Atmospheric oxygen and methane on the early Earth

The habitability of the early Earth was affected by various factors. We focus here on atmospheric oxygen (O 2 ) and methane (CH 4 ). O 2 is essential for supporting eukaryotic life, including plants and animals. CH 4 , along with CO 2 and H 2 O, is a powerful greenhouse gas that may have helped offset the lower luminosity of the young Sun. Its concentration is disputed, though, in both the Archean and Proterozoic eons. The CH 4 concentration depends strongly on atmospheric O 2 , which began to be produced biologically in geochemically significant amounts sometime during the Archean, probably near 2.7 Ga, based on the mass-independent sulphur isotope record and other (C and N) isotopic data. O 2 was produced by cyanobacteria living in terrestrial mats and in the surface ocean during this time, generating localized oxygen oases that vented O 2 into the atmosphere. Sometime between 2.2 and 2.4 Ga, a Great Oxidation Event occurred, causing O 2 to increase substantially, likely to approximately 10 per cent of present levels. Increases in O 2 near the beginning and end of the Proterozoic may have caused methane to decrease, triggering the Snowball Earth glaciations observed during both time intervals. This article is part of the discussion meeting issue ‘Chance and purpose in the evolution of biospheres’.