發現地球最古老化石的說法挑起眾多研究人員的關注
By
Alexandra Witze
格陵蘭岩石中的構造可能是由生活在37億年前的生物堆積而成
地質學家說他們挖掘出一些地球生命存在的最古老證據。雖然還尚未接受進一步的驗證，這項發現指出在星球年幼時生命很快就出現了。
在本周的期刊《自然》(Nature)當中，澳洲和英國的研究人員發表說他們從格陵蘭年代為37億年前的岩石中，發現了一種稱作「疊層石」(stromatolite)的層狀構造。疊層石是一種跟花椰菜有幾分相似的地質構造，當微生物抓住沉積物並往上疊而形成一層一層的圓丘時，就會形成疊層石。
但這項研究用到的岩石在數十億年的板塊地殼移動過程中，承受劇烈的擠壓及高溫而成為地球上扭曲變形得最嚴重的岩石之一。這樣的高溫高壓會造成岩石重新結晶，而抹去研究人員通常用來辨識疊層石的細小構造—故這項研究已經激起相當熱烈的爭議。
「在接受這篇研究的結果之前，我有14點需要他們妥善說明的問題及疑問。」西雅圖華盛頓大學的地質生物學家Roger Buick說。
這些岩石採自格陵蘭的 Isua，研究人員一直以來努力在這些岩石中尋找數十億年前有生命存在的可能跡象。過往對這些岩石化學性質的工作，像是1999年關於碳同位素的一篇論文指出這些岩石含有原始生物遺留下來的「生物指標」，但這些說法經過了數年仍然備受爭議。
重見天日的證據
融雪於今日帶來了新的證據。由澳洲臥龍崗大學的地質學家 Allen Nutman領導的研究團隊前往一處長年被雪堆掩蓋，直到最近才因氣溫上升融化而重見天日的岩石露頭。他們在此採集到一塊年代為37億年的岩石，並將它帶回澳洲進行深入研究。
他們在這塊岩石當中發現了他們所稱的疊層石，還有其他指向遠古生命的證據。「不同類型的證據組合在一起使得這個說法十分讓人信服。」團隊成員之一 Martin Van Kranendonk表示。他是澳洲肯辛頓的新南威爾斯大學的地質學家。
據Van Kranendonk 和他的同仁所言，這些構造是高度僅1-4公分的凸起，其外型和內部層狀結構跟古代及現存的疊層石十分相似。從周圍岩石的質地可以看出這些構造是堆積在淺海海底而形成，這跟今日在巴哈馬群島和澳洲西部當地的疊層石形成環境一致。另外這些岩石當中也含有白雲石之類的碳酸鹽礦物，這在近代的疊層石中也很常見。
之前廣為接受的最古老生命證據是澳洲西部皮爾布拉(Pilbara)地區的疊層石群，而格陵蘭構造的年代跟其相比還早了2.2億年。「跟澳洲的岩石相比，這篇研究提出的證據整體來說都薄弱許多。」美國加州帕薩迪納，NASA噴射推進實驗室的天體生物學家Abigail Allwood說。「雖然如此，從之前什麼證據都未能找著的岩石中發現任何事物都是相當驚人的消息。這也是這篇論文值得關注的原因。」
令人起疑的結晶
研究古代疊層石的問題之一便是層狀構造可以經由許多跟生命無關的作用來形成。就像浴缸內側的環狀水垢一樣，礦物沉澱在海床時也可以形成看起來像是疊層石，實際上卻不然的層狀構造。
「這些構造頂多只能被分類成『假疊層石』(pseudostromatolite)。」澳洲伯斯的古生物顧問專家，研究古代疊層石已有多年時間的Kathleen Grey表示。「令人遺憾的是，我不認為這項證據足以支持這麼重大的主張。」
美國劍橋市麻省理工學院的地質生物學家Tanja Bosak補充，她想看看據稱為疊層石的這些結構內部或周圍是否有少量的有機物。比對這些岩石中不同類型的碳可能有助於顯示這些結構是否由生物形成。
Allwood強調說，至少格陵蘭的岩石能幫助天體生物學家準備如何研究有史以來第一批從火星帶回來的樣品，此為NASA預計於2020年執行的任務內容之一。這項新發表的疊層石研究或許可以當作測試案例，來讓科學家討論出要擁有何種條件才能成為令人信服的遠古生命證據。
「如果我們在火星上找到像這樣的東西，是否能給它貼上標籤並稱呼其為『生命』？」她提出疑問。「我不認為我們可以這麼做。」
原始論文：Allen P. Nutman, Vickie C. Bennett, Clark R. L. Friend, Martin J. Van Kranendonk, Allan R. Chivas. Rapid emergence of life shown by discovery of 3,700-million-year-old microbial structures. Nature, 2016
Claims of Earth's oldest fossils tantalize researchers
Rocks in Greenland contain structures that could have been laid down by living organisms 3.7 billion years ago.
Geologists
say that they have unearthed some of the oldest known evidence for
life on Earth. The discovery, yet to be confirmed, suggests that life
arose quickly on the young planet.
In
this week's Nature,
Australian and British researchers report finding layered structures
called stromatolites in 3.7-billion-year-old rocks from Greenland1.
Stromatolites, which look a bit like geological cauliflowers, form
when microbes trap sediment and build up layer after dome-shaped
layer.
But
the discovery involves some of the most physically tortured rocks on
Earth, which have been squeezed and heated over billions of years as
crustal plates shifted. The pressure and heat recrystallizes the
rocks, erasing much of the fine-scale detail that researchers
normally use to identify fossilized stromatolites — so the work is
already triggering heated debate.
“I’ve
got 14 queries and problems that need addressing before I’ll
believe it,” says Roger Buick, a geobiologist at the University of
Washington in Seattle.
The
rocks hail from Isua, Greenland, where researchers have laboured to
tease out potential signs of life dating back billions of years.
Previous work on the rocks’ chemistry, such as a 1999 paper
analysing carbon isotopes2,
suggested that they contain ‘biomarker’ traces of early
organisms. But various claims over the years have remained
contentious.
Unearthing evidence
Now,
melting snow has revealed new clues. A team led by Allen Nutman, a
geologist at the University of Wollongong in Australia, visited a
rock outcrop that had been buried under a perennial snow patch until
warmer temperatures melted it away. They sawed out a chunk of
3.7-billion-year-old rock and took it back to Australia to study.
In
it they found the purported stromatolites, along with other clues to
ancient life. “It’s a combination of different types of evidence
that makes the story so compelling,” says team member Martin Van
Kranendonk, a geologist at the University of New South Wales in
Kensington, Australia.
The
structures are tiny bumps, just 1–4 centimetres tall, whose shape
and internal layering strongly resemble ancient and modern
stromatolites, Van Kranendonk and his colleagues say. The texture of
the surrounding rocks suggests that they were laid down at the bottom
of a shallow sea, much as stromatolites are today in places such as
the Bahamas and western Australia. And the rocks contain carbonate
minerals such as dolomite, which are also common in younger
stromatolites.
The
Greenland structures are about 220 million years older than the
oldest widely accepted evidence for life, a
set of stromatolites from the Pilbara region of western Australia3.
“The evidence is a whole lot thinner than the rocks in Australia,”
says Abigail Allwood, an astrobiologist at NASA’s Jet Propulsion
Laboratory in Pasadena, California. “Having said that, it’s
incredible that anything can be found in these rocks that are barely
a ghost of what they were before. That’s why it’s worthy of
attention.”
Doubts
crystallize
Part
of the problem with studying ancient stromatolites is
that layered structures can form through processes that have nothing
to do with life. Minerals precipitating out on the seafloor can leave
layers, like rings on a bathtub, that look like stromatolites but
aren't4.
“At
most, these structures should be classified as pseudostromatolites,”
says Kathleen Grey, a consulting palaeontologist in Perth, Australia,
who has worked on ancient stromatolites. “Sadly, I don’t feel the
evidence is convincing for such an important claim.”
Tanja
Bosak, a geobiologist at the Massachusetts Institute of Technology in
Cambridge, adds that she would like to see whether the proposed
stromatolites have small amounts of organic matter in or near them.
Comparing different types of carbon in the rock could help to reveal
whether the structures are biological or not.
At
a minimum, Allwood notes, the Greenland rocks should help
astrobiologists as they prepare for the first ever samples to be
returned from Mars, from a
NASA mission slated to launch in 2020.
The newly reported stromatolites may serve as a test case for
scientists to argue about what constitutes convincing evidence of
past life.
“If
we found something like this on Mars would we stick a flag in it and
call it life?” she asks. “I don’t think we would.”
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