研究顯示地球35億年前已經生機盎然

原文網址：https://www.sciencedaily.com/releases/2019/02/190208085855.htm

研究顯示地球35億年前已經生機盎然

地球在35億年前就已經擁有生命，但它們是勉強苟活或是欣欣向榮？由東京工業大學的地球生命研究所(ELSI)和其他機構領導的團隊，進行的新研究給出了答案。在硫同位素的比例中，紀錄了數十億年前微生物的代謝狀況，這份紀錄跟研究的預測相同，顯示古代海洋裡的生命已經蓬勃發展。科學家利用這類數據，可以從地球化學紀錄更加深入地探討古代的細胞狀態及生態。

科學家想要知道生命在地球已經存在了多久。如果生命的歷史跟地球的年紀幾乎一樣，代表形成生命並不困難，因此在整個宇宙應該相當常見。反之，如果生命花了很久的時間才形成，就代表生命需要相當特殊的條件才能誕生。雖然微生物在年代久遠的地質紀錄中有留下一些實體證據顯示它們曾經存在，但這些化石的保存狀況十分差勁，不像數十億年之後才出現的恐龍，留下的骨骸還在世界各地的博物館展出。因此，科學家便要運用其他方法來找出地質紀錄中是否含有生命的蹤跡。

週期表上的化學元素是以原子核內的質子數量來定義，比方氫原子有一個質子、氦有兩個、碳則有六個。除了質子之外，絕大多數的原子核內還有中子，它的重量跟質子差不多，但不帶電荷。質子數量相同但中子數量不同的原子，就稱為同位素。雖然許多同位素具有放射性並會因此衰變成別種元素，有些同位素卻不會發生這種反應，故稱作「穩定」同位素。比方說，碳的穩定同位素就有碳12(簡寫成¹²C，具有六個質子和六個中子)和碳13(簡寫成¹³C，具有六個質子和七個中子)這兩種。目前我們在地球上找到最古老的微生物證據，就是以穩定同位素的形式呈現。

包括人類在內，所有活著的生物都會「吃喝拉撒」；也就是說，生物都會攝取食物並把廢物排泄出來。微生物的食物通常都是在環境中能找到的簡單化合物，比方說某些微生物可以攝取二氧化碳，將其作為碳源來建構細胞。自然界產生的二氧化碳含有的碳12和碳13的比例大致上是固定的。然而，¹²CO₂的質量比¹³CO₂輕了約2%，造成¹²CO₂分子擴散得比較快而可以更快速地進行反應，使得微生物本身的同位素組成變輕，含有較多的碳12。這些微生物死掉的遺骸保留在化石紀錄的時候，它們的穩定同位素訊號也會跟著留存下來，而我們可以測量出來。這類反應造成的同位素組成，或稱「同位素訊號」可以非常精準地指出它們的成因是微生物。

除了碳以外，生物也需要別的元素才能生存，比方說硫。硫元素有16個質子，在自然界最常見的三種穩定硫同位素分別為擁有16個中子的³²S、17個中子的 ³³S和18個中子的³⁴S。許多微生物進行代謝時用的燃料為硫酸鹽，這是一種存在於自然界，跟四個氧原子鍵結的含硫化合物。反應過程中會排出另一種含硫化合物――硫化物。這些由古代微生物代謝後產生的硫化物廢物會儲存在地質紀錄中，而我們便可以分析像黃鐵礦(FeS₂)之類的礦物來得到硫同位素的比例。微生物留下的硫同位素記錄了以含硫化合物為主的生物代謝作用，最遠可追溯至大約35億年前。過往有數百篇研究探討硫酸鹽代謝後產生的硫同位素訊號，為何在古代和現代都有很大的變化。

這項新研究是首先探討遠古生物代謝旺盛程度的幾篇論文之一，科學家闡明了在微生物代謝硫的作用中一項很重要的生物變因，並釐清了什麼樣的細胞狀態會導致哪種類型的硫同位素分餾。得知代謝作用如何改變穩定同位素的比例後，可以預測生物會留下什麼樣的同位素訊號，因此可以把代謝作用和同位素訊號之間做出連結。30幾億年前記錄微生物硫酸鹽代謝作用的硫同位素比例，和這份論文中預測微生物代謝旺盛時會產生的硫同位素比例一致，顯示生命在遠古海洋的活動十分旺盛。ELSI的副教授Shawn McGlynn稱呼這項研究開拓的新研究領域為「演化和同位素酵素學」(evolutionary and isotopic enzymology)。利用這類資料，科學家接下來可以探討別的元素的代謝作用，像是碳和氮。此外，對酵素的演化和地球歷史有更多的瞭解之後，也能更全面的解釋地球化學紀錄跟細胞狀態和生態之間的關聯。

Research suggests life thrived on Earth 3.5 billion years ago

3.5 billion years ago Earth hosted life, but was it barely surviving, or thriving? A new study carried out by a multi institutional team with leadership including the Earth-Life Science Institute (ELSI) of Tokyo Institute of Technology (Tokyo Tech) provides new answers to this question. Microbial metabolism is recorded in billions of years of sulfur isotope ratios that agree with this study's predictions, suggesting life throve in the ancient oceans. Using this data, scientists can more deeply link the geochemical record with cellular states and ecology.

Scientists want to know how long life has existed on Earth. If it has been around for almost as long as the planet, this suggests it is easy for life to originate and life should be common in the Universe. If it takes a long time to originate, this suggests there were very special conditions that had to occur. Dinosaurs, whose bones are presented in museums around the world, were preceded by billions of years by microbes. While microbes have left some physical evidence of their presence in the ancient geological record, they do not fossilize well, thus scientists use other methods for understanding whether life was present in the geological record.

Presently, the oldest evidence of microbial life on Earth comes to us in the form of stable isotopes. The chemical elements charted on the periodic are defined by the number of protons in their nuclei, for example, hydrogen atoms have one proton, helium atoms have two, carbon atoms contain six. In addition to protons, most atomic nuclei also contain neutrons, which are about as heavy as protons, but which don't bear an electric charge. Atoms which contain the same number of protons, but variable numbers of neutrons are known as isotopes. While many isotopes are radioactive and thus decay into other elements, some do not undergo such reactions; these are known as "stable" isotopes. For example, the stable isotopes of carbon include carbon 12 (written as ¹²C for short, with 6 protons and 6 neutrons) and carbon 13 (¹³C, with 6 protons and 7 neutrons).

All living things, including humans, "eat and excrete." That is to say, they take in food and expel waste. Microbes often eat simple compounds made available by the environment. For example, some are able to take in carbon dioxide (CO₂) as a carbon source to build their own cells. Naturally occurring CO₂ has a fairly constant ratio of ¹²C to ¹³C. However, ¹²CO₂ is about 2 % lighter than ¹³CO₂, so ¹²CO₂ molecules diffuse and react slightly faster, and thus the microbes themselves become "isotopically light," containing more ¹²C than ¹³C, and when they die and leave their remains in the fossil record, their stable isotopic signature remains, and is measurable. The isotopic composition, or "signature," of such processes can be very specific to the microbes that produce them.

Besides carbon there are other chemical elements essential for living things. For example, sulfur, with 16 protons, has three naturally abundant stable isotopes, ³²S (with 16 neutrons), ³³S (with 17 neutrons) and ³⁴S (with 18 neutrons). Sulfur isotope patterns left behind by microbes thus record the history of biological metabolism based on sulfur-containing compounds back to around 3.5 billion years ago. Hundreds of previous studies have examined wide variations in ancient and contemporary sulfur isotope ratios resulting from sulfate (a naturally occurring sulfur compound bonded to four oxygen atoms) metabolism. Many microbes are able to use sulfate as a fuel, and in the process excrete sulfide, another sulfur compound. The sulfide "waste" of ancient microbial metabolism is then stored in the geological record, and its isotope ratios can be measured by analyzing minerals such as the FeS₂ mineral pyrite.

This new study reveals a primary biological control step in microbial sulfur metabolism, and clarifies which cellular states lead to which types of sulfur isotope fractionation. This allows scientists to link metabolism to isotopes: by knowing how metabolism changes stable isotope ratios, scientists can predict the isotopic signature organisms should leave behind. This study provides some of the first information regarding how robustly ancient life was metabolizing. Microbial sulfate metabolism is recorded in over a three billion years of sulfur isotope ratios that are in line with this study's predictions, which suggest life was in fact thriving in the ancient oceans. This work opens up a new field of research, which ELSI Associate Professor Shawn McGlynn calls "evolutionary and isotopic enzymology." Using this type of data, scientists can now proceed to other elements, such as carbon and nitrogen, and more completely link the geochemical record with cellular states and ecology via an understanding of enzyme evolution and Earth history.

原始論文：Min Sub Sim, Hideaki Ogata, Wolfgang Lubitz, Jess F. Adkins, Alex L. Sessions, Victoria J. Orphan, Shawn E. McGlynn. Role of APS reductase in biogeochemical sulfur isotope fractionation. Nature Communications, 2019; 10 (1) DOI: 10.1038/s41467-018-07878-4

引用自：Tokyo Institute of Technology. "Life thrived on Earth 3.5 billion years ago, research suggests: Scientists use stable sulfur isotopes to understand ancient microbial metabolism." ScienceDaily. ScienceDaily, 8 February 2019.