The Earth's biosphere represents the major source for important constituents of its current atmosphere including N₂. O₂, CH₄, dimethylsulfide (DMS), and certain halocarbons. Reactive species derived from an O₂-rich atmosphere (i.e., hydroxyl radicals) destroy reduced carbon gases in the troposphere thereby constraining their both their abundance and residence times. Methane, the most abundant hydrocarbon, has a mixing ratio of ~1.7 ppm and a residence time of ~8 years. However, the biosphere has existed for over 3.5 x 109 years, for much of which overall anaerobic conditions prevailed. Because of the ancient divergence of the 3 main biological kingdoms, it is likely that the Archaea, as represented by methanogens, have been present for all of the time the Earth's biosphere has been in existence. Because of the absence of oxidizing species in the Archaen troposphere (i.e., OH) and the lack of chemical oxidants available to bacteria for the destruction of CH₄ (e.g., SO₄, Fe³⁺), it is reasonable to assume that methane mixing ratios were much higher than present levels, possibly by orders of magnitude. A corollary of this line of reasoning is that volatile precursor substrates of methanogens, such as DMS, methane thiol, methylated amines, methanol, hydrogen, acetic and formic acids may also have been more abundant. Other gases of biogenic origin which could have entered into methanogenic pathways by nucleophilic exchange reactions would include halocarbons like methyl bromide.
A CH₄-rich Archaen atmosphere would also have significant acetylene present as a consequence of a complex series of photocatalytic reactions. Pelobacter acetylenicus is an anaerobe which grows on acetylene by first hydrating it to form acetaldehyde and then dismutating this intermediate to form ethanol and acetate. The formation of ethanol and acetate would have provided a uniform source of carbon and energy around which Proterozoic microbial communities could have developed. The appearance of oxidants like SO₄^-, SO, and Fe³⁺ as conditions became less reducing would have created niches for organisms capable of oxidizing acetate and ethanol to CO₂. This acetylene-based food chain has implications for the possibility for life on planets with Jovian-like atmospheres.
The difficulty for unequivocal detection of life on planets harboring microbial ecosystems analogous to this concept of an anoxic Archaen would be the discovery of "false positives." Methane, higher hydrocarbons, and many other simple organic compounds are abundant in the outer planets, comets and other heavenly bodies. The detection of other methylated compounds in addition to CH₄, such as DMS, methylhalides, and methylated amines, could provide supplemental data which would strengthen the case for the presence of microbial life on anoxic, extra-solar planets.