Letter

Nature 455, 1101-1104 (23 October 2008) | doi:10.1038/nature07381; Received 13 April 2008; Accepted 28 August 2008

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

Correspondence to: Birger Rasmussen1 Correspondence and requests for materials should be addressed to B.R. (Email: B.Rasmussen@curtin.edu.au).

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 approx2.7-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 (10–20permil) 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 10–20permil 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 approx2.2 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 approx2.15 Gyr ago10, 11, respectively. Our results eliminate the evidence for oxygenic photosynthesis approx2.7 Gyr ago and exclude previous biomarker evidence for a long delay (approx300 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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