原始論文:www.sciencedaily.com/releases/2017/03/170301132012.htm
發現世上最古老的化石
由倫敦大學學院的科學家(UCL)領導的國際團隊發現了年代至少有37.7億年的微生物遺骸。此發現是顯示地球最早生命形式之一為何的直接證據。
在加拿大魁北克的努夫亞吉圖克表岩帶(Nuvvuagittuq Supracrustal Belt,NSB)的石英地層中,發現嵌有以鐵維生的微生物形成的微小絲狀和管狀痕跡。
NSB有些岩石是世上已知最古老的沉積岩。這些岩石的年代為37.7億年前至43億年前,部份形成於富含鐵質的深海熱泉系統,此處提供了地球第一批生物生存的棲地。「我們的發現支持了生物在地球形成不久之後即出現在海底熱泉的說法。這項證據顯示地球生命出現地相當快,也跟最近發現年代為37億年,由微生物形成的丘狀沉積物互相吻合。」第一作者Matthew Dodd解釋(其為UCL地球科學系和英國倫敦奈米科技中心的博士生) 。
由UCL、NASA、加拿大卡內基研究院和英國工程暨物理科學研究委員會贊助,今日刊登於期刊《自然》(Nature)的這篇研究,詳細描述了團隊發現的遺骸及分析結果。此團隊由UCL、挪威地質調查所、美國地質調查所、西澳大學、渥太華大學和里茲大學的科學家組成。
在此發現之前,其他論文發表的最古老微體化石是發現於澳洲西部,定年結果為34.6億年。但有些科學家認為它們是非生物作用在岩石中形成的產物。因此由UCL領導的團隊的首要任務便是要確認從加拿大出土的遺骸來源是否為生物。
研究人員有系統的觀察樣本中由赤鐵礦(hematite)――鐵的一種氧化物,也就是鐵鏽――組成的絲線與管束,是否有可能由非生物作用形成,像是沉積物埋藏過程中發生的溫度壓力變化。他們發現由非生物作用產生的可能性為零。
這些由赤鐵礦形成的結構及其擁有分支的特徵,跟今日發現於熱泉系統附近的鐵氧化菌(iron-oxidising bacteria)如出一轍。另外這些結構周遭有石墨(graphite)、磷灰石(apatite)和碳酸鹽類礦物,它們可以在生物體內像是骨頭和牙齒中發現,因此常被視作跟化石有關的礦物。
他們還發現這些已被礦物取代的化石伴有年代較近的化石中常出現的球狀構造,代表很有可能是利用氧化鐵來得到能量的細菌在岩石裡變成化石之後,赤鐵礦才形成。
「我們認為在這些數公分寬稱作結核(concretion)或團塊(nodule)的結構內部,以及其他稱作薔薇組織(rosette)或米粒組織(granule)的更小型球狀結構中發現的絲線和微管是腐敗作用(putrefaction)的產物。它們的礦物組成跟挪威、北美五大湖區和澳洲西部較年輕的岩石一致。」領導這項研究的Dominic Papineau博士表示(其任職於UCL地球科學系和英國倫敦奈米科技中心)。
「從非常早期直到近代的地質紀錄中都有腐敗作用的詳細記載,而此結構和礦物組成都符合腐敗作用會導致的結果。我們從已知最古老岩層之一中挖掘到的事物,意味著發現了地球最古老生命形式之一為何的直接證據。這項發現有助於我們拼湊出地球的真實歷史以及其上的不凡生命是如何發展,甚至未來可以帶領我們找出宇宙其他地方的生命蹤跡。」
Matthew Dodd總結:「這些發現在在顯示當地球和火星表面皆有液態水時,地球的生命已經發展出來了,這帶出了許多關於地外生命的有趣問題。因此,我們預期終將找到火星40億年前曾有生命存在的證據,如果沒有找到的話,代表地球或許是個特別的例外。」
World's oldest fossils unearthed
Remains of microorganisms at least 3,770 million years old have
been discovered by an international team led by UCL scientists, providing
direct evidence of one of the oldest life forms on Earth.
Tiny filaments and tubes formed by
bacteria that lived on iron were found encased in quartz layers in the
Nuvvuagittuq Supracrustal Belt (NSB), Quebec, Canada.
The NSB contains some of the oldest
sedimentary rocks known on Earth which likely formed part of an iron-rich
deep-sea hydrothermal vent system that provided a habitat for Earth's first
life forms between 3,770 and 4,300 million years ago. "Our discovery
supports the idea that life emerged from hot, seafloor vents shortly after
planet Earth formed. This speedy appearance of life on Earth fits with other evidence
of recently discovered 3,700 million year old sedimentary mounds that were
shaped by microorganisms," explained first author, PhD student Matthew
Dodd (UCL Earth Sciences and the London Centre for Nanotechnology).
Published today in Nature and funded by UCL, NASA, Carnegie of
Canada and the UK Engineering and Physical Sciences Research Council, the study
describes the discovery and the detailed analysis of the remains undertaken by
the team from UCL, the Geological Survey of Norway, US Geological Survey, The
University of Western Australia, the University of Ottawa and the University of
Leeds.
Prior to this discovery, the oldest
microfossils reported were found in Western Australia and dated at 3,460
million years old but some scientists think they might be non-biological
artefacts in the rocks. It was therefore a priority for the UCL-led team to
determine whether the remains from Canada had biological origins.
The researchers systematically looked
at the ways the tubes and filaments, made of haematite -- a form of iron oxide
or 'rust' -- could have been made through non-biological methods such as
temperature and pressure changes in the rock during burial of the sediments,
but found all of the possibilities unlikely.
The haematite structures have the same
characteristic branching of iron-oxidising bacteria found near other
hydrothermal vents today and were found alongside graphite and minerals like
apatite and carbonate which are found in biological matter including bones and
teeth and are frequently associated with fossils.
They also found that the mineralised
fossils are associated with spheroidal structures that usually contain fossils
in younger rocks, suggesting that the haematite most likely formed when
bacteria that oxidised iron for energy were fossilised in the rock.
"We found the filaments and tubes
inside centimetre-sized structures called concretions or nodules, as well as
other tiny spheroidal structures, called rosettes and granules, all of which we
think are the products of putrefaction. They are mineralogically identical to
those in younger rocks from Norway, the Great Lakes area of North America and
Western Australia," explained study lead, Dr Dominic Papineau (UCL Earth
Sciences and the London Centre for Nanotechnology).
"The structures are composed of
the minerals expected to form from putrefaction, and have been well documented
throughout the geological record, from the beginning until today. The fact we
unearthed them from one of the oldest known rock formations, suggests we've
found direct evidence of one of Earth's oldest life forms. This discovery helps
us piece together the history of our planet and the remarkable life on it, and
will help to identify traces of life elsewhere in the universe."
Matthew Dodd concluded, "These
discoveries demonstrate life developed on Earth at a time when Mars and Earth
had liquid water at their surfaces, posing exciting questions for
extra-terrestrial life. Therefore, we expect to find evidence for past life on
Mars 4,000 million years ago, or if not, Earth may have been a special
exception."
原始論文:Matthew S.
Dodd, Dominic Papineau, Tor Grenne, John F. Slack, Martin Rittner, Franco
Pirajno, Jonathan O’Neil, Crispin T. S. Little. Evidence for early life
in Earth’s oldest hydrothermal vent precipitates. Nature, 2017;
543 (7643): 60 DOI: 10.1038/nature21377
引用自:University College London. "World's oldest fossils unearthed."
ScienceDaily. ScienceDaily, 1 March 2017.
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