Gaia Revisited: the Interplay Between Climate and Life on the Early Earth:

Earth’s atmosphere and climate have both undergone marked changes during the course of Earth’s history. Our climate has remained warm most of the time even though the Sun was considerably fainter in the distant past. This warmth was almost certainly caused by enhanced greenhouse gas concentrations. CO2 and H2O were probably the dominant greenhouse gases initially, but CH4 may have become an important contributor soon after life evolved, with atmospheric concentrations exceeding 1000 ppmv. CH4 is mostly biological in origin; hence, life may have played a direct role in controlling climate on the early Earth. The rise of O2 at ~2.3 Ga, now conclusively established by studies of mass independent fractionation (MIF) in S isotopes from ancient rocks, caused a marked decrease in methane, thereby triggering the Paleoproterozoic glaciations. An earlier, less well documented glaciation at 2.8-2.9 Ga corresponds to a second anomaly in the MIF record and may also have been triggered by biological innovations.