A shadow biosphere is the hypothesis of a hypothetical

microbial A microorganism, or microbe, is an organism of microscopic size, which may exist in its single-celled form or as a colony of cells. The possible existence of unseen microbial life was suspected from antiquity, with an early attestation in ...

biosphere The biosphere (), also called the ecosphere (), is the worldwide sum of all ecosystems. It can also be termed the zone of life on the Earth. The biosphere (which is technically a spherical shell) is virtually a closed system with regard to mat ...

of Earth that would use radically different

biochemical Biochemistry, or biological chemistry, is the study of chemical processes within and relating to living organisms. A sub-discipline of both chemistry and biology, biochemistry may be divided into three fields: structural biology, enzymology, ...

and

molecular A molecule is a group of two or more atoms that are held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions that satisfy this criterion. In quantum physics, organic chemistry, ...

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 micro-organisms targets primarily the biochemistry of macro-organisms.

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. 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."Life on Earth… but not as we know it"
Robin McKie, 14 April 2013, The Guardian Some argue that these alternative life forms could have become extinct, either by being out-competed by other forms of life, or they might have become one with the present day life via mechanisms like lateral gene transfer. Others, however, argue that this other form of life might still exist to this day. 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 university, public land-grant university, land-grant research university in Gainesville, Florida, United States. It is a senior member of the State University System of Florida and a preem ...

, argued that if organisms based on RNA once existed, they might still be alive today, unnoticed because they do not contain

ribosome Ribosomes () are molecular machine, macromolecular machines, found within all cell (biology), cells, that perform Translation (biology), biological protein synthesis (messenger RNA translation). Ribosomes link amino acids together in the order s ...

s, which are usually used to detect living microorganisms. They suggest searching for them in environments that are low in

sulfur Sulfur ( American spelling and the preferred IUPAC name) or sulphur ( Commonwealth spelling) is a chemical element; it has symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms ...

, environments that are spatially constrained (for example, minerals with pores smaller than one

micrometre The micrometre (English in the Commonwealth of Nations, Commonwealth English as used by the International Bureau of Weights and Measures; SI symbol: μm) or micrometer (American English), also commonly known by the non-SI term micron, is a uni ...

), 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 Chirality () is a property of asymmetry important in several branches of science. The word ''chirality'' is derived from the Greek (''kheir''), "hand", a familiar chiral object. An object or a system is ''chiral'' if it is distinguishable fro ...

opposite of ours, using some of the nonstandard amino acids, or using

arsenic Arsenic is a chemical element; it has Symbol (chemistry), symbol As and atomic number 33. It is a metalloid and one of the pnictogens, and therefore shares many properties with its group 15 neighbors phosphorus and antimony. Arsenic is not ...

instead of

phosphorus Phosphorus is a chemical element; it has Chemical symbol, symbol P and atomic number 15. All elemental forms of phosphorus are highly Reactivity (chemistry), reactive and are therefore never found in nature. They can nevertheless be prepared ar ...

, having a different

genetic code Genetic code is a set of rules used by living cell (biology), cells to Translation (biology), translate information encoded within genetic material (DNA or RNA sequences of nucleotide triplets or codons) into proteins. Translation is accomplished ...

, or even another kind of chemical for its genetic material that are not nucleic acids (

DNA Deoxyribonucleic acid (; DNA) is a polymer composed of two polynucleotide chains that coil around each other to form a double helix. The polymer carries genetic instructions for the development, functioning, growth and reproduction of al ...

nor

RNA Ribonucleic acid (RNA) is a polymeric molecule that is essential for most biological functions, either by performing the function itself (non-coding RNA) or by forming a template for the production of proteins (messenger RNA). RNA and deoxyrib ...

) chains or

biopolymer Biopolymers are natural polymers produced by the cells of living organisms. Like other polymers, biopolymers consist of monomeric units that are covalently bonded in chains to form larger molecules. There are three main classes of biopolymers, ...

s. 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 Life, also known as biota, refers to matter that has biological processes, such as Cell signaling, signaling and self-sustaining processes. It is defined descriptively by the capacity for homeostasis, Structure#Biological, organisation, met ...

has independently evolved on Earth more than once, which means that

microorganism A microorganism, or microbe, is an organism of microscopic scale, microscopic size, which may exist in its unicellular organism, single-celled form or as a Colony (biology)#Microbial colonies, colony of cells. The possible existence of unseen ...

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 Evolution of the biosphere Hypothetical life forms Philosophy of biology

Hypothesis

Criticism

See also

References

Further reading

External links