關(guān)于地球上能夠生成氧氣的光合作用,，被人們廣泛接受的最古老的證據(jù)來自從澳大利亞Pilbara Craton距今27億年前的頁巖中提取出的烴類生物標(biāo)記，它們被認(rèn)為是真核生物及光合作用藍(lán)藻的證據(jù),。
這么早的時(shí)間引起了一些爭議,，因?yàn)樵谀軌虍a(chǎn)生氧的藍(lán)藻在地球上最早出現(xiàn)的這個(gè)時(shí)間與大約3億年后引起大氣中氧含量增加的“大氧化事件”（great oxidation event）有一個(gè)很長的時(shí)間延遲。由Rasmussen等人所做的新的研究工作表明,，有機(jī)生物標(biāo)記不是太古代的,，一定是在晚些時(shí)候、在距今大約22億年前之后進(jìn)入巖石的,。因此,，真核生物及藍(lán)藻出現(xiàn)在地球上的最早的、明確無誤的化石證據(jù)分別應(yīng)該被糾正為在距今17.8億–16.8億年前和21.5億年前,。（生物谷Bioon.com）
生物谷推薦原始出處：
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.
