~2.8 Billion years ago
||Kihara S, Hartzler DA, Savikhin S. Oxygen concentration inside a functioning photosynthetic cell. Biophys J. 2014 May 6 106(9):1882-9. doi: 10.1016/j.bpj.2014.03.031 abstract and p.1882 left columnPubMed ID24806920
|| Olson, J.M. Photosynthesis in the Archean era. Photosynth. Res. 2006 88: 109–117 DOI: 10.1007/s11120-006-9040-5  Catling, D.C. and Claire, M.W. How Earth's atmosphere evolved to an oxic state: a status report. Earth Planet. Sci. Lett. 2005 237: 1–20PubMed ID16453059
||Abstract: "Such a low concentration suggests that the first oxygenic photosynthetic bacteria in solitary form could have evolved ~2.8 billion years ago without special mechanisms to protect them against reactive oxygen species." P.1882 left column: "The evolution of oxygenic photosynthesis by cyanobacteria ~2.8 billion years ago (primary source 1) introduced oxygen to the Earth’s atmosphere and oceans, causing dramatic changes in the ecosphere of our planet (primary source 2)."