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化学 1953

在可能的原始地球条件下生成氨基酸

斯坦利·米勒

让一道电火花穿过早期地球的气体,生命的基石便自行组装出来。

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In depth · the introduction

1953 年,一位研究生把一个玻璃回路灌满早期地球的气体,让一道火花在里面放电一周,然后眼看着那池水,变成了一锅由生命所用分子组成的浓汤。

核心想法

生命,是由一小套分子搭起来的——其中就有那些会彼此连成蛋白质的氨基酸。长久以来,有一个谜:在一颗没有生命的行星上,周围没有任何活物去制造它们,这些分子里最初的那一批,究竟是怎么出现的?斯坦利·米勒证明:它们根本不需要任何活物——只要给对的气体一个能量源,它们就会自己造出自己。

他重现了当时对早期大气的一个猜测——甲烷、氨、氢与水——把它封进玻璃,再用一道连续的电火花去轰击它,权当闪电。几天之内,清澈的水就变成了浑浊的红棕色,而漂在其中的,是真正的氨基酸。生命的基石,竟从寻常的化学里组装了出来。

它是如何诞生的

生命起于一锅化学「原始汤」的想法,早在 1920 年代就由亚历山大·奥巴林与 J. B. S. 霍尔丹提出过,可它当了三十年的故事,因为没人检验过。1952 年,23 岁的研究生米勒,向他的导师——诺贝尔化学奖得主哈罗德·尤里——请求一试。尤里觉得这是个胜算渺茫的尝试,拿一个博士学位去押注,未免太冒险。

米勒亲手搭起装置,让它运转。那场变化,一天之内便肉眼可见。1953 年他发表论文时,署的是自己一个人的名字——尤里坚持把功劳让给学生——结果登上了报纸头条:一个年轻人,竟在芝加哥的一张实验台上,某种意义上「煮」出了生命的原料。它成了那个世纪最著名的实验之一。

它为何重要

它把生命的起源,从纯粹的臆测,搬到了实验台上。在米勒之后,你可以提出精确的问题——哪些气体、多少能量、哪些分子——并得到可测量的答案。它没有说明生命是怎么开始的,但它表明:那最初、看上去最令人却步的一步——用死物造出生命的基石——是寻常的自然轻而易举就在做的事。仅这一次演示,就重塑了科学家、乃至所有人,对「我们从何而来」的想象。

一个可以想象的画面

把早期地球想成一间无人照看、却开着火的巨大厨房:简单的气体是食材,海洋是搅拌盆,闪电是炉灶。没有人在做饭。可那一遍又一遍、历经亿万年的加热与搅动,足以把生的食材合成出更多的东西——就像一锅汤,文火慢炖得够久,自会变稠成酱。米勒所做的,不过是把这间厨房缩进一个烧瓶,再把时钟快进。

米勒装置的可交互示意图:上方一只「大气」烧瓶里噼啪跳着火花,经冷凝管喂入下方一只「海洋」烧瓶。拖动一个滑块就能改变空气:滑向早期地球的还原性配方(甲烷、氨、氢),海洋便变深成一锅红棕色的氨基酸浓汤;滑向今天的氧化性空气(二氧化碳、氮、氧),水则保持清澈,几乎什么也不生成。

它的位置

米勒的这道火花,串起了本馆中两条漫长的故事。在它身后,站着达尔文——他曾私下遐想,生命或许始于「某个温暖的小池塘」——还有奥巴林与霍尔丹,把那一缕遐想变成了「原始汤」假说。在它前方,奔涌着遗传的分子——沃森与克里克的 DNA、克里克的中心法则——以及那个至今仍未解的问题:一锅基石的浓汤,究竟是怎样跨过那道界线,变成能够自我复制之物的?七十年过去,那一跨,仍是前沿;而米勒所证明的,仅仅、却也确凿地,是这趟旅程的第一步,并不难。

The original document
Original source text
Stanley L. Miller · Science 117 (1953): 528–529
The paper opens by placing itself inside an older idea — that life's first organic molecules formed while the young Earth was wrapped in a reducing atmosphere — associated with Oparin and Haldane and pressed more recently by Urey and Bernal.
The idea that the organic compounds that serve as the basis of life were formed when the earth had an atmosphere of methane, ammonia, water, and hydrogen … was suggested by Oparin and has been given emphasis recently by Urey and Bernal.
The experiment
A mixture of gases, CH4, NH3, H2O, and H2, which possibly made up the atmosphere of the Earth in its early stages, has been subjected to spark and silent discharges for times of the order of a week.
The apparatus is a sealed glass loop: water in a small flask is boiled so its vapour carries up into a larger flask holding the gas mixture, where tungsten electrodes strike a continuous spark — the stand-in for lightning. The products are cooled in a condenser and washed back down into the water, which plays the part of the primitive ocean, so that whatever forms accumulates there across the week-long run.
What appeared
Within a day the water turned pink, then a deep red-brown. Analysed by paper chromatography, glycine, α-alanine and β-alanine were identified with confidence, and aspartic acid and α-amino-n-butyric acid more tentatively — while a substantial fraction of the carbon, on the order of a tenth, had been drawn out of the gas and into organic compounds.
[ … ]
The short discussion argues that the amino acids were not stamped out directly by the discharge but were assembled in the water from simpler fragments — aldehydes and hydrogen cyanide — made by the spark. The full two pages, with the apparatus diagram and the chromatograms, are at the source below.
Stanley L. Miller · University of Chicago · 1953