Microstructures in the ~3.5 Gyr Apex Chert Formation were initially described as the oldest bacterial fossils on Earth over 20 years ago1. However, the identification of the structures (which resemble cyanobacteria) as biological in origin remains controversial1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13. Here we determine the petrology and geochemistry of similar structures from the original Apex Chert locality using thin sections and Raman spectroscopy. Based on the microscopic examination of thin sections, we identify features similar to those previously identified as microfossils as a series of quartz and haematite-filled fractures. Raman spectroscopy of the fractures shows that carbonaceous material is not, as previously reported, associated with the structures, but is instead disseminated in the surrounding quartz matrix. We suggest that although the microstuctures analysed are not microfossils, the presence of carbonaceous material in the surrounding matrix is consistent with the existence of microbial life at this time, and with evidence of early Archaean life14, 15 found at other sites. Furthermore, we caution against identifying microstructures as biological in origin without a full morphological and geochemical assessment.
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