A shadow biosphere is a hypothetical microbial biosphere of Earth that uses radically different biochemical and molecular processes than currently known life. Although life on Earth is relatively well-studied, the shadow biosphere may still remain unnoticed because the exploration of the microbial world targets primarily the biochemistry of the macro-organisms. The term was coined by Carol Cleland and Shelley Copley in 2005.
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 don't contain ribosomes, which are usually used to detect living organisms. 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.
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, having a chirality opposite of ours, using some of the non-standard amino acids, or using arsenic instead of phosphorus. Carol Cleland, a philosopher of science at the University of Colorado (Boulder), argues that desert varnish, whose status as living or nonliving has been debated since the time of Darwin, should be investigated as a potential candidate for a shadow biosphere.
The idea of a shadow biosphere is not widely accepted in biochemistry, and methods used by proponents and conclusions drawn from experiments that purport to show evidence of shadow biospheres have been criticized. For example, evidence that once seemed to support arsenic as a substitute for phosphorus in DNA could have resulted from lab or field contamination, and DNA that includes arsenic is chemically unstable.
- Cleland and Copley (2005)
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