地球生命的基石或許是由隕石帶來的

原文網址：https://www.sciencedaily.com/releases/2017/10/171002161248.htm

地球生命的基石或許是由隕石帶來的

By Wade Hemsworth

美國麥克馬斯特大學和德國馬克斯普朗克研究所的科學家表示，在37億年至45億年前生命必需的物質從隕石噴濺到許多溫暖的小池塘並溶解之後，於其中某處誕生了生命。

他們的計算結果顯示在富含養分的生命原湯中反覆出現的乾燥與濕潤環境，使得池塘中做為生命基石的基本分子鍵結成可以自我複製的RNA分子，構成了地球上第一組含有生命訊息的基因密碼。

研究人員從天文物理、地質、化學、生物和其他各領域的角度進行詳盡的研究和計算而得出此結論。雖然「溫暖的小池塘」的概念從達爾文那時就已經問世，研究人員如今才透過大量基於實證的計算證明其可行性。

Ben K.D. Pearce和Ralph Pudritz是這篇論文的主要作者，兩位皆任職於美國麥克馬斯特大學的起源研究所和天文物理系。他們表示現有證據顯示生命出現時地球仍在成形階段，當時陸地才剛從海面升起；隕石不斷轟炸地球，其中有些承裝著生命基石；可以過濾太陽紫外線的臭氧層保護罩也還沒有出現。

「之前從未有人真的去進行這些計算。」Pearce表示，「這是個相當宏大的起頭。我們對此感到十分興奮。」

Pudritz表示：「由於這項計算要輸入來自許多不同領域的大量數據，因此可以將所有資訊全部統籌起來實在非常驚人。過程中的每一步都自然而然地銜接到下一步。最終，它們結合起來呈現的是單獨一幅清晰的圖像，代表結果必有幾分真實。」

這項研究今日刊登於《美國國家科學院院刊》(Proceedings of the National Academy of Science)，合作研究人員包括了德國馬克斯普朗克天文研究所的Dmitry Semenov和Thomas Henning。

「要瞭解生命是如何開始，就要先瞭解地球在數十億年前是什麼樣子。如同我們研究顯示的，天文學提供了解答中很重要的一部份：太陽系形成過程中的細節會直接影響地球生命的起源。」其中一位共同作者，馬克斯普朗克天文研究所的Thomas Henning表示。

作者說生命的火苗點燃於RNA聚合物誕生的時刻。形成核苷酸的必要成分由隕石攜帶至這些池塘，而池水因為反覆地降雨、蒸發和流乾來回升降，使得這些成分的濃度提升到足以鍵結在一起的地步。論文表示乾燥與濕潤環境的結合，是發生鍵結的必要條件。

研究人員相信在部分案例中，某些分子鏈具備適合條件使得它們可以摺疊，並且從環境中吸引其他核苷酸而發生自發性複製，讓它們滿足了生命定義的其中一個條件。雖然這些聚合物還不算完美，但它們可以透過達爾文式的演化來逐漸進步，而這又滿足了另一項條件。

Pearce表示：「這是有關生命起源的實驗化學的聖杯。」

這種原始生命型式最終會發展出DNA。直到DNA演化出來以前，世界上僅有以RNA為基礎的生命。要到許久之後才會演化出以DNA作為基因藍圖的較高等生命。

「DNA太過複雜而無法成為第一種出現的生命型態。」Pudritz表示，「生命必須要以別的樣子開始，那就是RNA。」

研究人員的計算結果顯示在數以千計的池塘中都具有生命形成的必須條件，而關鍵合成過程所需的要件在其中一座池塘通通達成的可能性比在熱液噴泉大上許多。熱液噴泉是目前最具競爭力的理論認為生命出現的地方，其認為在海床熱液翻騰的裂隙當中，構成生命的元素在洶湧的熱水裡結合而成。作者在他們的新論文中表示熱泉不可能是生命誕生的環境，因為要讓RNA形成必備的鍵結作用，需要反覆出現的乾燥與濕潤環境。

計算結果也排除了形成生命的核苷酸來自宇宙塵的可能性。雖然這些塵埃確實會攜帶對的物質，但研究人員確定它們散落至地表的量不足以到達形成生命所需的濃度。在太陽系生命的早期，那時候的小行星遠比現在更加常見，它們可能會落到這數以千計的小池塘中，把生命的基石運送進來。

Pearce和Pudritz計畫於明年麥克馬斯特大學的生命起源實驗室開始運作時，將他們的理論付諸實驗，在此他們可以於密封環境下重現生命出現以前的地球環境。

Pudritz表示：「我們感到十分振奮可以將所有線索結合起來完成一篇理論論文，而對我們在實驗室能進行的工作提供清晰的預測與明確的想法。」

Meteorites may have brought building blocks of life to Earth

Life on Earth began somewhere between 3.7 and 4.5 billion years ago, after meteorites splashed down and leached essential elements into warm little ponds, say scientists at McMaster University and the Max Planck Institute in Germany.

Their calculations suggest that wet and dry cycles bonded basic molecular building blocks in the ponds’ nutrient-rich broth into self-replicating RNA molecules that constituted the first genetic code for life on the planet.

The researchers base their conclusion on exhaustive research and calculations drawing in aspects of astrophysics, geology, chemistry, biology and other disciplines. Though the “warm little ponds” concept has been around since Darwin, the researchers have now proven its plausibility through numerous evidence-based calculations.

Lead authors Ben K.D. Pearce and Ralph Pudritz, both of the McMaster’s Origins Institute and its Department of Physics and Astronomy, say available evidence suggests that life began when the Earth was still taking shape, with continents emerging from the oceans, meteorites pelting the planet – including those bearing the building blocks of life – and no protective ozone to filter the Sun’s ultraviolet rays.

“No one’s actually run the calculation before,” says Pearce. “This is a pretty big beginning. It’s pretty exciting.”

“Because there are so many inputs from so many different fields, it’s kind of amazing that it all hangs together,” Pudritz says. “Each step led very naturally to the next. To have them all lead to a clear picture in the end is saying there’s something right about this.”

Their work, with collaborators Dmitry Semenov and Thomas Henning of the Max Planck Institute for Astronomy, has been published today in the Proceedings of the National Academy of Science.

 “In order to understand the origin of life, we need to understand Earth as it was billions of years ago.  As our study shows, astronomy provide a vital part of the answer.  The details of how our solar system formed have direct consequences for the origin of life on Earth,” says Thomas Henning, from the Max Planck Institute for Astronomy and another co-author.

The spark of life, the authors say, was the creation of RNA polymers: the essential components of nucleotides, delivered by meteorites, reaching sufficient concentrations in pond water and bonding together as water levels fell and rose through cycles of precipitation, evaporation and drainage. The combination of wet and dry conditions was necessary for bonding, the paper says.

In some cases, the researchers believe, favorable conditions saw some of those chains fold over and spontaneously replicate themselves by drawing other nucleotides from their environment, fulfilling one condition for the definition of life. Those polymers were imperfect, capable of improving through Darwinian evolution, fulfilling the other condition.

“That’s the Holy Grail of experimental origins-of-life chemistry,” says Pearce.

That rudimentary form of life would give rise to the eventual development of DNA, the genetic blueprint of higher forms of life, which would evolve much later. The world would have been inhabited only by RNA-based life until DNA evolved.

“DNA is too complex to have been the first aspect of life to emerge,” Pudritz says. “It had to start with something else, and that is RNA.”

The researchers’ calculations show that the necessary conditions were present in thousands of ponds, and that the key combinations for the formation of life were far more likely to have come together in such ponds than in hydrothermal vents, where the leading rival theory holds that life began in roiling fissures in ocean floors, where the elements of life came together in blasts of heated water. The authors of the new paper say such conditions were unlikely to generate life, since the bonding required to form RNA needs both wet and dry cycles.

The calculations also appear to eliminate space dust as the source of life-generating nucleotides. Though such dust did indeed carry the right materials, it did not deposit them in sufficient concentration to generate life, the researchers have determined. At the time, early in the life of the solar system, meteorites were far more common, and could have landed in thousands of ponds, carrying the building blocks of life.

Pearce and Pudritz plan to put the theory to the test next year, when McMaster opens its Origins of Life laboratory that will re-create the pre-life conditions in a sealed environment.

“We’re thrilled that we can put together a theoretical paper that combines all these threads, makes clear predictions and offers clear ideas that we can take to the laboratory,” Pudritz says.

原始論文：Ben K. D. Pearce, Ralph E. Pudritz, Dmitry A. Semenov, Thomas K. Henning. Origin of the RNA world: The fate of nucleobases in warm little ponds. Proceedings of the National Academy of Sciences, 2017; 201710339 DOI: 10.1073/pnas.1710339114

引用自：McMaster University. "Did life on Earth start due to meteorites splashing into warm little ponds?."