Figure 2. Biological evolution, compared with a schematic representation of the atmospheric partial pressures of oxygen in the Earth and Martian atmospheres, as a function of geological time (horizontal axis). The horizontal lines along the top of the figure show the molecular clock estimates for the divergence times between the Bacteria, Archaea, and Eukaryotic Domains as recently estimated by Hedges et al. (2001), with the position of the Last Common Ancestor (LCA) of all life shown in the upper left. Error bars slightly below this labeled "Archaea vs. Bacteria" show the 1 and 2 error estimates for this LCA, and to the right of this similar errors are shown for the origin of the cyanobacteria (from Hedges et al. 2001). Note that the cyanobacteria (and hence the O₂-releasing complex of Photosystem-II) evolved significantly later than the LCA. Colored horizontal bars below this show the geological distribution of rocks and fossil types which have implications for the oxygen concentrations in the atmosphere and shallow waters of the oceans, including the Banded Iron Formations (BIFs), red beds, magnetofossils and eukaryotes (same distribution), phytoplankton, animals, and charcoal (adapted loosely from Kasting 1993). The blue bar on the left of this shows the probable time interval during which the Martian magnetic dynamo was active, shielding its atmosphere from erosion by the solar wind (partly constrained by the age of magnetization in ALH84001; Weiss, B. P., Vali, H., Baudenbacher, F. J., Stewart, S. T., and Kirschvink, J. L., unpublished manuscript). The box in the lower part of the diagram shows the perceived history of atmospheric oxygen for Earth and Mars. For Earth, the light blue field between 4.5 and 2.3 Ga reflects the presence of detrital, stream-rounded pyrite and uraninite, reducing paleosols, BIFs, and Mn⁺² incorporated into shallow-water carbonates, etc. The lower, deep-blue field shows the upper limit of 10^-8 bar O_{2 (}estimated by Pavlov et al. 2000) for a methane-dominated greenhouse atmosphere, which may have been promoted by more reducing source regions for volcanic gasses in Earth’s mantle during the first half of planetary history (Kump et al. 2001). The vertical black bar at about 2.3 Ga shows the ~70 Ma duration of the Paleoproterozoic Snowball Earth and the associated Kalahari Manganese Field which was deposited in its aftermath (Kirschvink et al. 2000; presumably as a result of the evolution of Photosystem-II in pre-existing cyanobacteria), with the resultant massive release of O₂ and the collapse of the methane greenhouse (Pavlov et al. 2000). Following this, the green field shows the transition from the post-Snowball episode until the depletion of ferrous iron at all levels of the world oceans (and an end to this buffering). The pink field indicates the subsequent rise in O₂ level to modern values, including 3± 1 spikes in O₂ anticipated from cyanobacterial blooms following the Neoproterozoic Snowball events (Kirschvink et al. 2000).

The red dotted line for Mars is anchored by the current atmospheric concentration of O₂, and is speculatively extended back in time for the inferred thicker atmosphere earlier in its history. We show it as a peak at 4.0 Ga to reflect the possible presence of ozone in the Martian atmosphere inferred from mass-independent fractionation of oxygen in ALH84001 carbonates (Farquhar et al. 1998), and infer a positive slope before this as a result of magnetic shielding of the Martian atmosphere by a geodynamo during which H (but not O) was lost to space.