"Most researchers imagine the initial oxygenation of the ocean and atmosphere to have been something like a staircase, but with steps only going up.  The first step, so the story goes, occurred around 2.4 billion years ago, and this, the so-called Great Oxidation Event, has obvious implications for the origins and evolution of the first forms of eukaryotic life.  The second big step in this assumed irreversible rise occurred almost two billion years later, coinciding with the first appearances and earliest diversification of animals."

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