Earth was mostly devoid of oxygen for much of its 4.5 billion year lifetime. That is, until certain processes started to allow for the eventual buildup of oxygen up to the levels we have now (around 21% of the atmosphere). While scientists have found evidence of the approximate timescales of rises in oxygen over time and are aware of some of the mechanisms behind it, the main driver behind Earth's long-term oxygenation is still unclear.
Earth was mostly devoid of oxygen for much of its 4.5 billion year lifetime. That is, until certain processes started to allow for the eventual buildup of oxygen up to the levels we have now (around 21% of the atmosphere). While scientists have found... [5566 chars]