关于地球上能够生成氧气的光合作用,被人们广泛接受的最古老的证据来自从澳大利亚Pilbara Craton距今27亿年前的页岩中提取出的烃类生物标记,它们被认为是真核生物及光合作用蓝藻的证据。
这么早的时间引起了一些争议,因为在能够产生氧的蓝藻在地球上最早出现的这个时间与大约3亿年后引起大气中氧含量增加的“大氧化事件”(great oxidation event)有一个很长的时间延迟。由Rasmussen等人所做的新的研究工作表明,有机生物标记不是太古代的,一定是在晚些时候、在距今大约22亿年前之后进入岩石的。因此,真核生物及蓝藻出现在地球上的最早的、明确无误的化石证据分别应该被纠正为在距今17.8亿–16.8亿年前和21.5亿年前。(生物谷Bioon.com)
生物谷推荐原始出处:
Nature 455, 1101-1104 (23 October 2008) | doi:10.1038/nature07381
Reassessing the first appearance of eukaryotes and cyanobacteria
Birger Rasmussen1, Ian R. Fletcher1, Jochen J. Brocks2,3 & Matt R. Kilburn4
1 Department of Applied Geology, Curtin University of Technology, Kent Street, Bentley, Western Australia 6102, Australia
2 The Research School of Earth Sciences,
3 Centre for Macroevolution and Macroecology, The Australian National University, Canberra, Australian Capital Territory 0200, Australia
4 Centre for Microscopy, Characterisation and Analysis, The University of Western Australia, 35 Stirling Highway, Crawley, Western Australia 6009, 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) ago1, 2 and the onset of extreme ice ages3. The oldest widely accepted evidence for oxygenic photosynthesis has come from hydrocarbons extracted from 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 cyanobacteria4, 5, 6, 7. The soluble hydrocarbons were interpreted to be indigenous and syngenetic despite metamorphic alteration and extreme enrichment (10–20) of 13C relative to bulk sedimentary organic matter5, 8. Here we present micrometre-scale, in situ13C/12C 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 13C, indicating that indigenous petroleum is 10–20 lighter than the extracted hydrocarbons5. These results are inconsistent with an indigenous origin for the biomarkers. Whatever their origin, the biomarkers must have entered the rock after peak metamorphism 2.2 Gyr ago9 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 2.15 Gyr ago10, 11, respectively. Our results eliminate the evidence for oxygenic photosynthesis 2.7 Gyr ago and exclude previous biomarker evidence for a long delay (300 million years) between the appearance of oxygen-producing cyanobacteria and the rise in atmospheric oxygen 2.45–2.32 Gyr ago1.