It is generally believed that the Earth's atmosphere did not contain oxygen until around 2500 million years ago (Mya) when oxygen-evolving photosynthetic bacteria arose. At around 800–500 Mya, the oxygen concentration increased sharply to reach the 21% we have today. So, it seems highly likely that life arose as anaerobic organisms, which then evolved to tolerate oxygen and finally, to use it as a terminal acceptor for the energy-producing oxidative processes in the respiration of aerobic bacteria and mitochondria in eukaryotic cells. The respiratory processes employed by typical aerobic organisms today have a wide range of mechanisms to deal with the troublesome side effects of living with a high oxygen concentration.

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