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In page Archean:

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The earliest identifiable fossils consist of stromatolites, which are microbial mats formed in shallow water by cyanobacteria. The earliest stromatolites are found in 3.48 billion-year-old sandstone discovered in Western Australia.[2][3] Stromatolites are found throughout the Archean[4] and become common late in the Archean.[5]: 307  Cyanobacteria were instrumental in creating free oxygen in the atmosphere.[citation needed]