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J. T. Staley (Microbiology/Astrobiology Program, U. Washington)
Although details of the first metabolic processes cannot yet be determined, the most important early biological process was primary production, i.e., the fixation of carbon dioxide into organic materials. Primary production on Earth today is dominated by photosynthesis, in which light is the energy source, but photosynthesis is very complex and undoubtedly required an extensive period of evolution. In contrast chemosynthesis, which uses chemical forms of energy and is carried out even today by many microorganisms, is simpler and likely evolved earlier than photosynthesis. The chemical disequilibria of early Earth provided energetically favorable reactions for utilization by early life forms.Among the energy sources available for chemosynthesis on primitive Earth was hydrogen gas reacting with oxidants such as carbon dioxide and sulfur dioxide. Contemporary microbes that use the hydrogen/carbon dioxide metabolism are methane producers, while others use the hydrogen/sulfite metabolism; examples of both can be found assocated with seafloor hydrothermal vents. Some evidence supports the evolution of these processes as long ago as 3.5 billion years.
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Bulletin of the American Astronomical Society, 34, #4
© 2002. The American Astronomical Soceity.