The rise of oxygen and complex life.

Mitochondria have been put forward as the saviours of anaerobes when their environment became oxygenated. However, despite oxygenic photosynthesis evolving around 2.7 billion years ago (Ga), followed by the "Great Oxidation" of the atmosphere ~ 2.4 Ga, the deep oceans remained largely anoxic and either iron-enriched or sulphidic until 580 million years ago, when the eukaryotic radiation was well underway. Atmospheric oxygen probably remained at an intermediate concentration (1-10% of the present level) from ~ 2.4 until ~ 0.8 Ga when a "lesser oxidation" began. This drastically changes the textbook view of the ecological conditions under which the mitochondrial endosymbiont established itself. It could explain the widespread distribution of anaerobic biochemistry in every eukaryotic supergroup: anaerobic biochemistry is hard-wired into the eukaryotes.