科學家發現地球上由生物移動留下來的最古老痕跡
法國普瓦捷環境與材料化學研究所(隸屬法國國家科學研究中心/普瓦捷大學)的Abderrazak El Albani領導的國際跨領域團隊，發現了年代最為古老，由生物運動留下的生痕化石。之前這類化石最老的年代為5.7億年，而這項新證據的年代則有21億年。發現地點為加彭的化石層，先前在此也找到了迄今年代最早的多細胞生物化石。研究結果發表於2019年2月11日出版的《美國國家科學院院刊》。
運用X光電腦微斷層掃描呈現這些管狀構造的立體樣貌，可以看出它們在沉積物內部的移動路徑。填充在這些管狀構造內部的是黃鐵礦晶體，這種礦物發現於黏土礦物形成的岩層當中，是細菌轉化生物組織的產物。附近一層一層的平行構造則是微生物氈的化石。
法國普瓦捷環境與材料化學研究所的地質學家Abderrazak El Albani和他的團隊，數年前在加彭弗朗斯維爾盆地的沉積岩層中發現了目前最古老的多細胞生物化石。這項發現讓科學家重新把地球出現多細胞生物的時間定在21億年前，比先前認為的6億年前還早了大約15億年。研究人員表示這座生物多樣性豐富的沉積岩層是在平靜的淺海環境形成，此時大氣中的dioxygenation(氧分子中的兩個氧原子皆參與的氧化作用)也達到高峰。
團隊最近在同一個地層中發現了生物移動留下的生痕化石，證明在此原始的海洋生態系中，某些多細胞生物已經進步到可以在富含有機質的泥巴中到處移動。
研究人員利用非破壞性的成像技術――X光電腦微斷層掃描來分析並重建生痕的立體影像。這些彎曲的管狀構造直徑通常相當一致，都只有幾釐米，並分布在細顆粒的沉積岩層中。分析幾何構造與化學性質之後，顯示它們是由生物製造，而且是在沉積當時一起生成的。
在生痕旁邊的是微生物薄膜的化石，這些毯子一般的構造形成於沉積岩層的交界面。研究人員認為此現象的成因是生物在沉積物裡到處移動，尋找藍綠菌製造出來的養分以及氧氣。
這些生物活著的時候看起來是什麼樣子？雖然很難給出肯定的答案，但它們看起來大概會像是變形蟲集合體。變形蟲在食物匱乏的時候會聚在一起而形成像是蛞蝓的生物，集體移動以尋找更適合生存的環境。
在此之前，科學家公認由生物移動留下的最古老痕跡形成於5.7億年前，而分子時鐘似乎也支持了這項預估。在21億年前的岩石中發現生物已經可以移動的新證據，也讓科學家對生命的歷史進程有了新的疑問：這是一項生物革新，引導之後的生物演化出更加完備的移動方式？或者只是一次實驗，在大約20.83億年前因為大氣氧化速率劇烈下降而被迫中止？
Discovery of the oldest evidence of mobility on
Earth
An
international and multi-disciplinary team coordinated by Abderrazak El Albani
at the Institut de chimie des milieux et matériaux de Poitiers (CNRS/Université
de Poitiers) has uncovered the oldest fossilised traces of motility. Whereas
previous remnants were dated to 570 million years ago, this new evidence is 2.1
billion years old. They were discovered in a fossil deposit in Gabon, where the
oldest multicellular organisms have already been found. These results
appear in the 11 February 2019 edition of PNAS.
A few years
ago, geologist Abderrazak El Albani and his team at the Institut de
chimie des milieux et matériaux de Poitiers (CNRS/Université de
Poitiers) discovered the oldest existing fossils of multicellular organisms in
a deposit in Gabon. Located in the Franceville Basin, the deposit allowed
scientists to re-date the appearance of multicellular life on Earth to 2.1
billion years – approximately 1.5 billion years earlier than previously thought
(600 million). At the time, researchers showed that this rich biodiversity
co-occurred with a peak in dioxygenation of the atmosphere, and developed
in a calm and shallow marine environment.
In this same
geological deposit, the team has now uncovered the existence of fossilised
traces of motility. This shows that certain multicellular organisms in this
primitive marine ecosystem were sophisticated enough to move through its mud,
rich in organic matter.
The traces
were analysed and reconstructed in 3D using X-ray computed micro-tomography, a
non-destructive imaging technique. The more or less sinuous structures are
tubular, of a generally consistent diameter of a few millimetres, and run
through fine layers of sedimentary rock. Geometrical and chemical analysis
reveals that they are biological in origin and appeared at the same time the
sediment was deposited.
The traces are
located next to fossilised microbial biofilms which formed carpets between the
superficial sedimentary layers. It is plausible that the organisms behind this
phenomenon moved in search of nutritive elements and the dioxygen, both
produced by cyanobacteria.
What did these
living elements look like? Though difficult to know for certain, they may have
been similar to colonial amoebae, which cluster together when resources become
scarce, forming a type of slug which moves in search of a more favourable
environment.
Until now, the
oldest traces of recognised movement were dated to 570 million years ago; an
estimate which appeared to be confirmed by the molecular clock. Evidence of
motility found in rock that is 2.1 billion years old raises new questions
regarding the history of life: was this biological innovation the prelude to
more perfected forms of movement, or an experiment cut short by the drastic
drop in atmospheric oxygen rates which occurred approximately 2.083 billion years
ago?
原始論文：Abderrazak El
Albani, M. Gabriela Mangano, Luis A. Buatois, Stefan Bengtson, Armelle
Riboulleau, Andrey Bekker, Kurt Konhauser, Timothy Lyons, Claire Rollion-Bard,
Olabode Bankole, Stellina Gwenaelle Lekele Baghekema, Alain Meunier, Alain
Trentesaux, Arnaud Mazurier, Jeremie Aubineau, Claude Laforest, Claude
Fontaine, Philippe Recourt, Ernest Chi Fru, Roberto Macchiarelli, Jean Yves
Reynaud, François Gauthier-Lafaye, Donald E. Canfield. Organism
motility in an oxygenated shallow-marine environment 2.1 billion years ago. Proceedings
of the National Academy of Sciences, 2019; 201815721 DOI: 10.1073/pnas.1815721116
引用自：CNRS. "Discovery of the oldest evidence of mobility on
Earth."
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