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Fig. 3. Milestones in evolution. (A) Paleontological periods of the Precambrian era. (B) Estimates of total earth biomass as a function of time. The graph is only a qualitative representation because it is not possible to establish or extrapolate precise levels of precambrian biomass from paleontological records (Kelly and Adams, 1994; Papagiannis, 1984; DeLong et al., 1994; Koch, 1998; O'Callaghan and Conrad, 1992; Reeve, 1992). BYA, billion years ago; Ph indicates initiation of the major increase in photosynthesis by cyanobacteria.