原文網址：https://www.ucl.ac.uk/news/2022/apr/diverse-life-forms-may-have-evolved-earlier-previously-thought
倫敦大學學院(UCL)的研究人員挑戰了一般對於生命何時出現的觀點，他們發表的新研究提出至少在37.5億年就已經出現多樣化的微生物形式。
這些一公分長，像是梳子一般分岔的平行排列細絲是由紅色的赤鐵礦組成，其中有些受到彎曲，或是具有微管和不同種類的赤鐵礦球體。它們是地球最古老的微體化石——活在海底熱泉附近的海床，以代謝鐵、硫、二氧化碳為生。採自加拿大魁北克的努夫亞吉圖克表岩帶。圖片來源：D. Papineau
在發表於《科學前緣》(Science Advances)的文章中，研究團隊分析了採自加拿大魁北克一塊拳頭大小的岩石，年代估計為37.5億年至42.8億年。在之前發表於《自然》(Nature)的文章，團隊在岩石裡發現了看似由細菌形成的微小細絲、扭結與微管。
相較於多數人認可的遠古生命留下的最初跡象，這些構造的年代還要再早個3億年左右；然而，並非所有科學家都同意它們是由生命形成。
進一步地全面分析這塊岩石之後，團隊最近發現了更大更複雜的構造：一條快要一公分的細枝，其中一端具有平行的分岔；還有數百個變形的球體(橢球)以及許多微管和細絲。
研究團隊表示雖然可以認定這些構造中的一部份是由化學反應隨機產生，但是具有平行的分岔、像是樹木一般的細枝最有可能的來源為生物，因為光由化學反應產生的構造中目前還沒有發現類似的。
團隊也提供證據指出細菌是透過哪些不同的方式獲取能量。他們發現岩石中已經變成礦物的化學反應副產物符合依靠鐵、硫維生的古代微生物；它們或許還能利用光和二氧化碳，而這種型態的光合作用並未牽扯到氧氣的生成。
研究人員表示這些新發現指出原始地球已經具有各式各樣的微生物，可能在地球形成的短短三億年後就出現了。
主要作者Dominic
Papineau博士(任職於UCL地球科學系、UCL倫敦奈米科技中心、UCL行星科學中心、中國地質大學)表示：「透過許多條不同的線索，我們的研究強力證明了在37.5億年前到42.8億年前，就已經有好幾種不同的細菌生存在地球上。」
「這代表生命可能在地球形成之後的短短3億年間就誕生了。從地質時間上來說這是相當快的――大約是太陽環繞銀河一周的時間。」
研究人員在這項研究中探討的對象為Papineau博士2008年從魁北克的努夫亞吉圖克表岩帶(Nuvvuagittuq
Supracrustal Belt ,NSB)採集的岩石。過往是一片海床的NSB含有地球上已知最古老的某些沉積岩。科學家認為它們形成於一座熱泉系統附近，在這種環境中，受到岩漿加熱而富含鐵的海水會從海床上的裂縫冒出。
研究團隊把岩石切成跟紙片一樣薄(100微米)以仔細觀察像是化石的微小構造。這些構造是由赤鐵礦(一種鐵氧化物，也就是鐵鏽)所組成，外面則包有石英。這次研究人員利用鑽石鋸切出來的薄片是之前的兩倍厚，因此能夠看到岩石中更加大型的赤鐵礦構造。
他們將這些構造和成分與年代較近的化石，以及現在熱泉系統附近的鐵氧化菌相比。結果發現這些彎曲的細絲、平行分岔的構造以及變形的球體(不規則橢球)在現代也能找到相應的構造，它們位在夏威夷附近的羅希海底山，以及北極海和印度洋的其他熱泉系統等地。
研究團隊除了運用許多種光學顯微鏡和拉曼顯微鏡(用來測量光的散射)來分析樣品，他們也利用兩種高解析度的影像技術拍攝數千張圖片之後，再透過超級電腦來製作岩石薄片的數位版本。這兩種技術的第一種為微電腦斷層掃描(microtomography,
micro-CT)，其運用X光來觀察岩石裡的赤鐵礦。第二場則是聚焦離子束，它可以把岩石削成只有200奈米厚的薄片，再結合電子顯微鏡來拍攝每張薄片之間的影像。
研究人員把兩種技術得到的圖像堆疊之後，創造出的三維模型可以用來探討不同的對象。這些模型讓他們確定赤鐵礦細絲有受到彎折並扭曲，而且含有有機碳，現代以鐵維生的細菌都有這些特徵。
在分析結果中，團隊的結論為赤鐵礦的構造不可能是岩石受到億萬年來的高溫與擠壓(變質作用)所產生。他們也指出這些構造在細粒石英(受到的變質作用較輕微)中保存得比粗粒石英(經歷較強的變質作用)更好。
研究人員也探討了含有化石的岩石裡有多少稀土元素，他們發現這些岩石和其他古老的岩石樣品一樣多。結果確認了這些海床沉積物和周圍的火山岩是形成於同一時間，而非像某些研究人員提出的，是較年輕的岩石滲入之後的假象。
岩石的層理因為這顆亮紅色的赤鐵礦燧石(一種富含鐵和矽的岩石)結核而彎曲，其中含有管狀與絲狀的微體化石。這種稱為碧玉的礦物與右上方的深綠色火山岩接觸，代表了海底熱泉在海床上沉澱下來的物質。攝自加拿大魁北克的努夫亞吉圖克表岩帶，以加拿大的25分硬幣為比例尺。圖片來源：D.
Papineau
在此研究之前，前人發表最老的化石發現於澳洲西部，年代為34.6億年。不過仍有部分科學家質疑其身分是否為化石，主張它們的來源並非生物。
UCL地球科學系、UCL化學工程系、UCL倫敦奈米科技中心、UCL行星科學中心、倫敦大學伯貝克學院，以及美國地質調查局、加拿大紐芬蘭紀念大學、卡內基科學研究院、里茲大學、中國武漢地質大學的研究人員參與了這項新研究。
研究經費來自UCL、加拿大卡內基研究院、中國武漢地質大學、中國國家科學基金會、中國科學院以及中國的111計畫。
Diverse life forms may have evolved
earlier than previously thought
Diverse microbial life existed on Earth
at least 3.75 billion years ago, suggests a new study led by UCL researchers
that challenges the conventional view of when life began.
For the study, published in Science Advances, the research team analysed a fist-sized rock from
Quebec, Canada, estimated to be between 3.75 and 4.28 billion years old. In an
earlier Nature paper*, the team found
tiny filaments, knobs and tubes in the rock which appeared to have been made by
bacteria.
However, not all scientists agreed that these
structures – dating about 300 million years earlier than what is more commonly
accepted as the first sign of ancient life – were of biological origin.
Now, after extensive further analysis of the rock,
the team have discovered a much larger and more complex structure – a stem with
parallel branches on one side that is nearly a centimetre long – as well as
hundreds of distorted spheres, or ellipsoids, alongside the tubes and
filaments.
The researchers say that, while some of the
structures could conceivably have been created through chance chemical
reactions, the “tree-like” stem with parallel branches was most likely
biological in origin, as no structure created via chemistry alone has been
found like it.
The team also provide evidence of how the bacteria
got their energy in different ways. They found mineralised chemical by-products
in the rock that are consistent with ancient microbes living off iron, sulphur
and possibly also carbon dioxide and light through a form of photosynthesis not
involving oxygen.
These new findings, according to the researchers,
suggest that a variety of microbial life may have existed on primordial Earth,
potentially as little as 300 million years after the planet formed.
Lead author Dr Dominic Papineau (UCL Earth Sciences,
UCL London Centre for Nanotechnology, Centre for Planetary Sciences and China
University of Geosciences) said: “Using many different lines of evidence, our
study strongly suggests a number of different types of bacteria existed on
Earth between 3.75 and 4.28 billion years ago.”
“This means life could have begun as little as 300
million years after Earth formed. In geological terms, this is quick – about
one spin of the Sun around the galaxy.”
For the study, the researchers examined rocks from
Quebec’s Nuvvuagittuq Supracrustal Belt (NSB) that Dr Papineau collected in
2008. The NSB, once a chunk of seafloor, contains some of the oldest
sedimentary rocks known on Earth, thought to have been laid down near a system
of hydrothermal vents, where cracks on the seafloor let through iron-rich
waters heated by magma.
The research team sliced the rock into sections about
as thick as paper (100 microns) in order to closely observe the tiny
fossil-like structures, which are made of haematite, a form of iron oxide or
rust, and encased in quartz. These slices of rock, cut with a diamond-encrusted
saw, were more than twice as thick as earlier sections the researchers had cut,
allowing the team to see larger haematite structures in them.
They compared the structures and compositions to more
recent fossils as well as to iron-oxidising bacteria located near hydrothermal
vent systems today. They found modern-day equivalents to the twisting
filaments, parallel branching structures and distorted spheres (irregular
ellipsoids), for instance close to the Loihi undersea volcano near Hawaii, as
well as other vent systems in the Arctic and Indian oceans.
As well as analysing the rock specimens under various
optical and Raman microscopes (which measure the scattering of light), the
research team also digitally recreated sections of the rock using a
supercomputer that processed thousands of images from two high resolution
imaging techniques. The first technique was micro-CT, or microtomography, which
uses X-rays to look at the haematite inside the rocks. The second was focused
ion beam, which shaves away miniscule - 200 nanometre-thick - slices of rock,
with an integrated electron microscope taking an image in-between each
slice.
Both techniques produced stacks of images used to
create 3D models of different targets. The 3D models then allowed the
researchers to confirm the haematite filaments were wavy and twisted, and
contained organic carbon, which are characteristics shared with modern-day
iron-eating microbes.
In their analysis, the team concluded that the
haematite structures could not have been created through the squeezing and
heating of the rock (metamorphism) over billions of years, pointing out that
the structures appeared to be better preserved in finer quartz (less affected
by metamorphism) than in the coarser quartz (which has undergone more
metamorphism).
The researchers also looked at the levels of rare
earth elements in the fossil-laden rock, finding that they had the same levels
as other ancient rock specimens. This confirmed that the seafloor deposits were
as old as the surrounding volcanic rocks, and not younger imposter
infiltrations as some have proposed.
Prior to this discovery, the oldest fossils
previously reported were found in Western Australia and dated at 3.46 billion
years old, although some scientists have also contested their status as
fossils, arguing they are non-biological in origin.
The new study involved researchers from UCL Earth
Sciences, UCL Chemical Engineering UCL London Centre for Nanotechnology, and
the Centre for Planetary Sciences at UCL and Birkbeck College London, as well
as from the U.S. Geological Survey, the Memorial University of Newfoundland in
Canada, the Carnegie Institution for Science, the University of Leeds, and the
China University of Geoscience in Wuhan.
The research received support from UCL, Carnegie of
Canada, Carnegie Institution for Science, the China University of Geoscience in
Wuhan, the National Science Foundation of China, the Chinese Academy of
Sciences, and the 111 project of China.
原始論文：
1.
Dominic Papineau, Zhenbing She,
Matthew S. Dodd, Francesco Iacoviello, John F. Slack, Erik Hauri, Paul
Shearing, Crispin T. S. Little. Metabolically diverse primordial
microbial communities in Earth’s oldest seafloor-hydrothermal jasper. Science
Advances, 2022; 8 (15) DOI: 10.1126/sciadv.abm2296
2.
Matthew S. Dodd, Dominic Papineau, Tor
Grenne, John F. Slack, Martin Rittner, Franco Pirajno, Jonathan O’Neil, Crispin
T. S. Little. Evidence for early life in Earth’s oldest hydrothermal
vent precipitates. Nature, 2017; 543 (7643): 60 DOI: 10.1038/nature21377
引用自：University College London. "Diverse life
forms may have evolved earlier than previously thought."
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