原文網址：http://www.nature.com/news/oldest-traces-of-life-on-earth-may-lurk-in-canadian-rocks-1.22685
地球最古老的生命跡象或許就潛藏在加拿大的岩石當中
研究人員發表了存活在39.5億年前的生物留下的化學訊號，但有眾多學者對此抱持懷疑
顯微鏡下的黑色石墨顆粒，可能具有地球遠古生命留下的訊號。 Credit: Komiya et al. Nature, 10.1038/nature24019
一項新研究提出加拿大東北方的古老岩石含有超過39.5億年前的生物留下的化學痕跡。如果此發現屬實，就會成為地球所有已知生命跡象中最早的紀錄。
對於某些人來說，此研究增添了更多證據暗示地球初期充斥各種不同形式的生物。「接受這項事實吧！」倫敦大學學院的地球化學家Dominic Papineau如此表示。他在三月跟其他學者發表的一篇研究中，於加拿大魁北克發現可能為微生物的化石，其年代至少可以追溯至37.7億年前。
但是其他人則懷疑這項最新研究是否禁得起更仔細的檢驗。先前許多聲稱找到遠古生命的研究都遭到相當猛烈的質疑聲浪——部份是因為數十億年前形成的岩石歷經劇烈高溫作用且嚴重擠壓，使得其中蘊含的地質語言變得難以解讀；另一部份則是因為生物留下的化學訊號跟沒有生物參與的化學反應，實際上相當難以區分開來。
「當我讀到這篇研究我想說『又來了。』」斯德哥爾摩瑞典自然史博物館的地質學家Martin Whitehouse表示。2002年他參與的研究批評了一篇在格陵蘭發現遠古生命的類似報告。
關鍵岩塊
這篇最新研究調查了位在加拿大北方的一套岩石，統稱為Saglek岩塊。由東京大學小宮剛和佐野有司領導的團隊在2011至2013年間探訪此區，在武裝警衛持續警戒北極熊的同時，團隊分散在露頭各處的岩石蒐集樣品。
於9月28日當期的《自然》，研究人員發表他們對Saglek地區進行的碳同位素分析結果，樣品類型分別是整個磨碎的岩石和個別石墨顆粒。在某些樣品中，他們發現其含有的碳-13對碳-12的比例相對來說較低。碳-12是比較輕的同位素，它的原子核比碳-13少了一顆中子。生物傾向於利用碳-12來製造有機化合物，因此微生物曾經居住過的場所，像是Saglek的岩石，就會含有豐富的輕同位素。
小宮表示在這些古老且高度變形的岩石中找到生命跡象「令他們相當驚訝且興奮」。他說他和團隊排除了其他會讓碳同位素比例產生偏差的可能解釋，像是菱鐵礦的分解。此外，石墨結晶時的溫度似乎跟他們附近的岩石在擠壓加熱的過程中經歷的溫度大致相同，顯示石墨並非是之後才進入岩石內部的汙染物質。
研究人員從加拿大拉布拉多稱作Saglek岩塊的地區採集岩石樣品。Credit: Komiya
et al. Nature, 10.1038/nature24019
在之前發表的論文中，小宮的團隊描述了Saglek岩塊的地質史，並利用一種變質岩――片麻岩中的古老鋯石晶體進行鈾―鉛定年，結果顯示片麻岩的年代為39.5億年。他們肯定含有生命訊息的石墨至少有這麼老，因為石墨位在看似被39.5億年的片麻岩穿過的岩石之中，所以團隊推測石墨的年代會早於片麻岩。
但此描述反而促使其他科學家提出警語。「這些石墨沉積物的年代比作者宣稱的年輕許多。」華沙波蘭科學院地質科學研究所的地質學家Monika Kusiak，與澳洲伯斯科廷大學的地質學家Daniel Dunkley表示。他們都曾經進行研究企圖闡明Saglek岩塊的地質史，並主張研究人員推論中被片麻岩穿過的岩石，實際年代比片麻岩還要年輕。
「這篇論文中其他所有內容都構築在地質年代之上，但地質年代本身並沒有建造得十分穩固。」Whitehouse補充，「整篇論文就像是一棟搖搖欲墜的危樓。」
小宮表示他支持由他的團隊做出的解釋。
跟拉布拉多相關的研究
對此成果的爭議跟1996年開始發生的爭論十分相似，當時一組美國―澳洲―英國的聯合團隊發表他們在格陵蘭西南部外海Akilia島取得的岩石中，內部的磷灰石顆粒含有生物作用過的石墨，年代至少有38.6億年之久。這項研究在許多方面都受到批判，但還是有些人堅稱Akilia島的岩石含有遠古生命留下的蹤跡。
拉布拉多包括Saglek岩塊在內的古老岩石可以直接跨過拉布拉多海連結到格陵蘭。「它們有許多相似之處。」哥本哈根丹麥自然史博物館的地質學家Minik Rosing表示。他的研究領域為格陵蘭的岩石中可能由生命留下來的化學訊號。「這邊的地質狀況極為複雜。」
他在讚賞小宮團隊的同時表示仍有許多研究尚待完成。他說：「這是一個良好例子呈現出相當精細且品質高超的分析成果。」然而，他也表明當地岩石十分複雜，使得研究用的石墨「就算通過各種方法驗證，其年代可能僅有27億年這般年輕。」
Oldest traces of life on Earth may lurk in Canadian
rocks
Researchers report chemical evidence of organisms
that lived 3.95 billion years ago, but scepticism abounds.
The microscope view shows dark grains
of graphite that could contain traces of ancient life on Earth. Credit: Komiya et al.
Nature, 10.1038/nature24019
Ancient rocks in
northeastern Canada could contain chemical traces of life from more than 3.95
billion years ago, a new study suggests1. If confirmed, the finding would be among the earliest known
signs of life on Earth.
To some, the work adds to
growing evidence that the young Earth was teeming with many different kinds of
organism. “Accept it!” says Dominic Papineau, a geochemist at University
College London who, in March, co-authored a report of possible fossilized
microbes from Quebec that date back at least 3.77 billion years2.
But others are sceptical
that the latest work will hold up to scrutiny. Many previous claims of ancient
life have been hotly
contested — in part
because rocks that formed billions of years ago have been severely heated and squished,
making the geological context hard to interpret, and in part because the
chemical traces of life can be difficult to distinguish from reactions that do
not involve living organisms.
“When I read this I
thought, ‘here we go again,’” says Martin Whitehouse, a geologist at the
Swedish Museum of Natural History in Stockholm who, in 2002, co-authored a
study3 that criticized a similar report of ancient life in
Greenland.
Rock block
The latest work
investigated a set of rocks from northern Labrador, known collectively as the
Saglek block. A team led by Tsuyoshi Komiya and Yuji Sano of the University of
Tokyo visited the area between 2011 and 2013, fanning out among the rocky
outcrops to gather samples while armed guards kept watch for polar bears.
In the 28 September issue
of Nature, the researchers report analysing carbon isotopes in
powdered rock and in individual graphite grains from the Saglek area. In some
of the samples, they found relatively low amounts of the isotope carbon-13
compared to carbon-12, a lighter isotope with one less neutron in its nucleus.
Organisms prefer to use carbon-12 to make organic compounds, and so material in
which microbes once lived — like the Saglek rocks — becomes enriched in the
lighter isotope.
Finding evidence of life in
these ancient, highly deformed rocks “is surprising and exciting”, says Komiya.
He says that he and his group have ruled out other possible explanations for
the skewed ratio of carbon isotopes, such as decomposition of the mineral
siderite. And the graphite seems to have crystallized at roughly the same
temperature as that experienced by the rocks around it as they were squeezed
and heated, which suggests the graphite isn’t just contamination that arrived
later.
Researchers sampled rocks
from an area in Labrador, Canada, called the Saglek block. Credit: Komiya et al. Nature, 10.1038/nature24019
In previous papers4, 5, Komiya’s team described the geological history of the Saglek
block, and used uranium–lead dating on ancient zircon crystals inside a type of
metamorphic rock called gneiss to conclude that the gneiss was 3.95 billion
years old. They say that the graphite containing hints of life must be at least
that old, because it lies within rocks that are apparently shot through with —
and thus presumably older than — the 3.95-billion-year-old gneiss.
Yet that scenario prompts
other scientists to raise a warning flag. “The graphite is in much younger
sediment than the authors claim,” say geologists Monika Kusiak of the Institute
of Geological Sciences at the Polish Academy of Sciences in Warsaw and Daniel
Dunkley of Curtin University in Perth, Australia, who have been working to
unravel the geological history of the Saglek block. They argue that the rocks
that are supposedly shot through with the gneiss are not, in fact, older than
it.
“Everything else in this
paper is resting on the geochronology, and the geochronology is just not
well-founded,” adds Whitehouse. “It’s a house of cards.”
Komiya says that he stands
by his team’s interpretation.
Labrador links
The debate over the results
resembles one that began in 1996, when a US–Australian–British team reported
finding biologically altered graphite from at least 3.86 billion years ago
inside grains of a mineral called apatite, in rocks on Akilia island off southwestern
Greenland6. That work was criticized on many
fronts7, although some still argue that Akilia's rocks contain traces
of ancient life.
Labrador’s ancient rocks,
including the Saglek block, lie directly across the Labrador Sea from
Greenland. “There are a lot of similarities,” says Minik Rosing, a geologist at
the Natural History Museum of Denmark in Copenhagen who has studied possible
chemical traces of life in Greenland rocks8. “It's extremely complex geology.”
He applauds Komiya's team,
but says that more remains to be done. “It's a good example of very
sophisticated and high-quality analytical work,” he says. But the rocks are so
complicated, he explains, that the graphite being studied “could be as young as
2.7 billion years old, and still pass all the tests”.
原始文章：Alexandra Witze. Oldest traces of life on Earth may lurk in Canadian rocks. Nature,
2017. doi:10.1038/nature.2017.22685
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