The existence of a terrestrial Precambrian (more than 542 Myr ago) biota has been largely inferred from indirect chemical and geological evidence associated with palaeosols1,2, the weathering of clay minerals3 and microbially induced sedimentary structures in siliciclastic sediments4. Direct evidence of fossils within rocks of non-marine origin in the Precambrian is exceedingly rare5,6. The most widely cited example comprises a single report of morphologically simple mineralized tubes and spheres interpreted as cyanobacteria, obtained from 1,200-Myr-old palaeokarst in Arizona5. Organic-walled microfossils were first described from the non-marine Torridonian (1.2–1.0 Gyr ago) sequence of northwest Scotland in 19077. Subsequent studies8,9,10 found few distinctive taxa—a century later, the Torridonian microflora is still being characterized as primarily nondescript “leiospheres”11. We have comprehensively sampled grey shales and phosphatic nodules throughout the Torridonian sequence. Here we report the recovery of large populations of diverse organic-walled microfossils extracted by acid maceration, complemented by studies using thin sections of phosphatic nodules that yield exceptionally detailed three-dimensional preservation. These assemblages contain multicellular structures, complex-walled cysts, asymmetric organic structures, and dorsiventral, compressed organic thalli, some approaching one millimetre in diameter. They offer direct evidence of eukaryotes living in freshwater aquatic and subaerially exposed habitats during the Proterozoic era. The apparent dominance of eukaryotes in non-marine settings by 1 Gyr ago indicates that eukaryotic evolution on land may have commenced far earlier than previously thought.
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J. Rosenberg produced the confocal laser scanning image (Supplementary Fig. 6a); we thank O. Green for preparation of phosphatic nodules at Oxford. We thank J. Antcliffe, R. Callow and S. Moorhouse for field assistance and the people of Scoraig and Bill (the boatman) for access to Cailleach Head. This research was supported by NASA grant NNX07AU79G (P.K.S.), NERC NE/G015716/1 (C.H.W.) and NERC NE/G524060/1 (L.B.).
The authors declare no competing financial interests.
All materials (rock sample, remaining organic residues, palynological slides, thin sections) are curated in the collections of the Centre for Palynology of the University of Sheffield, UK.
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Strother, P., Battison, L., Brasier, M. et al. Earth’s earliest non-marine eukaryotes. Nature 473, 505–509 (2011). https://doi.org/10.1038/nature09943
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