Shadow biosphere
Encyclopedia
The term "shadow biosphere" was coined by Carol Cleland and Shelley Copley. A shadow biosphere is a hypothetical microbial biosphere
of Earth that uses radically different biochemical
and molecular
processes than currently known life. While life on Earth is relatively well-studied, the shadow biosphere may still remain unnoticed because our exploration of the microbial world targets our biochemistry primarily.
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 may still be alive today, unnoticed because they don't contain ribosome
s, 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 , a chirality
opposite of ours, that use some of the non-standard amino acids, or that use 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.
Biosphere
The biosphere is the global sum of all ecosystems. It can also be called the zone of life on Earth, a closed and self-regulating system...
of Earth that uses radically different biochemical
Biochemistry
Biochemistry, sometimes called biological chemistry, is the study of chemical processes in living organisms, including, but not limited to, living matter. Biochemistry governs all living organisms and living processes...
and molecular
Molecular biology
Molecular biology is the branch of biology that deals with the molecular basis of biological activity. This field overlaps with other areas of biology and chemistry, particularly genetics and biochemistry...
processes than currently known life. While life on Earth is relatively well-studied, the shadow biosphere may still remain unnoticed because our exploration of the microbial world targets our biochemistry primarily.
Steven A. Benner, Alonso Ricardo, and Matthew A. Carrigan, biochemists at the University of Florida
University of Florida
The University of Florida is an American public land-grant, sea-grant, and space-grant research university located on a campus in Gainesville, Florida. The university traces its historical origins to 1853, and has operated continuously on its present Gainesville campus since September 1906...
, argued that if organisms based on RNA
RNA world hypothesis
The RNA world hypothesis proposes that life based on ribonucleic acid pre-dates the current world of life based on deoxyribonucleic acid , RNA and proteins. RNA is able both to store genetic information, like DNA, and to catalyze chemical reactions, like an enzyme protein...
once existed, they may still be alive today, unnoticed because they don't contain ribosome
Ribosome
A ribosome is a component of cells that assembles the twenty specific amino acid molecules to form the particular protein molecule determined by the nucleotide sequence of an RNA molecule....
s, which are usually used to detect living organisms. They suggest searching for them in environments that are low in sulfur
Sulfur
Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...
, environments that are spatially constrained (for example, minerals with pores smaller than one micrometre
Micrometre
A micrometer , is by definition 1×10-6 of a meter .In plain English, it means one-millionth of a meter . Its unit symbol in the International System of Units is μm...
), 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 , a chirality
Chirality (chemistry)
A chiral molecule is a type of molecule that lacks an internal plane of symmetry and thus has a non-superimposable mirror image. The feature that is most often the cause of chirality in molecules is the presence of an asymmetric carbon atom....
opposite of ours, that use some of the non-standard amino acids, or that use arsenic
Arsenic
Arsenic is a chemical element with the symbol As, atomic number 33 and relative atomic mass 74.92. Arsenic occurs in many minerals, usually in conjunction with sulfur and metals, and also as a pure elemental crystal. It was first documented by Albertus Magnus in 1250.Arsenic is a metalloid...
instead of phosphorus
Phosphorus
Phosphorus is the chemical element that has the symbol P and atomic number 15. A multivalent nonmetal of the nitrogen group, phosphorus as a mineral is almost always present in its maximally oxidized state, as inorganic phosphate rocks...
.. Carol Cleland, a philosopher of science at the University of Colorado (Boulder), argues that desert varnish
Desert varnish
Desert varnish, or rock varnish is a orange-yellow to black coating found on exposed rock surfaces in arid environments. Desert varnish is usually around one micron thick and present nanometre-scale layering...
, 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.
Criticism
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 seems to support arsenic as a substitute for phosphorus in DNA could have resulted from lab or field contamination, and that DNA that includes arsenic is chemically unstable.Further reading
- Cleland, C.E. (2006) Astrobiology Magazine A Shadow Biosphere
- 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.
- Cleland, C. E. and Copley, S. D. (2005) The possibility of alternative microbial life on Earth. International Journal of Astrobiology 4(4), 165-173.
- Wolfe-Simon,F., Davies,P.C.W. and Anbar, A.D. (2009) Did nature also choose arsenic? International Journal of Astrobiology, Cambridge University Press doi:10.1017/S1473550408004394
- Kuhn, T. (1962) The structure of scientific revolutions. University of Chicago Press: Chicago.
- Pace, N. R. (1997) A molecular view of microbial diversity and the biosphere. Science, 274, 734-740.
- Woese, C. R. (2004) The archaeal concept and the world it lives in: a retrospective. Photosynthesis Research, 80, 361-372.