生命留下的最古老痕跡：科學家在古代岩石中發現殘存下來的微生物
Isabelle Dubach
澳洲西部的某些岩石是世上最古老的岩石，科學家在其中發現了保存十分良好的微生物遺骸――這對該領域來說是相當重大的進展，提供了地球生命是如何誕生的線索。
年代為35億年的德雷瑟層中的黃鐵礦化疊層石在顯微鏡底下的照片。又稱作愚人金的黃鐵礦勾勒出疊層石的輪廓。圖片來源：UNSW
新南威爾斯大學的研究人員在澳洲西部皮爾巴拉地區，年代久遠的德雷瑟層中發現疊層石(stromatolite，微生物形成的化石構造)裡面具有有機物。
自從在1980年代發現疊層石之後，科學家就一直認為它們是由生物形成的。然而，將近四十年來卻一直未能證明這項理論，原因是疊層石的結構儘管是相當強烈的證據，科學家卻遲遲無法找到決定性的證物：成功保存下來的有機物殘骸。但是最近發表在知名期刊《地質》(Geology)的研究改變了此點。
領導這項研究的Raphael
Baumgartner博士說：「這項發現令我們相當興奮。有史以來我們首次能向世人證明，這些疊層石是地球極為古老的生命留下來的絕佳證據。」Baumgartner博士是新南威爾斯大學澳洲天體生物學中心Martin
Van Kranendonk教授團隊裡的副研究員。
Van Kranendonk教授表示在尋找如此古老的生命時，這項發現是團隊迄今找到的證據中最為強力的。
Van Kranendonk教授說：「結果讓我們對這些岩石有更加深入的認識，探索遠古生命的科學也因此有了重大進展；更特別的是，對於尋找火星生命來說這同樣具有十分重大的幫助。我們現在有了新的目標與方法，來尋找遠古生命留下的痕跡。」
挖得更深，看得更細
自從在1980年發現德雷瑟層之後，科學家就一直猜測這些構造是否真由微生物形成，也就是生物留下的最古老痕跡。
Baumgartner博士表示：「遺憾的是，學術圈對結構型的生命跡象抱持懷疑的態度。因此德雷瑟層疊層石的起源究竟為何，長久以來一直是個極具爭議的主題。」
「進行這項研究時，我花了許多時間在實驗室裡運用顯微分析技術來仔細觀察岩石樣品，希望可以證明我們的理論無懈可擊。」
德雷瑟層的疊層石通常發現於岩石表面，因此風化得十分嚴重。在這項研究中，科學家探討的疊層石樣品來自於更加深層、位於風化剖面下方的岩石，因此保存得相當良好。
Baumgartner博士說：「透過觀察鑽井得到的岩芯樣品，使我們可以看到將遠古微生物完美捕捉下來的瞬間影像。」
Baumgartner分析岩石的時候運用了各種尖端的顯微分析工具與技術，像是高倍率電子顯微鏡、光譜分析和同位素分析法。
他發現疊層石的組成基本上是黃鐵礦(又稱「愚人金」的一種礦物)，而且其中含有有機物。
Baumgartner表示：「發現疊層石裡的黃鐵礦把有機物保存下來讓我們十分興奮。我們看見保存相當完整而且黏在一起的絲線，這是微生物膜遺留下來的典型特徵。」
研究人員表示之前從未在德雷瑟層觀察到這類殘留物，而在顯微鏡底下實際看到真的令他們無比興奮。
「我真的非常驚訝――在計畫開始之前，我們從來沒有預期到可以發現如此強力的證據。我還記得那個晚上，我終於明瞭我在用電子顯微鏡看著的，正是生物膜留下的痕跡。那時大概是晚上11點，一切都豁然開朗了起來。之後我一直待到凌晨三四點，因為太過興奮所以不停記下一張又一張的影像，完全忘了時間的流逝。」
皮爾巴拉的岩石，這些遠古疊層石的產地。圖片來源：UNSW
尋找火星生命的線索
新南威爾斯大學的博士候選人Tara
Djokic 是Baumgartner博士的同事，就在兩年多前她也在澳洲西部同一地區的溫泉沉積物中發現疊層石，因而把陸地上微生物出現的最古老證據往前推進5.8億年。
Baumgartner博士說：「Tara的重要發現是在此處找到保存格外良好的矽華(geyserite)沉積物，代表當時這裡有間歇泉，也就是會有流體從陸地噴出來。」
「她的研究著重在古代環境整體的地質背景，結果佐證了生命是起源於陸地而非海洋的理論；而我的研究則是對此處找到的疊層石構造進行更為深入的細節探討。」
科學家表示這兩項研究可以幫我們解答一個核心問題：人類來自何方？
Baumgartner博士說：「解開生命起源於何處，對於了解人類的祖宗來說非常重要。由此也可以幫我們找出生命還有可能在哪些地方誕生，像是其他星球什麼樣的環境中有機會孕育出生命。」
NASA和歐洲太空總署(ESA)上個月才跟Martin
Van Kranendonk在皮爾巴拉進行為期一周的訓練，學習在這些古老的岩石中辨認生命留下來的痕跡。這是Van
Kranendonk首度和專門研究火星的團隊分享他對皮爾巴拉地區的瞭解，其中包括NASA和ESA負責2020火星任務的主持人。
Van Kranendonk教授說：「在人類搜尋地外生命以及解開火星的奧秘時，澳洲擁有的古代岩石以及我們對它們的專業知識能做出如此重大的貢獻，實在讓我感到相當自豪。」
Earliest signs of life: scientists find microbial remains
in ancient rocks
Scientists have found exceptionally
preserved microbial remains in some of Earth’s oldest rocks in Western
Australia – a major advance in the field, offering clues for how life on Earth
originated.
The UNSW researchers found the organic matter in
stromatolites – fossilised microbial structures – from the ancient Dresser
Formation in the Pilbara region of Western Australia.
The stromatolites have been thought to be of biogenic
origin ever since they were discovered in the 1980s. However, despite strong
textural evidence, that theory was unproven for nearly four decades, because
scientists hadn’t been able to show the definitive presence of preserved
organic matter remains – until today’s publication in prestigious journal Geology.
“This is an exciting discovery – for the first time,
we’re able to show the world that these stromatolites are definitive evidence
for the earliest life on Earth,” says lead researcher Dr Raphael Baumgartner, a
research associate of the Australian Centre for Astrobiology in Professor
Martin Van Kranendonk’s team at UNSW.
Professor Van Kranendonk says the discovery is the
closest the team have come to a “smoking gun” to prove the existence of such
ancient life.
“This represents a major advance in our knowledge of
these rocks, in the science of early life investigations generally, and – more
specifically – in the search for life on Mars. We now have a new target and new
methodology to search for ancient life traces,” Professor Van Kranendonk says.
Drilling deep,
looking closely
Ever since the Dresser Formation was discovered in
the 1980, scientists have wondered whether the structures were truly microbial
and therefore the earliest signs of life.
“Unfortunately, there is a climate of mistrust of
textural biosignatures in the research community. Hence, the origin of the
stromatolites in the Dresser Formation has been a hotly debated topic,” Dr
Baumgartner says.
“In this study, I spent a lot of time in the lab,
using micro-analytical techniques to look very closely at the rock samples, to
prove our theory once and for all.”
Stromatolites in the Dresser Formation are usually
sourced from the rock surface, and are therefore highly weathered. For this
study, the scientists worked with samples that were taken from further down
into the rock, below the weathering profile, where the stromatolites are
exceptionally well preserved.
“Looking at drill core samples allowed us to look at
a perfect snapshot of ancient microbial life,” Dr Baumgartner says.
Using a variety of cutting-edge micro-analytical
tools and techniques – including high-powered electron microscopy, spectroscopy
and isotope analysis – Dr Baumgartner analysed the rocks.
He found that the stromatolites are essentially
composed of pyrite – a mineral also known as ‘fool’s gold’ – that contains
organic matter.
“The organic matter that we found preserved within
pyrite of the stromatolites is exciting – we’re looking at exceptionally
preserved coherent filaments and strands that are typically remains of
microbial biofilms,” Dr Baumgartner says.
The researchers say that such remains have never been
observed before in the Dresser Formation, and that actually seeing the evidence
down the microscope was incredibly exciting.
“I was pretty surprised – we never expected to find
this level of evidence before I started this project. I remember the night at
the electron microscope where I finally figured out that I was looking at
biofilm remains. I think it was around 11pm when I had this ‘eureka’ moment,
and I stayed until three or four o'clock in the morning, just imaging and
imaging because I was so excited. I totally lost track of time,” Dr Baumgartner
says.
Clues for search
for life on Mars
Just over two years ago, Dr Baumgartner’s colleague
Tara Djokic, a UNSW PhD candidate, found stromatolites in hot spring deposits
in the same region in WA, pushing back the earliest known existence of
microbial life on land by 580 million years.
“Tara's main findings were these exceptional
geyserite deposits that indicate that there have been geysers in this area, and
therefore fluid expulsions on exposed land surface,” Dr Baumgartner says.
“Her study was focused on the broader geological
setting of the paleo-environment – lending support to the theory that life
originated on land, rather than in the ocean – whereas my study really went
deeper on the finer details of the stromatolite structures from the area.”
The scientists say that both studies are helping us
answer a central question: where did humanity come from?
“Understanding where life could have emerged is
really important in order to understand our ancestry. And from there, it could
help us understand where else life could have occurred – for example, where it
was kick-started on other planets,” Dr Baumgartner says.
Just last month, NASA and European Space Agency (ESA)
scientists spent as week in the Pilbara with Martin Van Kranendonk for
specialist training in identifying signs of life in these same ancient rocks.
It was the first time that Van Kranendonk shared the region’s insights with a
dedicated team of Mars specialists – a group including the Heads of NASA and
ESA Mars 2020 missions.
“It is deeply satisfying that Australia’s ancient
rocks and our scientific know-how is making such a significant contribution to
our search for extra-terrestrial life and unlocking the secrets of Mars,” says
Professor Van Kranendonk.
原始論文：Raphael J.
Baumgartner, Martin J. Van Kranendonk, David Wacey, Marco L Fiorentini, Martin
Saunders, Stefano Caruso, Anais Pages, Martin Homann, Paul Guagliardo. Nano−porous
pyrite and organic matter in 3.5-billion-year-oldstromatolites record
primordial life. Geology, 2019; DOI: 10.1130/G46365.1
引用自：University of New South Wales. "Earliest
signs of life: Scientists find microbial remains in ancient rocks."
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