The introduction of oxygen into the Earth's atmosphere was a double-edged sword. It provided a fuel that would allow the evolution of complex organisms with high energy demands, but also represented a new source of toxins. Oxygen-respiring eukaryotes needed not only to develop machinery to harness the power of oxygen, which they gained through the acquisition of mitochondria, but also to build up defences against its toxic effects.

Oxygen has been a trouble-maker from the very beginning. Although by nature a sluggish reactant, it has a tendency to 'radicalize', forming incompletely reduced reactive oxygen species (ROS, such as O·–, H2O2 and HO·), which are highly potent oxidants. Within the cell, ROS can cause genetic degeneration and physiological dysfunction, eventually leading to cell death and progressive ageing of the organism.

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