Figure 1.
Estimated evolution of atmospheric oxygen content over Earth’s history (e.g., Rye and Holland, 1998; Catling and Claire, 2005; Canfield, 2005; Holland, 2006). The likely timing of the evolution of oxygenic photosynthesis, indicated by bar A (Schopf, 1993) to B (Brocks et al., 1999) significantly precedes the large increase in atmospheric oxygen indicated by the end of mass-independent S-isotope fractionation (C; Domagal-Goldman et al., 2008). During this time, local oxygen oases may have existed.