Pincemail and colleagues  say that oxygen appeared in the Earth's atmosphere 3 million years ago. Perhaps 'million' is a misprint for 'billion' (in the American sense of 109) but even that is not strictly correct.
Cyanobacteria (blue-green algae) and the resultant stomatolites have been dated as early as 3.5 × 109 years ago. Although it is presumed that these organisms produced oxygen, the atmospheric level probably remained less than 10−2 × present atmospheric level until about 2 × 109 years ago. During this long interval, oxygen was utilized in large quantities for the oxidation of ferrous iron to form the immense banded iron formations. Ferrous iron could not have been transported in an oxidizing environment. The major increase in atmospheric oxygen is dated to between 2.0 and 1.8 × 109 years ago, after which banded iron deposits rarely occur. Red beds of iron deposited in the ferric (oxidized) state began to appear 2 × 109 years ago . The subsequent rate of rise of oxygen concentrations towards present atmospheric levels is still uncertain but it probably exceeded present atmospheric levels 300 × 106 years ago .
JOHN. F. NUNN
Northwood, Middlesex, UK
1 Pincemail J, Defraigne JO, Limet R. Oxidative stress in clinical situations—fact or fiction? Eur J Anaesthesia
2 Kasting JF. Earth's early atmosphere. Science
3 Graham JB, Dudley R, Aquilar NM, Gans C. Implications of the late Palaeozoic oxygen pulse for physiology and evolution. Nature