科學家找到了也許是地球生命出現的最早證據

Scientists may have found the earliest evidence of life on Earth

科學家找到了也許是地球生命出現的最早證據

When did life on Earth begin? Scientists have dug down through the geologic record, and the deeper they look, the more it seems that biology appeared early in our planet’s 4.5-billion-year history. So far, geologists have uncovered possible traces of life as far back as 3.8 billion years. Now, a controversial new study presents potential evidence that life arose 300 million years before that, during the mysterious period following Earth’s formation.

地球上的生命何時誕生？科學家長久以來不停地深堀地質記錄以期找到解答。隨著他們越加深入，生物在地球45億年的歷史上似乎顯得越早出現。目前為止，地質學家找到疑似由生命留下來的蹤跡可追溯至38億年前。現今，一則頗受爭議的新研究發表的證據顯示生命可能比這還要早3億年誕生—在那段地球甫形成，至今仍充滿謎團的時期。

The clues lie hidden in microscopic flecks of graphite—a carbon mineral—trapped inside a single large crystal of zircon. Zircons grow in magmas, often incorporating other minerals into their crystal structures of silicon, oxygen, and zirconium. And although they barely span the width of a human hair, zircons are nearly indestructible. They can outlast the rocks in which they initially formed, enduring multiple cycles of erosion and deposition.

這些線索隱藏在石墨(一種碳質礦物)的微小顆粒之中，而它們又被包覆在體積較大的鋯石(zircon)結晶裡面。在鋯石結晶於岩漿逐漸生長的過程中，常有其他礦物會被困在它由矽、氧、鋯組成的結晶構造內。雖然鋯石甚少成長至比人類毛髮還要寬的尺寸，但它幾乎是堅不可摧的。它們的壽命遠遠長過它們形成時所待的岩石，且經歷多次侵蝕沉積循環後仍能毫髮無傷。

In fact, although the oldest rocks on Earth date back only 4 billion years, researchers have found zircons up to 4.4 billion years old. These crystals provide a rare glimpse into the first chapter of Earth’s history, known as the Hadean eon. “They are pretty much our only physical samples of what was going on on the Earth before 4 billion years ago,” says Elizabeth Bell, a geochemist at the University of California, Los Angeles (UCLA), and lead author of the new study, published online today in the Proceedings of the National Academy of Sciences.

事實上，雖然現今地球上已知最古老岩石的年齡僅僅40億年，研究人員早已發現年代為44億年的鋯石顆粒。這些晶體讓我們有機會驚鴻一瞥地球歷史序章，科學家稱之為冥古宙時(Hadean eon)的樣貌。「或許能讓我們一窺地球40億年之前有何事物發生的實際樣品僅只如此。」加州大學洛杉磯分校(UCLA)的地球化學家Elizabeth Bell說，她是此篇發表在10/26《美國國家科學院院刊》(Proceedings of the National Academy of Sciences)的新研究的第一作者。

In the study, Bell and her colleagues examined zircons from the Jack Hills in Western Australia, a site that has yielded more Hadean samples than anywhere else on Earth, searching for inclusions of carbon minerals like diamonds and graphite. The mere presence of these minerals does not prove biology existed when the zircons formed, but it does provide the opportunity to look for chemical signs of life. The team eventually found small bits of potentially undisturbed graphite in one 4.1 billion-year-old crystal. The graphite has a low ratio of heavy to light carbon atoms—called isotopes—consistent with the isotopic signature of organic matter. “On Earth today, if you were looking at this carbon, you would say it was biogenic,” Bell says. “Of course, that’s more controversial for the Hadean.”

在此研究中，Bell和她的同僚檢視了來自澳洲西部Jack Hills的鋯石。於此地區發現的冥古宙樣品數量之多為世上首屈一指。他們的目標是尋找鋯石內的碳質礦物包裹體(inclusion)，像是石墨或鑽石。僅是出現這些礦物並不能證明鋯石形成當下有生物存在，但我們有機會能從中找到生命存在的蛛絲馬跡。研究團隊最終找到了一顆41億年前的鋯石，其中含有的微量石墨可能尚未受到汙染。這些石墨的碳原子較重和較輕者(同位素(isotope))之間的比例偏低，與有機物的碳同位素訊號相當一致。「如果今日在地球上看到帶有這種訊號的碳，你會說他來自生物體。」Bell說，「當然，在時空背景是冥古宙的情況下，爭議會大上許多。」

The authors list several nonbiological processes that could explain their findings, but they favor the idea that the graphite started out as organic matter in sediments that got dragged into the Earth’s mantle during the collision of tectonic plates. As the sediments melted to form magma, the elevated temperatures and pressures transformed the carbon into graphite, which eventually found its way into a zircon crystal.

作者雖然也有列出數種非生物作用或許能解釋他們的發現，但他們仍偏好這些石墨一開始是沉積物中的有機物，之後在板塊碰撞時被拖入地球地函中的說法。隨著沉積物慢慢熔融成岩漿，逐漸升高的溫度和壓力會把有機物轉變成石墨，最後被鋯石晶體囚禁於其中。

If this story is true, and life existed 4.1 billion years ago, Bell says that the new results would corroborate growing evidence of a more hospitable early Earth than scientists once imagined. “The traditional view of the Earth’s first few hundred million years was that this was a sterile, lifeless, hot planet that was constantly being bombarded by meteorites,” she says. But partly thanks to the wealth of information revealed by the Jack Hills zircons in recent years, scientists have come to see the early Earth as much milder and more amenable to life.

若此故事屬實，即代表了生命41億年前就已存在。Bell說這跟越來越多的證據結合起來，顯示了早期地球比科學家曾經認為的還要宜人。「傳統觀點認為地球在最初數億年是顆貧瘠、了無生氣、熾熱的星球，且時常被隕石雨轟炸。」她說，「然而，部分歸功於近幾年來Jack Hills鋯石提供的豐富資訊，科學家已經開始認為早期地球是處更溫和，且更適合生命成長的地方。」

“We know there was liquid water,” says Mark van Zuilen, a geomicrobiologist at the Paris Institute of Earth Physics. “There’s nothing that holds us back from assuming life was there.” However, van Zuilen and others say they’re not sure the new study provides compelling evidence that it was.

「我們知道那時候的地球有液態水。」Mark van Zuilen說，他是巴黎地球物理研究所的地質微生物學家。「沒有任何證據可以遏止我們推論當時生命已經存在。」然而，van Zuilen和其他人並不確定此篇新研究提出的證據具備足夠說服力，能夠證明生命已經出現。

Some of this circumspection has roots in recent history. In 2008, researchers announced that diamond-graphite inclusions in 4.3-billion-year-old zircons had potentially biological signatures, inspiring Bell and her team to start looking through UCLA’s own collection of Jack Hills crystals. But subsequent analysis showed the 2008 inclusions came from lab contamination, not early Earth. In the new study, the researchers took measures to prevent similar problems.

這樣慎重的態度其來有自。發生在最近的2008年，另一組研究人員聲稱他們在43億年前的鋯石中，發現的鑽石-石墨包裹體可能含有生物訊號。這也啟發了Bell和她的團隊開始仔細檢視UCLA擁有的Jack Hills結晶樣品。但隨後的分析結果顯示2008年的包裹體來自於實驗室汙染而非早期地球。因此在這項新研究中，研究人員採取許多措施來預防類似的問題發生。

“That one negative experience doesn’t mean nobody should try again,” says John Eiler, a geologist at the California Institute of Technology in Pasadena. “But let’s just say, I’m cautious.” For one, he says, researchers need to settle some important debates, like whether the inclusions in Hadean zircons truly preserve original material, or if they’ve been altered, for example, during a later bout of metamorphism. He also questions whether organic matter can survive in magma chambers long enough to form graphite, casting doubt on the proposed mechanism.

「一次失敗的經驗並非表示不應該有任何人再去嘗試。」John Eiler說，他是加州理工學院的地質學家。「但這麼說吧，對此我十分保留。」他說，身為一名研究人員必須提出一些重要的論點，比方說冥古宙鋯石中的包裹體是否真得保有原本的物質，或者他們其實已經被置換掉了，像是在形成之後的連串變質作用可能造成的結果。他同時也質疑作者提出的機制：有機物是否能在岩漿庫存活夠久而形成石墨。

Those issues aside, most scientists—including the authors—agree that the data do not yet exclude nonbiological explanations. Many abiotic processes can produce carbon with isotopic signatures similar to organic matter. For instance, the graphite could contain carbon from certain kinds of meteorites, which have light isotopic compositions. Alternatively, some invoke chemical processes, like the so-called Fischer-Tropsch reactions, in which carbon, oxygen, and hydrogen react with a catalyst like iron to form methane and other hydrocarbons. Such reactions probably occurred near hydrothermal vents in the Hadean, van Zuilen says, and can impart isotopic signatures that are indistinguishable from biological materials.

除了這些議題之外，大部分的科學家，包括作者本身也同意這些數據並不能完全排除非生物來源的解釋。許多非生物作用生成的碳可以與有機物有類似的同位素訊號。比如某些種類隕石中的石墨會含有較輕的碳同位素組成。另外也有人提出一些化學作用也會生成類似的產物，像是費雪－闕布希反應(Fischer-Tropsch reaction)，這是一種碳、氧、氫在環境中有鐵這類催化劑的情況下，生成甲烷和其他碳氫化合物的反應。van Zuilen說在冥古宙的熱泉附近可能會發生這類反應，它們產生的碳同位素訊號可能會無法與生物物質區分開來。

One way to settle the question that doesn’t rely on isotopes involves studying Mars, which, unlike Earth, still has rocks older than 4 billion years on its surface. “If we can find evidence for the existence of life on Mars at that time, then it will be easier to argue the case that it was also present on Earth,” says Alexander Nemchin, a geochemist at Curtin University in Bentley, Australia, and lead author of the 2008 study on diamond inclusions.

要解決這件問題而不依靠同位素的方法之一，便是研究火星上的岩石。因為火星與地球不同，它的表面仍然有許多年代超過40億年的岩石。「如果我們可以找到證據說明那時火星上有生命存在，那我們要證明地球也是如此便簡單的多。」澳洲科廷大學的地球化學家Alexander Nemchin說，他是2008年鑽石包裹體研究的第一作者。

For now, scientists must make do with zircons, the only materials that preserve any record—however cryptic—of the Hadean eon. Bell acknowledges the need to test her team’s hypothesis on additional samples. She says researchers must make a concerted effort to find more Hadean carbon in Jack Hills zircons and see if it too has potentially biological origins. “Hopefully we didn’t just chance on the one freak zircon that had graphite in it,” she says. “Hopefully there is actually a fair amount of it.”

目前科學家仍須仰賴鋯石來研究冥古宙。儘管是如此的難以解讀，它們畢竟是唯一保存並記錄當時資訊的物質。Bell坦承她的團隊仍須更多樣品來驗證他們的假說。她說研究人員必須要同心協力尋找更多包裹在Jack Hills鋯石中的碳，並且檢視它們是否有可能來自生物。「我希望我們找到一顆含有石墨的奇特鋯石並非偶然。」她說，「希望這樣的鋯石事實上為數眾多。」

原文網址：http://news.sciencemag.org/biology/2015/10/scientists-may-have-found-earliest-evidence-life-earth

原作者：Julia Rosen