Is the early evolution of life on Earth the key to resolving the Faint Young Sun paradox?
Submitted by Linda Sohl, Columbia UniverityWhat we know...
Clues to the composition of Earth's pre-biotic atmosphere composition as well as surface temperatures prior to the end of late bombardment ca. 3.9 Ga are sparse and inconclusive. Recent perspectives have suggested that an initial CO2/H2-rich atmosphere (Tian et al., 2005, Science, 308, 1014) would have countered the effects of a Sun that was 25-30% dimmer than at present, and permitted a temperate surface environment. An atmosphere composed chiefly of CO and H2 would also avoid the photo-dissociation problems that a methane-rich atmosphere would have had so early in solar system history, when the Sun would have been emitting large amounts of extreme UV radiation (e.g. Rai et al. 2005 Science 309:1062). However, many researchers have adopted the view that a very thick CO2 atmosphere was not plausible or sufficient to have fully compensated for the lower solar luminosity. The addition of abundant biogenic methane to the atmosphere is seen as necessary for providing an adequate greenhouse effect and avoiding a permanent icehouse condition (e.g., Zahnle and Sleep, 2002, Geol. Soc. London Spec. Pub. 199:231-257).
The requirement for biogenic methane implies that anaerobic methane-generating organisms (methanogens) would have evolved very early in Earth history, and would have been present in sufficient mass to alter the chemistry of the atmosphere "in time" to compensate for loss of H2 via thermal escape and the inadequacy of CO2 as a sole greenhouse gas. Battistuzzi et al. (2004, BMC Evol. Biol., 4:44) have estimated a genomic timescale of metabolic innovations and prokaryote evolution that suggests an origin of life by 4.1 Ga and the existence of methanogens by 3.8 Ga. Such dates could mesh with estimates for the rate of loss of an early CO2/H2 atmosphere, in terms of when greenhouse compensation via methane would be needed.
However, there have been as yet no definitive fossils (biochemical or otherwise) recovered from rocks to support genomic timescales such as that of Battistuzzi et al. (2004). It is also not clear as to whether the evolution of methanogens fortuitously allowed the Earth to avoid a permanent icehouse as greenhouse capacity was reduced by purely external and/or physical means (e.g. heavy bombardment thermal escape), or if the replacement of an early CO2/H2 atmosphere by methane wasn't in fact largely biologically mediated.
An interesting line of questioning for students could include a calculation of the amount of methanogen biomass needed to produce an atmosphere with as much as 100 ppm methane, and how quickly that might have developed given any sinks for atmospheric methane and nutrient limitations for the methanogens.
References and other Resources
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Battistuzzi et al. 2004, BMC Evol. Biol., 4:4
Fowler C.M.R. Ebinger C.J. and Hawkesworth C.J. eds.. 2002. The Early Earth: Physical Chemical and Biological Development. Geological Society London Special Publications 199.
Rai et al. 2005 Science 309:1062
Tian et al., 2005, Science, 308:1014
Zahnle and Sleep, 2002, Geol. Soc. London Spec. Pub. 199:231-257