English: A schematic view of the changing concentration of atmospheric oxygen as a function of geological time in billions of years (Ga). Correlation between the estimated oxygen concentration changes and the major evolutionary events on Earth are based on data presented by Falkowski (2006; 2011), Tomitani et al. (2006), Kump (2008), Blankenship (2002; 2010), and Hohmann-Marriott and Blankenship (2011). The dates in the figure are not to scale and are approximations. Uncertainties of the selected events in the evolution of life are depicted with bars. A number of hypotheses exist as to when oxygenic photosynthesis was invented, by primitive cyanobacteria-like organisms. In this scheme, we have summarized currently available data and depict (by a black bar) that the first cyanobacteria could have evolved as early as 3.2 Ga or as late as 2.4 Ga ago. Therefore, an upper boundary of 3.2 Ga for the age of oxygenic photosynthesis and the initial rise of atmospheric oxygen in the atmosphere as indicated in this figure is still under discussion in the literature (see e.g., Tomitani et al., 2006; Allen and Martin, 2007; Blankenship et al., 2007; Buick, 2008). The figure is modified and adapted from Shevela and Messinger (2011).