Signatures of early microbial life from the Archean (4 to 2.5 Ga) eon

The Archean era (4 to 2.5 billion years ago, Ga) yielded rocks that include the oldest conclusive traces of life as well as many controversial occurrences. Carbonaceous matter is found in rocks as old as 3.95 Ga, but the oldest (graphitic) forms may be abiogenic. Due to the metamorphism that altered the molecular composition of all Archean organic matter, non-biological carbonaceous compounds such as those that could have formed in seafloor hydrothermal systems are difficult to rule out. Benthic microbial mats as old as 3.47 Ga are supported by the record of organic laminae in stromatolitic (layered) carbonates, in some stromatolitic siliceous sinters, and in some siliciclastic sediments. In these deposits, organic matter rarely preserved fossil cellular structures (e.g., cell walls) or ultrastructures (e.g., external sheaths) and its simple textures are difficult to attribute to either microfossils or coatings of cell-mimicking mineral templates. This distinction will require future nanoscale studies. Filamentous-sheath microfossils occur in 2.52 Ga rocks, and may have altered counterparts as old as 3.47 Ga. Surprisingly large spheres and complex organic lenses occur in rocks as old as 3.22 Ga and ~ 3.4 Ga, respectively, and represent the best candidates for the oldest microfossils. Titaniferous microtubes in volcanic or volcanoclastic rocks inferred as microbial trace fossils have been reevaluated as metamorphic or magmatic textures. Microbially-induced mineralization is supported by CaCO₃ nanostructures in 2.72 Ga stromatolites. Sulfides 3.48 Ga and younger bear S-isotope ratios indicative of microbial sulfate reduction. Ferruginous conditions may have fueled primary production via anoxygenic photosynthesis-as suggested by Fe-isotope ratios-possibly as early as 3.77 Ga. Microbial methanogenesis and (likely anaerobic) methane oxidation are indicated by C-isotope ratios as early as 3.0 Ga and ~ 2.72 Ga, respectively. Photosynthetic production of O₂ most likely started between 3.2 and 2.8 Ga, i.e. well before the Great Oxidation Event (2.45-2.31 Ga), as indicated by various inorganic tracers of oxidation reactions and consistent with morphology of benthic deposits and evidence for aerobic N metabolism in N-isotope ratios at ~ 2.7 Ga.

This picture of a wide diversification of the microbial biosphere during the Archean has largely been derived of bulk-rock geochemistry and petrography, supported by a recent increase in studied sample numbers and in constraints on their environments of deposition. Use of high-resolution microscopy and micro- to nanoscale analyses opens avenues to (re)assess and decipher the most ancient traces of life.