Abstract
Although the notion of an early origin and diversification of life on Earth during the Archaean eon has received increasing support in geochemical, sedimentological and palaeontological evidence, ambiguities and controversies persist regarding the biogenicity and syngeneity of the record older than Late Archaean1,2,3. Non-biological processes are known to produce morphologies similar to some microfossils4,5, and hydrothermal fluids have the potential to produce abiotic organic compounds with depleted carbon isotope values6, making it difficult to establish unambiguous traces of life. Here we report the discovery of a population of large (up to about 300 μm in diameter) carbonaceous spheroidal microstructures in Mesoarchaean shales and siltstones of the Moodies Group, South Africa, the Earth’s oldest siliciclastic alluvial to tidal-estuarine deposits7. These microstructures are interpreted as organic-walled microfossils on the basis of petrographic and geochemical evidence for their endogenicity and syngeneity, their carbonaceous composition, cellular morphology and ultrastructure, occurrence in populations, taphonomic features of soft wall deformation, and the geological context plausible for life, as well as a lack of abiotic explanation falsifying a biological origin. These are the oldest and largest Archaean organic-walled spheroidal microfossils reported so far. Our observations suggest that relatively large microorganisms cohabited with earlier reported benthic microbial mats8 in the photic zone of marginal marine siliciclastic environments 3.2 billion years ago.
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Acknowledgements
We thank M. Giraldo, J. Laval and N. Decloux for sample preparation; P. Compère for TEM imaging; C. Henrist for environmental SEM imaging and energy-dispersive X-ray analyses; T. Prokopiuk for carbon isotope analyses; J. Robertson for information on geology of the Agnes Gold Mine; and A. H. Knoll for comments on an earlier version of the manuscript. The study was supported by a University of Liège Impulsion Grant (CFRA0805) and a University of Liège grant (RCFRA0036-J) to E.J., a National Science Foundation grant (EAR-937 05-45484), a NASA Astrobiology Institute award (NNA04CC09A), a Natural Sciences and Engineering Research Council of Canada 938 Discovery grant to A.B., and Australian Research Council funding to C.M.
Author Contributions E.J. and A.B. conceived the study and wrote the paper. E.J. discovered the microfossils and performed the microscopic, SEM and TEM analyses, and interpreted the data. C.M. performed the Raman analyses and interpreted and wrote the results. A.B. conducted field work and carbon isotope analyses and interpreted the sedimentary structures. All authors commented on the manuscript.
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Javaux, E., Marshall, C. & Bekker, A. Organic-walled microfossils in 3.2-billion-year-old shallow-marine siliciclastic deposits. Nature 463, 934–938 (2010). https://doi.org/10.1038/nature08793
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DOI: https://doi.org/10.1038/nature08793
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