FIGURE 1. The evolution of eukaryotes and cyanobacteria, and the rise of atmospheric oxygen.

There are large gaps in time between the first morphological fossil evidence for cyanobacteria and eukaryotes, and the molecular-fossil occurrences inferred by Brocks et al.^{3, 4}from lipid biomarkers. This discrepancy may disappear if the results of Rasmussen et al.⁵, showing that the biomarkers are younger than their host rocks, are confirmed — as may the 300-million-year delay before the later rise in atmospheric oxygen. Because oxygenic photosynthesis is the ultimate source of environmental oxygen, cyanobacteria must have evolved by a minimum of 2.4 billion years ago. This still leaves a gap of about 400 million years in the other direction, between the rise of oxygen and the first firm fossil evidence of cyanobacteria.