Evolution of life and the atmosphere

Modern methanogens can all reduce carbon dioxide with molecular hydrogen, while synthesizing carbohydrates (which can be represented by their empirical, i.e. simplest, formula CH20)  

The initial step is actually the formation of pyruvic acid with the liberation of free hydrogen atoms, which can then be combined with other groups  

Fermentation is one of a number of anaerobic respiration (i.e. energy-liberating) processes that are discussed more fully in Section 3.3.2b. 

Other plausible candidates for the first prokaryotes are the ancestors of chemotrophes colonizing hydrothermal vents. Today they use the oxygen dissolved in ocean water to obtain energy from the sulphides emanating from the hot springs in chemosynthetic reactions (see Box 1.10) that can be summarized by  

Clearly, anaerobic ancestors would have had to utilize a chemical energy source other than oxygen, but there is no clear evidence that anaerobic chemotrophes existed. 

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The geological era in which the physical reconfiguration of oceans and continents caused major deviations in the evolution of life and the atmosphere. 

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