這些微化石或許含有世界上最古老的生物特徵
科學家確認年代為34億年的澳洲斯特尼湖(Strelley Pool)微化石具有跟現代細菌類似的化學特徵，這讓科學家幾乎確定這些化石的來源為生物，並使它們名列世上最古老的微化石。此研究發表於波士頓舉行的歌德施密特大會，並同時刊登在同儕審查期刊《Geochemical Perspectives Letters》。
左圖為斯特尼湖的微化石在電子顯微鏡之下的影像。從右圖我們可以看到斯特尼湖、崗夫林特(Gunfilnt)和現代微生物的X光吸收光譜，顯示出多種分子官能基造成的特有峰值(包括芳香基/烯烴基的285.1 eV、亞胺/腈基/羰基/酚基的286.7 eV、醯胺基288.2 eV、羧基/酯基/縮醛基的288.6 eV、羥基的289.4 eV)。
由Julien
Alleon博士(隸屬法國巴黎第四大學的IMPMC以及美國麻省理工學院)領導的科學團隊，成功證實古代微化石中殘留的化學訊號符合年代較近細菌化石中的化學訊號，因此這些微化石可能是由原始生命形式遺留下來。
他們比較了同步輻射X光吸收光譜(synchrotron-based X-ray absorption spectroscopy)對下列樣品的分析結果：分別為斯特尼湖的微化石、在崗夫林特層(Gunflint Formation)中年代較近的微化石(發現於加拿大安大略省蘇必略湖湖岸，年代為19億年)以及現代細菌。結果顯示三者擁有類似的吸收光譜特徵，代表這些殘留的化學物質都是由同一種來源建造而成，因此支持斯特尼湖的微化石是生物來源的說法。
Jullien Alleon博士表示：
「此研究有幾個重點。首先，我們證實這些年代34億年的微化石中，含有的元素和分子特性跟受到石化過程而輕微分解的生物遺骸的特性一致。這充分支持了斯特尼湖微化石的來源確實為生物。關於誰才是世上最古老的微化石還有其他競爭者，將此分析策略運用到其他古代樣品上有助於解決這項爭議。
其次，這些遠古生命的殘響竟然可以歷經34億年的時光而留存下來，令我們感到十分驚訝。我們從這些微化石的分子結構知道它們曾長時間處在高達300 °C的環境，但我們仍然可以看出它們原始化學組成留下的痕跡。
這讓我們更有信心地確定它們真的是迄今發現的最古老化石。」
國立澳洲大學的教授Vickie Bennett對此研究的評論為：
「這項研究的成果相當令人興奮，他們運用新的分析方法提供了強而有力的證據，顯示這些燧石含有生物成因的微化石。連同在斯特尼湖岩石中觀察到的其他早期生命跡象，像是代表微生物氈的疊層石，此研究更進一步地確認地球上的生命最早在34億年前就已經出現。
其他年代更老、宣稱含有最古老陸域生物的岩石無法運用此篇研究的技術，因為這些岩石經歷過的溫度高出許多，比方說格陵蘭伊蘇阿(Isua)年代為37億年的疊層石以及加拿大年代為41億年的微化石。然而，這篇研究顯示出該領域的發展速度相當驚人，而在不久之後也許我們就有方法可以分析並找出更古老的生命證據。」
Microfossils, possibly world's oldest, had biological characteristics
Scientists have confirmed that the 3.4-billion-year-old Strelley
Pool microfossils had chemical characteristics similar to modern bacteria. This
all but confirms their biological origin and ranks them amongst the world's
oldest microfossils. The work is presented at the Goldschmidt geochemistry
conference in Boston, with simultaneous publication in the peer-reviewed
journal Geochemical
Perspectives Letters.
A team of scientists, led
by Dr Julien Alleon (IMPMC, Paris, France; and MIT, Cambridge, MA, USA) have
been able to show that the chemical residuals from ancient microfossils match
those of younger bacterial fossils, and so are likely to have been laid down by
early life forms.
They compared the results
of synchrotron-based X-ray absorption spectroscopy analysis of the Strelley
Pool microfossils with more recent ones from the Gunflint Formation (1.9
billion years old, found on the shores of Lake Superior, Ontario, Canada) and
with modern bacteria. All showed similar absorption features, indicating that
the residual chemicals were made from the same building blocks, thereby
supporting a biological origin (see illustration below).
Dr Jullien Alleon said:
"There are a couple of
important points which come out of this work. Firstly, we demonstrate that the
elemental and molecular characteristics of these 3.4 Ga microfossils are
consistent with biological remains, slightly degraded by fossilization
processes. This effectively supports the biological origin of the Strelley Pool
microfossils. There are competing claims over which microfossils are actually
the world's oldest, this analytical strategy needs to be applied to other
ancient samples to help settle the controversy.
Secondly, it is remarkable
that these echoes of past life have survived the extreme conditions they have
experienced over the last 3.4 billion years: we know from the molecular
structure of the microfossils that they have been exposed to temperatures of up
to 300 °C for long periods. And yet we are still able to see signs of their
original chemistry.
This is a step forward to
confirming that these are indeed the oldest fossils yet discovered."
Commenting, Professor
Vickie Bennett (Australian National University) said:
"This is exciting work
with the new types of analyses providing compelling evidence that the cherts
contain biogenic microfossils. This is in line with other observations for
early life from the Strelley Pool rocks, including stromatolites interpreted as
microbial mats, and further confirming that the minimum age for life on Earth
is 3.4 billion years.
The techniques used here
are not applicable to the older rocks that host the claims for the oldest
terrestrial life, as these rocks were exposed to much higher temperatures.
These samples include the 3.7 billion year old stromatolites from Isua,
Greenland and the 4.1 billion year old Canadian microfossils. However, this
work shows how quickly the field is developing and that new capabilities for
testing and confirming earlier evidence of life are in reach."
相關論文：J. Alleon, S. Bernard, C. Le Guillou, O. Beyssac, K. Sugitani,
F. Robert. Chemical
nature of the 3.4 Ga Strelley Pool microfossils. Geochemical Perspectives Letters,
2018; 37 DOI: 10.7185/geochemlet.1817
Kenichiro Sugitani, Koichi Mimura, Tsutomu Nagaoka, Kevin Lepot,
Makoto Takeuchi. Microfossil
assemblage from the 3400Ma Strelley Pool Formation in the Pilbara Craton,
Western Australia: Results form a new locality. Precambrian Research,
2013; 226: 59 DOI: 10.1016/j.precamres.2012.11.005
引用自：Goldschmidt Conference. "Microfossils,
possibly world's oldest, had biological characteristics."
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