![]() Oxygenic photosynthesis has enabled the oxygenation of oceanic and terrestrial niches, and the diversification of complex life. They are the only prokaryotic organisms that perform oxygenic photosynthesis, and are thus generally held responsible for the rise of oxygen in the atmosphere and oceans around 2.4 Ga, during the so-called Great Oxidation Event (GOE ), facilitated by geological processes. ![]() Cyanobacteria also played a major role in early biogeochemical fluxes and in Life and Earth evolution. They play a key role in food webs as primary producers performing oxygenic photosynthesis. Modern cyanobacteria constitute an ancient and well-diversified bacterial phylum, with unique complex morphologies and cellular differentiation. Finally, we suggest possible new calibration points for molecular clocks, and strategies to improve our understanding of the timing and pattern of the evolution of cyanobacteria and oxygenic photosynthesis. We also assess the reliability of the previously described cyanobacteria fossil record and the challenges of molecular approaches on modern cyanobacteria. Here, we review the classic and new cyanobacterial biosignatures. ![]() Recently, new biosignatures specific to cyanobacteria were proposed. Nevertheless, the identification of cyanobacteria in the early fossil record remains ambiguous because the morphological criteria commonly used are not always reliable for microfossil interpretation. They are also major primary producers in past and present oceans, and the ancestors of the chloroplast. ![]() They are responsible for the oxygenation of the atmosphere and oceans since the Great Oxidation Event around 2.4 Ga, debatably earlier. Cyanobacteria played an important role in the evolution of Early Earth and the biosphere. ![]()
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