Figure 3. Typical redox couples in neutral seawater (adapted from Gaidos et al. 1999). Half-reactions on the left couple spontaneously with those below them on the right. The red couple on the lower right indicates the lowest redox level (down to molecular O2) achieved by respiratory chains in the Last Common Ancestor (Castresana and Moreira 1999) prior to about 4 Ga (Hedges et al. 2001).