在地球的遙遠過去，適合生命發展的條件也許曾經出現卻又消失

原文網址：www.sciencedaily.com/releases/2017/01/170117163939.htm

在地球的遙遠過去，適合生命發展的條件也許曾經出現卻又消失

由華盛頓大學領導的新研究發現，在生命真正穩定下來的十幾億年之前，地球海洋曾發展出適合複雜生命生長的環境，但之後卻逐漸消失。

這項研究以硒元素為工具來測量許久之前的氧氣含量，或許也能幫助尋找地球之外的生命訊號。

刊登於1月18日的《美國國家科學院院刊》(Proceedings of the National Academy of Sciences)的這篇論文，主要作者華盛頓大學地球與太空科學系的博士生Michael Kipp分析了沉積岩中硒元素的同位素比例，來測量20至24億年前地球大氣中氧含量的多寡。

Kipp於華盛頓大學的共同作者包括先前在地球與太空科學系擔任博士後研究員，現在則任教於蘇格蘭聖安德魯斯大學的Eva Stüeken，以及同時參與華盛頓大學天體生物計畫的教授Roger Buick。另一位共同作者是加州大學河濱分校的Andrey Bekker。研究人員表示這項研究有助於證實Bekker首創的理論。

Buick說：「複雜細胞存在的化石證據或許可追溯至17.5億年前。但是最早的化石並非代表曾經存活過的最古老生物，因為要成為化石而保存下來的機率是非常低的。」

「這項研究顯示在有化石紀錄之前，環境中已經有足夠的氧氣讓複雜細胞得以演化出來，甚至在生態系中具有重要地位。」他補充，「這不代表複雜細胞必定發生過上述現象，但它們的確可以發生。」

Kipp和Stüeken利用華盛頓大學同位素地球化學實驗室的質譜儀，來分析特定時期的頁岩(一種沉積岩)中微量的硒。他們藉此可以得知硒是否受到氧氣的出現，也就是氧化作用的影響而改變。氧化硒的化合物會在形成過後受到還原，而在岩石中留下同位素比例改變的紀錄。同時，當環境中含有許多氧氣時，硒的含量也會增加。

Buick表示之前認為地球氧氣含量變化的歷史是「沒有，接著有一些，然後變成相當多。但現在看起來氧氣在長達2.5億年左右的一段時期中曾經相當地多，之後又降回去。」

氧氣曾長時間保持在高水平具有相當重要的意義。Kipp強調：「僅管在此前後或許曾短暫出現偶爾可以支持這些生命的環境，但要讓它們繼續演化並成為生態系中的重要一份子，就得讓氧氣長時間維持在一定含量。」

Stüeken表示之前已經有人猜測曾發生氧氣增加的情形，但卻不清楚它的分布範圍有多廣。這項研究更加清楚地呈現出氧氣大量「溢出」的情形會是如何：「在大氣和海洋表層當中具有相當大量的氧氣，但在深海卻完全不是如此。」

是什麼原因造成氧氣含量戲劇性地增加，卻以同樣劇烈的速度減少？

「這是一則重要卻難以回答的問題。」Stüeken表示。「我們不知道該現象為什麼發生，也不知道為什麼結束。」

Buick說：「這在地球歷史上是一段前所未見的時期。如果你去觀察硒同位素隨時間變化的情形，此區間真的相當特殊。跟前後比較起來，一切都顯得如此不同。」

研究人員利用硒——由希臘文的月亮來命名的元素——作為探測久遠之前氧含量的有力工具，他們表示此方法也能幫助尋找地球以外的氧氣，甚至可能藉此找到生物。

他們指出未來世代的太空望遠鏡可以給予天文學家遠方行星的大氣成分。其中某些行星大小可能與地球相仿，或許大氣還含有相當可觀的氧氣。

「找出在地球遙遠的過去曾有段時間氧氣含量跟現今相差不遠，但是當時棲息的生物卻大相逕庭，意味著一個世界從遠端探測出具有豐富氧氣，不一定就保證也擁有複雜的生物圈。」Kipp表示。

Buick總結說：「這種新方法可以測量一個行星過去歷史當中的氧氣含量，由此可以了解此處是否有複雜的生物演化出來，且持續夠久而能夠演化出智慧生物。」

 

Conditions right for complex life may have come and gone in Earth's distant past

Conditions suitable to support complex life may have developed in Earth's oceans -- and then faded -- more than a billion years before life truly took hold, a new University of Washington-led study has found.

The findings, based on using the element selenium as a tool to measure oxygen in the distant past, may also benefit the search for signs of life beyond Earth.

In a paper published Jan. 18 in the Proceedings of the National Academy of Sciences, lead author Michael Kipp, a UW doctoral student in Earth and Space Sciences, analyzed isotopic ratios of the element selenium in sedimentary rocks to measure the presence of oxygen in Earth's atmosphere between 2 and 2.4 billion years ago.

Kipp's UW coauthors are former Earth and space sciences postdoctoral researcher Eva Stüeken -- now a faculty member at the University of St. Andrews in Scotland -- and professor Roger Buick, who is also a faculty member with the UW Astrobiology Program. Their other coauthor is Andrey Bekker of the University of California, Riverside, whose original hypothesis this work helps confirm, the researchers said.

"There is fossil evidence of complex cells that go back maybe 1 ¾ billion years," said Buick. "But the oldest fossil is not necessarily the oldest one that ever lived -- because the chances of getting preserved as a fossil are pretty low.

"This research shows that there was enough oxygen in the environment to have allowed complex cells to have evolved, and to have become ecologically important, before there was fossil evidence." He added, "That doesn't mean that they did -- but they could have."

Kipp and Stüeken learned this by analyzing selenium traces in pieces of sedimentary shale from the particular time periods using mass spectrometry in the UW Isotope Geochemistry Lab, to discover if selenium had been changed by the presence of oxygen, or oxidized. Oxidized selenium compounds can then get reduced, causing a shift in the isotopic ratios which gets recorded in the rocks. The abundance of selenium also increases in the rocks when lots of oxygen is present.

Buick said it was previously thought that oxygen on Earth had a history of "none, then some, then a lot. But what it looks like now is, there was a period of a quarter of a billion years or so where oxygen came quite high, and then sunk back down again."

The oxygen's persistence over a long stretch of time is an important factor, Kipp stressed: "Whereas before and after maybe there were transient environments that could have occasionally supported these organisms, to get them to evolve and be a substantial part of the ecosystem, you need oxygen to persist for a long time."

Stüeken said such an oxygen increase has been guessed at previously, but it was unclear how widespread it was. This research creates a clearer picture of what this oxygen "overshoot" looked like: "That it was moderately significant in the atmosphere and surface ocean -- but not at all in the deep ocean."

What caused oxygen levels to soar this way only to crash just as dramatically?

"That's the million-dollar question," Stüeken said. "It's unknown why it happened, and why it ended."

"It is an unprecedented time in Earth's history," Buick said. "If you look at the selenium isotope record through time, it's a unique interval. If you look before and after, everything's different."

The use of selenium -- named after the Greek word for moon -- as an effective tool to probe oxygen levels in deep time could also be helpful in the search for oxygen -- and so perhaps life -- beyond Earth, the researchers said.

Future generations of space-based telescopes, they note, will give astronomers information about the atmospheric composition of distant planets. Some of these could be approximately Earth-sized and potentially have appreciable atmospheric oxygen.

"The recognition of an interval in Earth's distant past that may have had near-modern oxygen levels, but far different biological inhabitants, could mean that the remote detection of an oxygen-rich world is not necessarily proof of a complex biosphere," Kipp said.

Buick concluded, "This is a new way of measuring oxygen in a planet's historical past, to see whether complex life could have evolved there and persisted long enough to evolve into intelligent beings."

原始論文：Michael A. Kipp, Eva E. Stüeken, Andrey Bekker, and Roger Buick. Selenium isotopes record extensive marine suboxia during the Great Oxidation Event. Proceedings of the National Academy of Sciences, January 2017 DOI:10.1073/pnas.1615867114

引用自：University of Washington. "Conditions right for complex life may have come and gone in Earth's distant past." ScienceDaily. ScienceDaily, 17 January 2017.