The shaded regions represent cases when the biogenic O2 fluxes are either implausibly high or low. It is important to realize that for the results of our biogeochemical model, we are considering a non-steady-state, time-dependent Earth system. Sulphate built up in the ocean going from the Archaean to the Proterozoic, so that the summed flux of sulphate-producing weathering plus input from photochemical sulphate rainout must have outpaced sulphide deposition in the ocean.

Thus, a curious subtlety is that marine microbial sulphate reduction should have exceeded the flux of sulphate from weathering in the Archaean because the sulphate at this time primarily derived from photochemical oxidation of volcanic gases. Hydrogen escape to space supported a sulphate rainout flux.

Life on Earth is thought to have coevolved with the chemistry of the oceans and atmosphere, and the shift from an anoxic to an oxic world across the Archean-Proterozoic boundary represents a fundamental step in this process.

Mitochondria have been put forward as the saviours of anaerobes when their environment became oxygenated. However, despite oxygenic photosynthesis evolving around 2. This drastically changes the textbook viewof the ecological conditions under which the mitochondrial endosymbiont established itself. It could explain the widespread distri-bution of anaerobic biochemistry in every eukaryotic supergroup: Eukaryote, evolution, hydrogenosome, mitochondrion, syntrophy.

T HE traditional model of eukaryotic evolution states thateukaryotes arose by gradual changes to a prokaryoticprogenitor cell. This new, or primitive, eukaryote did notcontain any organelles and has been suggested to be a hetero-trophic anaerobic amoeboagellate Sagan The serialendosymbiosis theory proposed that several eukaryotic fea-tures such as mitochondria, plastids and agella arose by sub-sequent symbioses with dierent prokaryotes.

It is nowgenerally accepted that mitochondria and plastids are derivedfrom once free-living prokaryotes Gray , although theagella hypothesis lacks supporting data beyond resemblances.