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A shadow biosphere is the hypothetical microbe biosphere of Earth that would use radically different biochemistry and molecular processes from those of known life. Although life on Earth is relatively well studied, if a shadow biosphere exists, it may be unnoticed because the exploration of micro-organisms targets primarily the biochemistry of macro-organisms.
Steven A. Benner, Alonso Ricardo, and Matthew A. Carrigan, biochemists at the University of Florida, argued that if organisms based on RNA once existed, they might still be alive today, unnoticed because they do not contain , which are usually used to detect living microorganisms. They suggest searching for them in environments that are low in sulfur, environments that are spatially constrained (for example, minerals with pores smaller than one micrometre), or environments that cycle between extreme hot and cold.Benner, S. A., Ricardo, A. and Carrigan, M. A. (2004) Is there a common chemical model for life in the universe? . Curr. Opin. Chem. Biol., 8, 672-689. .
Other proposed candidates for a shadow biosphere include organisms using different suites of amino acids in their proteins or different molecular units (e.g., bases or sugars) in their nucleic acids,Carol Cleland and Copley, S. D. (2005) The possibility of alternative microbial life on Earth. International Journal of Astrobiology 4(4), 165-173. having a chirality opposite of ours, using some of the nonstandard amino acids, or using arsenic instead of phosphorus,Davies, P. C. W., Benner, S.A., Carol Cleland, Lineweaver, C.H., McKay, C.P. and Wolfe-Simon, F. Signatures of a Shadow Biosphere (2009) Astrobiology. 9(2): 241-249. . having a different genetic code, or even another kind of chemical for its genetic material that are not nucleic acids (DNA nor Ribonucleic acid) chains or . Carol Cleland, a philosopher of science at the University of Colorado (Boulder), argues that desert varnish, whose status as biological or nonbiological has been debated since the time of Darwin, should be investigated as a potential candidate for a shadow biosphere.
Existence of a shadow biosphere could mean that life has independently evolved on Earth more than once, which means that may exist on Earth which have no evolutionary connection with any other known form of life. It is suggested that if an alternate form of microbial life on Earth is discovered, the odds are good that life is also common elsewhere in the universe.
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