A shadow biosphere is a hypothetical microbial

biosphere The biosphere (from Greek βίος ''bíos'' "life" and σφαῖρα ''sphaira'' "sphere"), also known as the ecosphere (from Greek οἶκος ''oîkos'' "environment" and σφαῖρα), is the worldwide sum of all ecosystems. It can also ...

of Earth that would use radically different biochemical and molecular processes from that of currently known life. Although life on Earth is relatively well studied, if a shadow biosphere exists it may still remain unnoticed, because the exploration of the microbial world targets primarily the biochemistry of the macro-organisms.

The hypothesis

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."Life on Earth… but not as we know it"
Robin McKie, 14 April 2013, The Guardian Steven A. Benner, Alonso Ricardo, and Matthew A. Carrigan, biochemists at the

University of Florida The University of Florida (Florida or UF) is a public land-grant research university in Gainesville, Florida. It is a senior member of the State University System of Florida, traces its origins to 1853, and has operated continuously on its ...

, 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 The micrometre ( international spelling as used by the International Bureau of Weights and Measures; SI symbol: μm) or micrometer (American spelling), also commonly known as a micron, is a unit of length in the International System of Unit ...

), 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, Cleland, C. E. 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 Arsenic is a chemical element with the symbol As and atomic number 33. Arsenic occurs in many minerals, usually in combination with sulfur and metals, but also as a pure elemental crystal. Arsenic is a metalloid. It has various allotropes, ...

instead of

phosphorus Phosphorus is a chemical element with the symbol P and atomic number 15. Elemental phosphorus exists in two major forms, white phosphorus and red phosphorus, but because it is highly reactive, phosphorus is never found as a free element on Ear ...

, having a different

genetic code The genetic code is the set of rules used by living cells to translate information encoded within genetic material ( DNA or RNA sequences of nucleotide triplets, or codons) into proteins. Translation is accomplished by the ribosome, which links ...

, or even another kind of chemical for its genetic material that are not nucleic acids ( DNA nor RNA) chains or biopolymers.

Carol Cleland Carol Edith Cleland (born 1948) is an American philosopher of science known for her work on the definition of life and the shadow biosphere, on the classification of minerals by their geological history, on the distinction between historical an ...

, 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 Life is a quality that distinguishes matter that has biological processes, such as Cell signaling, signaling and self-sustaining processes, from that which does not, and is defined by the capacity for Cell growth, growth, reaction to Stimu ...

has evolved on Earth more than once, which means that

microorganism A microorganism, or microbe,, ''mikros'', "small") and ''organism'' from the el, ὀργανισμός, ''organismós'', "organism"). It is usually written as a single word but is sometimes hyphenated (''micro-organism''), especially in olde ...

s 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.

Criticism

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.

References

External links

A Shadow Biosphere
*[https://web.archive.org/web/20140407081517/http://archived.thebioblog.com/2010/02/alternate-life-styles-scientists-predict-the-possibility-of-a-shadow-biosphere/ Alternate "Life" Styles: Scientists Predict The Possibility Of A Shadow Biosphere] {{DEFAULTSORT:Shadow biosphere Astrobiology Ecology Philosophy of biology Hypothetical life forms

The hypothesis

Criticism

See also

References

Further reading

External links