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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.[1]


The hypothesisEdit

It has been proposed that the early Earth hosted multiple origins of life, some of which produced chemical variations on life as we know it.[2][3] 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 ribosomes, 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.[4]

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,[1] having a chirality opposite of ours, using some of the non-standard amino acids, or using arsenic instead of phosphorus.[5] 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.[6][7]


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.[8]

See alsoEdit

  • Abiogenesis – The natural process by which life arises from non-living matter
  • Astrobiology – Science concerned with life in the universe
  • Biological dark matter – An informal term for unclassified or poorly understood genetic material
  • Extremophile – Organisms capable of living in extreme environments
  • GFAJ-1
  • Hypothetical types of biochemistry – Possible alternative biochemicals used by life forms
  • Panspermia – Hypothesis that life exists throughout the Universe, distributed by space dust, meteoroids, asteroids, comets, planetoids, and also by spacecraft carrying unintended contamination by microorganisms
  • Purple Earth hypothesis – An astrobiological hypothesis that life forms of early Earth were retinal-based rather than chlorophyll-based, making Earth appear purple rather than green
  • RNA world hypothesis
  • Shadow life – A hypothesis that if life has evolved on Earth more than once, microorganisms may exist on Earth which have no evolutionary connection with any other known form of life
  • DNA – Molecule that encodes the genetic instructions used in the development and functioning of all known organisms and many viruses


  1. ^ a b Cleland and Copley (2005)
  2. ^ Cleland, C.E. (2007) Epistemological issues in the study of microbial life: alternative biospheres. Studies in the History and Philosophy of Biological and Biomedical Sciences 38:847–861.
  3. ^ "Life on Earth… but not as we know it", Robin McKie, 14 April 2013, The Guardian
  4. ^ Benner, S. A., Ricardo, A. and Carrigan, M. A. (2004) Is there a common chemical model for life in the universe? Archived 2010-12-14 at the Wayback Machine. Curr. Opin. Chem. Biol., 8, 672-689.
  5. ^ Davies, P. C. W. , Benner, S.A., Cleland, C.E., Lineweaver,C.H., McKay,C.P. and Wolfe-Simon,F. Signatures of a Shadow Biosphere (2009) Astrobiology. 9(2): 241-249. doi:10.1089/ast.2008.0251.
  6. ^ Cleland (2007)
  7. ^ "Life on Earth… but not as we know it", Robin McKie, 14 April 2013, The Guardian
  8. ^ Vastag, Brian (27 May 2011). "Debate over arsenic-based life enters a new chapter". Washington Post.

Further readingEdit

External linksEdit