State of the planet· The columns of time
Oxygenated world
2.22 Ga → 717 Ma
A billion and a half years of air that contains oxygen, and very little of it. What that ceiling made impossible may be why large animals took so long to appear.
What the evidence shows
Oxygen had been produced for hundreds of millions of years before this regime opens; what changes is that it stops being consumed as fast as it is made. Then almost nothing changes again for 1.5 billion years, from 2.22 billion to 717 million. Levels stay far below the modern ones throughout, probably under a few per cent of today's, which is enough for a cell and not enough for a body: a large active animal has to move oxygen from its surface to its centre, and below a certain concentration that cannot be done at all. This ceiling is one of the standing explanations for why animals appear only at the very end of the regime, and not in the billion years of opportunity that preceded it.
Bounds and conventions
The regime marks a presence, not an abundance, and the distinction carries the whole argument: to say the Proterozoic air held oxygen is true and to imagine it breathable is not. How much there was is estimated from trace metals and isotopes in sediments, methods that give a ceiling more readily than a value, and the published ceilings differ from one another by a factor of ten.