原文
Conditions on Early Earth and the Beginnings of Life
①A little more than 3.8 billion years ago is a good estimate of when life began on Earth. How it began remains speculative. There is no standard theory; there is instead a confusion of conflicting theories that attack the problem from different angles. This is a change from 1953 when a classic experiment on the origin of life was published. Then, Stanley Miller and Harold Urey had just completed their famous laboratory simulation of the conditions of an early Earth at the University of Chicago. When Miller and Urey let electric sparks course like lightning through an “atmosphere” of methane, ammonia, and hydrogen, which circulated above an “ocean” of boiling water, they found that a reddish substance, rich in amino acids, accumulated in their glass apparatus. Amino acids, when strung together in long folded chains, form proteins, and proteins are the building blocks of the living cell. From the spontaneous synthesis of amino acids to the spontaneous origin of life on the primitive Earth did not seem such a long way to go.
②That early optimism has proven profoundly mistaken, for at least two reasons. The first is simply that it is, in fact, a long way from amino acids to life. The hardest part about creating life is not making the amino acids that go into proteins; or the sugars, phosphates, and bases that go into DNA, which carries the cell’s genetic blueprint; or the lipids that form its protective membrane. The hardest part about creating life is not making the “bricks”: it is assembling them into a finished structure. That is what all the theories that have emerged since the Miller-Urey experiment are primarily about, and the conflict among them shows no signs of being resolved soon.
③Furthermore, in recent years even the fundamental premise of that landmark experiment has been called into question. Today most researchers who study early Earth do not believe that its atmosphere was primarily methane and ammonia, which would have been a strongly reducing atmosphere, where reducing means hydrogen-rich. Methane and ammonia are both comparatively fragile molecules that might easily have been broken apart by the ultraviolet sunlight that bathed the young Earth, which had not yet evolved an ozone shield. More important, the idea that Earth was hot to begin with as a result of its viol
ent birth, when large asteroids collided to form it, implies that its early atmosphere was rich in carbon dioxide rather than methane. That is the form in which carbon would be released by exploding asteroids.
④The bottom line is that the early atmosphere is not likely to have been a giant Miller-Urey experiment; it would have been mostly nitrogen and carbon dioxide. In such an atmosphere it is indeed hard to make the molecular bricks of life, let alone a living organism. It is hard even to make the chemical compounds necessary for life. The most important compounds are formaldehyde and hydrogen cyanide, which, brought together in the presence of water, react to produce amino acids, from which the bricks are made. Formaldehyde and hydrogen cyanide, then, seem to be essential stages on the chemical road to life, and hydrogen cyanide especially cannot be made in great quantities in a carbon-dioxide atmosphere. Both compounds, however, are abundant in comets like Halley, Hyakutake, and Hale-Bopp. Presumably they are in other comets as well.
⑤Here, then, is an elegant solution to the dilemma. The dilemma is that the old view of ho
w life began conflicts with the new view of how Earth began and how it acquired an ocean. The solution, perhaps, is to deliver the organic precursors of life with the same vehicles that almost certainly helped create the ocean: icy comets. Researchers have calculated that over the course of Earth’s history, comets have delivered an amount of organic matter to the planet that is nearly a million times its present biomass—the total mass of all living things. Most of the organic matter would have arrived during the heavy bombardment that ended 3.8 billion years ago.
译文
早期地球的条件以及生命的开始
①react to翻译38 亿多年前是地球上生命开始的一个很好的估计。它是如何开始的仍然是推测性的。没有标准的理论；相反，存在相互矛盾的理论的混淆，这些理论从不同的角度解决这个问题。这是来自于1953年的一个改变，当时一个经典的对于生命起源的实验被发表了。当时，斯坦利米勒和哈罗德尤里刚刚在芝加哥大学完成了他们著名的早期地球条件实验室模拟。当 Miller 和 Urey 让电火花像闪电一样穿过由甲烷、氨和氢组成的“大气层”，“大气层”在由沸水组成的“
海洋”上方循环，他们发现一种富含氨基酸的微红物质积聚在他们的玻璃杯中仪器。氨基酸以长折叠链串在一起时形成蛋白质，而蛋白质是活细胞的组成部分。从氨基酸的自发合成到原始地球上生命的自发起源似乎并没有多远的路要走。
②至少出于两个原因，这种早期的乐观情绪已被证明是大错特错的。事实上，第一个理由很简单，从氨基酸到生命还有很长的路要走。创造生命最困难的部分不是制造用以形成蛋白质的氨基酸，或者用以形成DNA（DNA携带着细胞的遗传蓝图）的糖、磷酸盐和碱基，或形成其保护膜的脂质。创造生命最困难的部分不是制造“砖块”： 而是将它们组装成一个完整的结构。这就是自Miller-Urey实验以来出现的所有理论的主要内容，而且它们之间的冲突没有迹象表明会很快得到解决。
③此外，近年来，甚至这个具有里程碑意义的实验的基本前提也受到质疑。今天，大多数研究早期地球的研究人员都认为它的大气层主要不是甲烷和氨气，甲烷和氨气可能是一种强还原性大气层，还原性意味着富含氢气。甲烷和氨都是相对脆弱的分子，很容易被照射尚未形成臭氧层的年轻地球的紫外线分解掉。更重要的是，有种说法说，地球诞生时情况非常剧烈，当时大型小行星碰撞形成地球，因此地球一开始的时候是非常热的。这意味着它早期的大气层富含二氧化碳而不是甲烷。这种情况中，碳会因爆炸的小行星而被释放出来。
④最重要的是，早期的大气不太可能是一个巨大的Miller-Urey实验；它可能主要是氮气和二氧化碳。在这样的大气中，确实很难制造出构成生命的分子“砖块”，更不用说一个活的有机体了。甚至制造生命所必需的化合物也很困难。最重要的化合物是甲醛和氰化氢，它们在有水的情况下聚集在一起，反应生成氨基酸，而生命的“砖块”是由氨基酸制成的。因此，甲醛和氰化氢似乎是通往生命的化学道路上必不可少的阶段，在二氧化碳大气中，尤其无法大量制造氰化氢。然而，这两种化合物在哈雷、百武和海尔波普等彗星中都很丰富。据推测，它们也存在于其他彗星中。
⑤那么，这里有一个解决困境的优雅的方法。困境在于，关于生命如何开始的旧观点与关于地球如何开始以及它如何获得海洋的新观点相冲突。或许，解决方案是，形成生物有机的前身的东西是跟几乎肯定帮助创建了海洋的载体是一个东西：冰彗星。研究人员计算出，在地球的历史进程中，彗星向地球输送的有机物数量几乎是其目前生物量（所有生物的总质量）的一百万倍。大部分有机物是在 38 亿年前结束的猛烈轰炸期间到达的。

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