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Reassessing the first appearance of eukaryotes and cyanobacteria


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 situ 13C/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.

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Figure 1: Reflected light photomicrographs of kerogen.
Figure 2: Reflected light photomicrographs (a–e, g) and a plane-polarized light photomicrograph (f) of pyrobitumen.
Figure 3: Carbon isotopic composition of organic matter.


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We thank S. Bengtson, G. A. Logan, J. R. Muhling, S. Revets and S. Sheppard for discussion and comments; and A. C. Cook for organic reflectivity measurements. We acknowledge the facilities of the Australian Microscopy and Microanalysis Research Facility at the Centre for Microscopy, Characterisation and Analysis, University of Western Australia, a facility funded by the University, State and Commonwealth Governments.

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Correspondence to Birger Rasmussen.

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Rasmussen, B., Fletcher, I., Brocks, J. et al. Reassessing the first appearance of eukaryotes and cyanobacteria. Nature 455, 1101–1104 (2008).

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