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) ago and the onset of extreme ice ages. The oldest widely accepted evidence for oxygenic photosynthesis has come from hydrocarbons extracted from approximately 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 cyanobacteria. The soluble hydrocarbons were interpreted to be indigenous and syngenetic despite metamorphic alteration and extreme enrichment (10-20 per thousand) of (13)C relative to bulk sedimentary organic matter. Here we present micrometre-scale, in situ (13)C/(12)C 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 (13)C, indicating that indigenous petroleum is 10-20 per thousand lighter than the extracted hydrocarbons. These results are inconsistent with an indigenous origin for the biomarkers. Whatever their origin, the biomarkers must have entered the rock after peak metamorphism approximately 2.2 Gyr ago 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 approximately 2.15 Gyr ago, respectively. Our results eliminate the evidence for oxygenic photosynthesis approximately 2.7 Gyr ago and exclude previous biomarker evidence for a long delay (approximately 300 million years) between the appearance of oxygen-producing cyanobacteria and the rise in atmospheric oxygen 2.45-2.32 Gyr ago.

The record of Archaean microfossils is sparse. Of the few bona fide fossil assemblages, most are from shallow-water settings, and they are typically associated with laminated, stromatolitic sedimentary rocks. Microfossils from deep-sea hydrothermal systems have not been reported in Precambrian rocks (> 544 million years old), although thermophilic microbes are ubiquitous in modern sea-floor hydrothermal settings, and apparently have the most ancient lineages. Here, I report the discovery of pyritic filaments, the probable fossil remains of thread-like microorganisms, in a 3,235-million-year-old deep-sea volcanogenic massive sulphide deposit from the Pilbara Craton of Australia. From their mode of occurrence, the micro-organisms were probably thermophilic chemotropic prokaryotes, which inhabited sub-sea-floor hydrothermal environments. They represent the first fossil evidence for microbial life in a Precambrian submarine thermal spring system, and extend the known range of submarine hydrothermal biota by more than 2,700 million years. Such environments may have hosted the first living systems on Earth, consistent with proposals for a thermophilic origin of life.

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