.The presence of steranes in Australian shales indicates that eukaryotes were present 2.7 billion years ago
Pour réaliser ces estimations d'apparitions des "eucaryotes", les chercheurs essayent de mettre en évidence des traces/cicatrices de leurs présences au sein de rochers datant de l'ère précambrienne. Pour cela, des biomarqueurs à base d'hydrocarbones sont utilisés.
Le souci majeur de cette technique est bien évidemment la "contamination" par des hydrocarbones non "indigènes" de la roche.
Les termes techniques pour les puristes, avec les mots clefs surlignés pour les "flemmards".
Recent geochemical studies, using rigorous laboratory procedures designed to minimize this problem, recovered traces of hydrocarbon biomarkers (4) from organic-rich shales as old as 2.7 Gyr (4, 5, 6, 7). The extracted bitumens mainly consisted of n-alkanes, methylalkanes and acyclic isoprenoids, but also included complex polycyclic biomarkers such as 2-methylhopanes and steranes, suggesting the presence of cyanobacteria and eukaryotes, respectively (4, 5, 6). The biomarker results set new minimum ages for fossil evidence of cyanobacteria (previously 2.15 Gyr old) (11) and eukaryotes (previously between 1.78 Gyr ago and 1.68 Gyr ago) (10). The presence of biomarkers linked to oxygen-producing cyanobacteria as well as steranes, the biosynthesis of which in extant eukaryotes requires molecular oxygen, also provides the most reliable minimum age for the onset of oxygenic photosynthesis (4, 5, 6. )Although the weight of evidence pointed to a probable syngenetic origin for the biomarkers, the possibility of contamination was not entirely ruled out (5).
Mais cette semaine dans Nature, une nouvelle technique, basée sur l'analyses des carbones 13C/12C met en évidence une discordance entre l'âge de ces tout premiers biomarqueurs et l'âge de la roche elle-même. Ces eucaryotes vieux de 2,7 milliards d'années ne sont en fait que des petits jeunes.
Et l'âge de l'apparition du premier eucaryote est maintenant évalué à 1,7 milliard d'années...
Ca fait mal, non ?
M'enfn, d'un autre côté, si cela veut dire que le dernier eucaryotes va disparître de la Terre un milliard d'années plus tard que prévu, "je signe tout de suite" !
Nature. 2008 Oct 23;455(7216):1101-4.
Reassessing the first appearance of eukaryotes and cyanobacteria.
Rasmussen B, Fletcher IR, Brocks JJ, Kilburn MR.
Department of Applied Geology, Curtin University of Technology, Kent Street, Bentley, Western Australia 6102, Australia.
The evolution of oxygenic photosynthesis had a profound impact on the Earth's surface chemistry, leading to a sharp rise in atmospheric oxygen between 2.45 and 2.32 billion years (Gyr) ago and the onset of extreme ice ages. The oldest widely accepted evidence for oxygenic photosynthesis has come from hydrocarbons extracted from approximately 2.7-Gyr-old shales in the Pilbara Craton, Australia, which contain traces of biomarkers (molecular fossils) indicative of eukaryotes and suggestive of oxygen-producing cyanobacteria. The soluble hydrocarbons were interpreted to be indigenous and syngenetic despite metamorphic alteration and extreme enrichment (10-20 per thousand) of (13)C relative to bulk sedimentary organic matter. Here we present micrometre-scale, in situ (13)C/(12)C measurements of pyrobitumen (thermally altered petroleum) and kerogen from these metamorphosed shales, including samples that originally yielded biomarkers. Our results show that both kerogen and pyrobitumen are strongly depleted in (13)C, indicating that indigenous petroleum is 10-20 per thousand lighter than the extracted hydrocarbons. These results are inconsistent with an indigenous origin for the biomarkers. Whatever their origin, the biomarkers must have entered the rock after peak metamorphism approximately 2.2 Gyr ago and thus do not provide evidence for the existence of eukaryotes and cyanobacteria in the Archaean eon. The oldest fossil evidence for eukaryotes and cyanobacteria therefore reverts to 1.78-1.68 Gyr ago and approximately 2.15 Gyr ago, respectively. Our results eliminate the evidence for oxygenic photosynthesis approximately 2.7 Gyr ago and exclude previous biomarker evidence for a long delay ( approximately 300 million years) between the appearance of oxygen-producing cyanobacteria and the rise in atmospheric oxygen 2.45-2.32 Gyr ago.
