本篇文章共分 3 段，說明從 20 世紀初，科學對於生命初始時地球環境的推論以及實驗，以及目前對於早期地球溫度以及大氣成分的分析，說明當時環境為何有利於生命的出現。

本篇考題英文原文與對應之中文翻譯整理如下。練習作答解題時若有對語意不清楚之處，請仔細查閱對照，以提升閱讀理解能力。

Conditions on Earth When Life Began 生命開始時地球的狀態

1. 奧帕林與米勒的早期理論與實驗

   In the 1920s, Aleksandr Oparin, a Russian biochemist, proposed and developed the idea that life originated in the warm, watery environment of early Earth’s surface, under an atmosphere mostly composed of methane. The early seas were believed by Oparin to be rich in simple organic molecules, which reacted to form more complex molecules, eventually leading to proteins and life. Then, almost 30 years after Oparin published his ideas, Stanley Miller demonstrated that amino acids, the building blocks of the proteins necessary for life, could form under conditions thought to prevail on early Earth. Miller’s experiment was elegant. He passed electric discharges through a mixture of methane, hydrogen, ammonia, and steam, and when he analyzed the results, found that he had made amino acids. The discharges were a proxy for lightning, the gas mixture an educated guess about what the early atmosphere may have been like. Amino acids cannot replicate themselves, and are not themselves alive. Nevertheless, this experiment has long been recognized as a landmark for understanding a process that must have been one of the important steps in the evolution of life on Earth, the natural synthesis of amino acids. However, it now seems likely that Miller’s experiments may not be directly applicable to the events of the early Archean (that is, early in the geologic eon that lasted from Earth’s formation until about 2.5 billion years ago).

   20 世紀 20 年代，俄羅斯生物化學家亞歷山大．奧帕林 (Aleksandr Oparin) 提出並發展了這樣一個觀點：生命起源於早期地球表面溫暖、多水的環境，此環境籠罩在主要由甲烷組成的大氣中。奧帕林認為早期的海洋富含簡單的有機分子，這些分子經過反應後形成更複雜的分子，最終導致蛋白質和生命。然後，在奧帕林發表他的觀點近 30 年後，斯坦利．米勒證明了氨基酸，即生命所需的蛋白質的組成部分，可以在被認為是早期地球上的條件下形成。米勒的實驗很優雅。他使電流通過甲烷、氫氣、氨氣和蒸汽的混合物，當他分析結果時，發現他已經製造了氨基酸。放電是閃電的代表，氣體混合物是對早期大氣層可能是什麼樣子的一種有根據的猜測。氨基酸不能自我複製，本身也沒有生命。儘管如此，這個實驗長期以來一直被認為是理解地球上生命進化的重要步驟之一—氨基酸的自然合成過程的一個里程碑。然而，現在看來，米勒的實驗可能並不直接適用於早期太古代（即從地球形成到大約 25 億年前的地質年代初期）。

2. 地球溫度

   One of the problems hindering understanding of the origin of life is that environmental conditions on early Earth are not known with any certainty. It is possible to make only reasoned estimates. For example, for some fairly long period of time after formation, perhaps as much as several hundred million years, the surface must have been much hotter than it is today. Continued impacts of meteorites, large and small, would have added further heat energy, and in the earliest part of Earth history the larger impacting bodies may have broken through the cooling crust to expose underlying molten material. Large quantities of volcanic gases would have been released into the atmosphere as lavas erupted onto the surface, producing a greenhouse effect far more severe than anything likely to result from human activity. It is quite possible that the early atmosphere was many times as dense as today’s, and that the seas and oceans were hot. Some have even suggested that because of the high atmospheric pressure, the oceans could have been hotter than the boiling point of water today. However, life as we know it is quite sensitive to temperature, and no modern organisms are known to survive much above 100°C. It is unlikely that life became established until surface temperatures had decreased to this level, or lower.

   阻礙理解生命起源的問題之一是，早期地球的環境條件並不確定。只可能做出合理的估計。例如，在形成後的某個相當長的時期內，也許多達幾億年，表面一定比今天熱得多。大大小小的隕石的持續撞擊，會進一步增加熱能，在地球歷史的最早階段，較大的撞擊體可能會突破冷卻了的地殼，暴露出下面的熔融物質。大量的火山氣體會隨著熔岩噴發到地表而被釋放到大氣中，產生的溫室效應比人類活動可能產生的任何效應都要嚴重得多。很有可能早期大氣的密度是今天的許多倍，海洋也很熱。有些人甚至認為，由於大氣壓力高，海洋可能比今天水的沸點還要熱。然而，我們所知道的生命對溫度相當敏感，沒有任何現代生物能在 100 攝氏度以上生存。在地表溫度下降到這個程度或更低程度之前，生命不太可能建立起來。

3. 大氣成分

   Although we do not know the precise composition of the early atmosphere, there has been enough progress made on this subject in recent years that it is possible to say with some certainty that the methane-rich composition envisioned by Oparin, and the methane-ammonia-hydrogen mixture used by Miller in his experiments, are probably not very realistic. Based on studies of our closest neighbor planets, Mars and Venus, and also considering evidence from Earth’s sedimentary rocks, it seems probable that Earth’s early atmosphere was rich in carbon dioxide rather than methane. On both Mars and Venus, carbon dioxide is by far the most abundant gas in the atmosphere. On Earth it is a minor constituent. But there is an enormous amount of this compound buried in the sedimentary rocks of Earth’s crust, enough so that, if it were all released, our atmosphere would be much more like those of our neighboring planets. How did carbon dioxide gas end up in the crust? The answer lies in what geologists refer to as the carbon cycle. Through a series of chemical reactions, carbon dioxide from the atmosphere finds itself, in dissolved form, in the oceans. In seawater it combines with calcium to precipitate as calcium carbonate, the main constituent of limestone. Over geologic time so much carbon dioxide from the atmosphere has been converted to limestone in this fashion that there is more than 100,000 times as much stored as limestone as there is in the atmosphere.

   儘管我們不知道早期大氣的確切成分，但近年來在這個問題上已經取得了足夠的進展，可以肯定地說，奧帕林設想的富含甲烷的成分，以及米勒在他的實驗中使用的甲烷-氨-氫混合物，可能都不是非常現實的。根據對我們最近的鄰居行星，火星和金星的研究，同時考慮到來自地球沉積岩的證據，地球的早期大氣似乎很可能富含二氧化碳而不是甲烷。在火星和金星上，二氧化碳是迄今為止大氣中最豐富的氣體。在地球上，它是一個次要的成分。但是有大量的這種化合物埋藏在地殼的沉積岩中，足以使它全部釋放出來，我們的大氣層將更像我們鄰近的行星。二氧化碳氣體最後是如何出現在地殼中的？答案在於地質學家所稱的碳循環。通過一系列的化學反應，大氣中的二氧化碳以溶解的形式出現在海洋中。在海水中，它與鈣結合，沉澱為碳酸鈣，這是石灰石的主要成分。在地質時期，因為許多來自大氣層的二氧化碳已經以這種方式轉化為石灰石，以至於儲存在石灰石中的二氧化碳數量是大氣中的 100,000 倍以上。