and her colleagues have identified and named five hitherto-unknown species of tetrapod from the gap, and gathered material from seven other, as-yet-unnamed ones. This suggests the gap is a product of incomplete collecting in the past rather than an actual hiatus in animal history brought about by the Devonian mass extinction.

这段2500万年的黑暗时期被称为“柔默空缺”，是以20世纪美国的一位古生物学家阿尔弗雷德·柔默的名字命名的。他是这一空白时期最早的发现者。但现在这一黑暗时期已经揭开了神秘的面纱。由来自剑桥大学的詹妮弗·克拉克领队的一个化石搜寻小组一直在苏格兰对早石炭纪时期的化石材料进行搜集和分析。克拉克博士和她的同事们已经成功发现并命名5种该空白时期的迄今为止未知的四足动物，而且还成功搜集到另外7个未命名物种的化石材料，并将发现成果发表在《自然生态与进化》杂志中。这表明柔默空缺是过去科学家未能收集到证据造成，而并非泥盆纪物种大灭绝事件造成了动物史的断裂。

• Hiatus: 裂缝， 空隙

早期维管植物

The team’s discoveries range from species the size of newts to ones the size of crocodiles (pictured in the artist’s above). Crucially, some were clearly adapted to be able to walk for long periods on land in a way their Devonian ancestors had not been. Romer’s gap thus seems to be the time when tetrapods became unequivocally terrestrial.

该搜索小组的发现成果包罗万象，物种从小型的蝾螈类到大型的鳄鱼类都应有尽有（请参见上图美术师的画作）。关键在于，柔默空缺时期的一些生物明显已经能在陆地长时间行走了，它们的行走方式是泥盆纪的祖先做不到的。由此看来，四足动物在柔默空缺似乎正式成为陆生动物。

白垩纪早期哺乳动物毛兽的独特演化

But that is not all. One of the team’s most intriguing findings came as a result of an analysis of the fossils’ anatomies, to determine how they were related to each other and to earlier and later animals. This concluded that a great evolutionary split, between the amphibians and what are known as the amniotes, probably happened during the gap. The amniotes are those animals (including modern reptiles, birds and mammals) that have complex eggs surrounded by a membrane which cushions and protects the developing embryo. It was amniotes that evolved the eggshell, a development which let them sever all connection with the water by laying their eggs on land.

不仅如此，该搜寻小组最吸引人的发现之一是化石解剖分析结果，这一结果旨在确定这些生物之间的联系，以及它们与早期生物和后期生物之间的联系。由此可以推断，在此期间两栖动物和羊膜动物之间出现了进化分裂。羊膜动物包括现代爬行类、鸟类和哺乳类动物。它们卵由一层薄膜包裹结构很复杂。这层薄膜可以对发育中的胚胎起到很好的保护和缓冲作用。正是羊膜动物进化出了蛋壳，这一进化使其可以在陆地上产卵，由此彻底切断与水的所有联系。

• Amphibians：两栖动物

• Amniote：羊膜动物，脊椎动物

• Membrane：薄膜

恐龙蛋

Romer’s gap, in other words, now seems bridged—and this, in turn, bridges not only the gap in understanding of when tetrapods became terrestrial, but also that concerning when the amniotes evolved. And since, as mammals, human beings are also amniotes, that, from a human point of view, is an evolutionary twofer.

换句话说，柔默空缺的神秘面纱似乎已经被揭开了。这一发现不仅揭开了四足动物正式进化为陆生生物的时间之谜，并且还揭示了羊膜动物是何时进化而来的。由于人类是哺乳动物，因此也同样属于羊膜动物。所以从人类的角度而言，这一发现就人类对进化之谜的探索而言，是个一石二鸟的好事情。

• Twofer: 买一送一的半价票

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