New types of carbonaceous filamentous microstructures have been identified in silica veins at two new localities in the ˜3.5 Ga North Pole area of Western Australia. Their carbon isotopic compositions were measured in situ by secondary-ion mass spectrometry. The carbonaceous filaments are ˜1 μm wide, 10 to 100 μm long, and are permineralized in a fine-grained (˜1 μm) silica matrix. They are morphologically divided into three types (i.e., spiral, thread-like, and branched filaments). Their sizes and morphologies resemble modern and previously reported fossil bacteria. These similarities and their complex three-dimenstional geometry suggest that they may represent morphologically preserved fossil bacteria. δ13C values of the carbonaceous filaments range from -42 to -32‰, which strongly suggest that they are composed of biologically fixed organic compounds, possibly via the reductive acetyl-CoA pathway or the Calvin cycle. This is consistent with the hypothesis that autotrophs already existed on the Archean Earth.
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