In the narrative of life on Earth we have now reached the Archaean Eon (3.8 to 2.5 billion years ago); the age when chemistry came alive. During this period the Earth day increased from about 15 hours long to about 18 hours and the Sun brightened to 80% of its current level. At present, few data are available that are able to specify the atmospheric, oceanic or geological conditions on the early (prebiological) Earth. It can be reasonably assumed that conditions were very hostile due to volcanism, radiation, and continued bombardment by objects large and small from space; but it is likely that the average climate was temperate rather than extremely cold or hot (Kasting & Howard, 2006). As I have discussed in Chapter 6, there is no agreement on the gaseous composition of the primeval atmosphere apart from the general acceptance that oxygen was absent. According to Lazcano & Miller (1996): ‘atmospheric chemists mostly favor high CO2 + N2, whereas prebiotic chemists mostly favor more reducing conditions’. In fact high levels of carbon dioxide in the early atmosphere are indicated by the high level of carbonate minerals in rocks of that age.

One of the reasons for postulating reduced gases in the atmosphere of the early Earth is that such an atmosphere would have had a greenhouse effect, which would compensate for the dimmer Sun of the day and keep the Earth from freezing over (Sagan & Chyba, 1997). It is thought that there was a global glaciation in the mid Archaean, approximately 2.9 billion years ago, which might have been caused by a hydrocarbon haze shielding solar radiation, the haze resulting from methane photolysis after the newly evolved methanogens increased the atmospheric methane/carbon dioxide ratio. This glaciation ended as continued production of methane produced greenhouse warming. Subsequent to the success of photosynthetic organisms, increase in atmospheric oxygen decreased greenhouse warming by methane and probably caused global glaciation just after the end of the Archaean (approximately 2.4 billion years ago) in the early Palaeoproterozoic (Kasting & Howard, 2006). These glaciations show the fine balance required to avoid global freezing (‘snowball Earth’; see Fig. 11.2) primarily because of the faint early Sun. Sagan & Chyba (1997) calculated that an atmospheric mixing ratio of ammonia (the mixing ratio CX of a gas X is defined as the number of moles of X per mole of air) of only about 10–6 to 10–4 would have been sufficient on the early Earth to cause enough greenhouse warming to counteract the effects of reduced solar radiation.