Hypothetical types of biochemistry

Several forms of biochemistry are agreed to be scientifically viable, but are not proven to exist at this time. The kinds of living organisms known on Earth, as of 2026, all use carbon compounds for basic structural and metabolic functions, water as a solvent, and deoxyribonucleic acid or ribonucleic acid to define and control their form. If life exists on other celestial bodies, it may be chemically similar, though it is also possible that there are organisms with quite different chemistries – for instance, involving other classes of carbon compounds, compounds of another element, and/or another solvent in place of water.
The possibility of life-forms being based on "alternative" biochemistries is the topic of an ongoing scientific discussion, informed by what is known about extraterrestrial environments and about the chemical behaviour of various elements and compounds. It is of interest in synthetic biology and is also a common subject in science fiction.
The element silicon has been much discussed as a hypothetical alternative to carbon. Silicon is in the same group as carbon on the periodic table and, like carbon, it is tetravalent. Hypothetical alternatives to water include ammonia, which, like water, is a polar molecule, and cosmically abundant; and non-polar hydrocarbon solvents such as methane and ethane, which are known to exist in liquid form on the surface of Titan.

Overview of hypothetical types of biochemistry

Type	Basis	Brief description	Details
Alternative-chirality biomolecules	Alternative biochemistry	Mirror image biochemistry	Alternative-chirality biomolecules refer to biomolecules with reflected chirality. In known Earth-based life, amino acids are almost universally of the L form and sugars are of the D form; however, the mirror-image forms could equally form the basis for alternative biochemistries. Synthetic biologists have proposed creating mirror-image versions of existing organisms, using entirely mirror-image biochemistry; these would behave identically to their template organisms except when interacting with existing biomolecules. Mirror-image microorganisms would be resistant to the immune systems of existing organisms. Scientists have stated concern over this risk and discouraged the creation of them.
Alternative nucleic acids	Alternative biochemistry	Different genetic storage	Xeno nucleic acids may possibly be used in place of RNA or DNA. XNA is the general term for a nucleic acid with an altered sugar backbone. Examples of XNA are: TNA, which uses threose; HNA, which uses 1,5-anhydrohexitol; GNA, which uses glycol; CeNA, which uses cyclohexene; LNA, which utilizes a form of ribose that contains an extra linkage between its 4' carbon and 2' oxygen; FANA, which uses arabinose, but with a single fluorine atom attached to its 2' carbon; PNA, which uses, in place of sugar and phosphate, N--glycine units connected by peptide bonds. In comparison, Hachimoji DNA changes the base pairs instead of the backbone. These new base pairs are P, Z, B, and S.
Ammonia biochemistry	Non-water solvents	Ammonia-based life	Ammonia is relatively abundant in the universe and has chemical similarities to water. The possible role of liquid ammonia as an alternative solvent for life is an idea dating back to 1954 at least, when J. B. S. Haldane raised the topic at a symposium about life's origin.
Arsenic biochemistry	Alternative biochemistry	Arsenic-based life	Arsenic, which is chemically similar to phosphorus, while poisonous for most life forms on Earth, is incorporated into the biochemistry of some organisms.
Borane biochemistry	Alternative biochemistry	Borane-based life	Boranes are dangerously explosive in Earth's atmosphere but would be more stable in a reducing atmosphere, one with no oxygen or other oxidizing gases, and which may contain actively reductant gases such as hydrogen, carbon monoxide, methane, and hydrogen sulfide. Molecular structures containing alternating boron and nitrogen atoms share some properties with hydrocarbons. However, boron is far rarer in the universe than its neighbours of carbon, nitrogen, and oxygen.
Cosmic necklace-based biology	Nonplanetary life	Non-chemical life	In 2020, Luis A. Anchordoqu and Eugene M. Chudnovsky hypothesized that life composed of magnetic semipoles connected by cosmic strings could evolve inside stars.
Dusty plasma-based biology	Nonplanetary life	Non-chemical life	In 2007, Vadim N. Tsytovich and colleagues proposed that lifelike behaviours could be exhibited by dust particles suspended in a plasma, under conditions that might exist in space.
Extremophiles	Alternative environment	Life in variable environments	It would be biochemically possible to sustain life in environments that are only periodically consistent with life as we know it, such as extremely high or low temperatures, pressures, or pH; or the presence of high levels of salt or nuclear radiation.
Heteropoly acid biochemistry	Alternative biochemistry	Heteropoly acid-based life	Various metals can form complex structures with oxygen, such as heteropoly acids.
Hydrogen fluoride biochemistry	Non-water solvents	Hydrogen fluoride-based life	Hydrogen fluoride has been considered as a possible solvent for life by scientists such as Peter Sneath.
Hydrogen sulfide biochemistry	Non-water solvents	Hydrogen sulfide-based life	Hydrogen sulfide is a chemical analog of water but is less polar and a weaker inorganic solvent.
Methane biochemistry	Non-water solvents	Methane-based life	Methane is relatively abundant in the Solar System and the Universe and is known to exist in liquid form on Titan, the largest moon of Saturn. Though highly unlikely, it is considered to be possible for Titan to harbour life. If so, it will most likely be methane-based life.
Non-green photosynthesizers	Other speculations	Alternate plant life	Physicists have noted that, although photosynthesis on Earth generally involves green plants, a variety of other-coloured plants could also support photosynthesis, essential for most life on Earth, and that other colours might be preferred in places that receive a different mix of stellar radiation than Earth. In particular, retinal is capable of, and has been observed to, perform photosynthesis. Bacteria capable of photosynthesis are known as microbial rhodopsins. A plant or creature that uses retinal photosynthesis is always purple.
Shadow biosphere	Alternative environment	A hidden life biosphere on Earth	A shadow biosphere is a hypothetical microbial biosphere of Earth that uses radically different biochemical and molecular processes than currently known life. It could exist, for example, deep in the crust or sealed in ancient rocks.
Silicon biochemistry	Alternative biochemistry	Silicon-based life	Like carbon, silicon can create molecules that are sufficiently large to carry biological information; however, the scope of possible silicon chemistry is far more limited than that of carbon.
Silicon dioxide biochemistry	Non-water solvents	Silicon dioxide-based life	Gerald Feinberg and Robert Shapiro have suggested that molten silicate rock could serve as a liquid medium for organisms with a chemistry based on silicon, oxygen, and other elements such as aluminium.
Sulfur biochemistry	Alternative biochemistry	Sulfur-based life	The biological use of sulfur as an alternative to carbon is purely hypothetical, especially because sulfur usually forms only linear chains rather than branched ones.

Shadow biosphere

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.

Alternative-chirality biomolecules

Perhaps the least unusual alternative biochemistry would be one with differing chirality of its biomolecules. In known Earth-based life, amino acids are almost universally of the form and sugars are of the form. Molecules using amino acids or sugars may be possible; molecules of such a chirality, however, would be incompatible with organisms using the opposing chirality molecules. Amino acids which chirality is opposite to the norm are found on Earth, and these substances are generally thought to result from decay of organisms of normal chirality. However, physicist Paul Davies speculates that some of them might be products of "anti-chiral" life.
It is questionable, however, whether such a biochemistry would be truly alien. Although it would certainly be an alternative stereochemistry, molecules that are overwhelmingly found in one enantiomer throughout the vast majority of organisms can nonetheless often be found in another enantiomer in different organisms such as in comparisons between members of Archaea and other domains, making it an open topic whether an alternative stereochemistry is truly novel.

Non-carbon-based biochemistries

On Earth, all known living things have a carbon-based structure and system. Scientists have speculated about the advantages and disadvantages of using elements other than carbon to form the molecular structures necessary for life, but no one has proposed a theory employing such atoms to form all the necessary structures. However, as Carl Sagan argued, it is very difficult to be certain whether a statement that applies to all life on Earth will turn out to apply to all life throughout the universe. Sagan used the term "carbon chauvinism" for such an assumption. He regarded silicon and germanium as conceivable alternatives to carbon ; but, on the other hand, he noted that carbon does seem more chemically versatile and is more abundant in the cosmos. Norman Horowitz devised the experiments to determine whether life might exist on Mars that were carried out by the Viking Lander of 1976, the first U.S. mission to successfully land a probe on the surface of Mars. Horowitz argued that the great versatility of the carbon atom makes it the element most likely to provide solutions, even exotic solutions, to the problems of survival on other planets. He considered that there was only a remote possibility that non-carbon life forms could exist with genetic information systems capable of self-replication and the ability to evolve and adapt.