The 3,430-million-year-old Strelley Pool Chert (SPC) (Pilbara Craton, Australia) is a sedimentary rock formation containing laminated structures of probable biological origin (stromatolites). Determining the biogenicity of such ancient fossils is the subject of ongoing debate. However, many obstacles to interpretation of the fossils are overcome in the SPC because of the broad extent, excellent preservation and morphological variety of its stromatolitic outcrops--which provide comprehensive palaeontological information on a scale exceeding other rocks of such age. Here we present a multi-kilometre-scale palaeontological and palaeoenvironmental study of the SPC, in which we identify seven stromatolite morphotypes--many previously undiscovered--in different parts of a peritidal carbonate platform. We undertake the first morphotype-specific analysis of the structures within their palaeoenvironment and refute contemporary abiogenic hypotheses for their formation. Finally, we argue that the diversity, complexity and environmental associations of the stromatolites describe patterns that--in similar settings throughout Earth's history--reflect the presence of organisms.

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 Archaean. Non-biological processes are known to produce morphologies similar to some microfossils, and hydrothermal fluids have the potential to produce abiotic organic compounds with depleted carbon isotope values, making it difficult to establish unambiguous traces of life. Here we report the discovery of a population of large (up to about 300 mum 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 deposits. 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 mats in the photic zone of marginal marine siliciclastic environments 3.2 billion years ago.

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