Hypothetical types of biochemistry
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Hypothetical types of biochemistry are forms of biochemistry speculated to be scientifically viable but not proven to exist at this time. While the kinds of living beings we know on Earth commonly use carbon for basic structural and metabolic functions, water as a solvent and DNA or RNA to define and control their form, it may be possible that undiscovered life-forms could exist that differ radically in their basic structures and biochemistry from that known to science.
Scientists who have considered this topic 
Scientists who have considered possible alternatives to carbon-water biochemistry include:
- J.B.S. Haldane (1892 to 1964), a geneticist noted for his work on abiogenesis.
- V.Axel Firsoff (1910 to 1981), British astronomer.
- Peter Sneath (1923 to 2011), microbiologist, author of the book Planets and Life.
- The physicist Gerald Feinberg (1933-1992) and the chemist Robert Shapiro (1935-2011), co-authors of the book Life Beyond Earth.
- The American chemist George Pimentel of the University of California, Berkeley.
- Isaac Asimov, biochemist and science fiction writer.
- Carl Sagan, astronomer.
- Robert A. Freitas Jnr, specialist in nano-technology and nano-medicine; author of the book Xenology.
- The Cambridge biologist William Bains, a contributor to the journal Astrobiology.
- John Baross, oceanographer and astrobiologist, who chaired a committee of scientists under the United States National Research Council which in 2007 published a report on life's limiting conditions. The report addresses the concern that a space agency might conduct a well-resourced search for life on other worlds "and then fail to recognize it if it is encountered".
Earth as a possible habitat 
Apart from the prospect of finding different forms of life on other planets or moons, Earth itself has been suggested as a place where a shadow biosphere of biochemically unfamiliar micro-organisms may have lived in the past, or may still exist today.
Alternative-chirality biomolecules 
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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 L form and sugars are of the D form. Molecules of opposite chirality have identical chemical properties to their mirrored forms, so life that used D amino acids or L sugars may be possible; molecules of such a chirality, however, would be incompatible with organisms using the opposing chirality molecules. It is questionable, however, whether such a biochemistry would be truly alien; while it is certainly 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 (often basal) organisms such as in comparisons between members of Archea and other domains, making it an open topic whether an alternative stereochemistry is truly novel.
Non-carbon-based biochemistries 
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On Earth, all living things have a molecular machinery of carbon compounds. Scientists have speculated about the pros and cons of using atoms 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. Carl Sagan 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.
Silicon biochemistry 
The most commonly proposed basis for an alternative biochemical system is the silicon atom, because silicon has many chemical properties similar to those of carbon and is in the same group of the periodic table, the carbon group. Like carbon, silicon can create molecules that are sufficiently large to carry biological information.
However, silicon has several drawbacks as an alternative to carbon. Silicon, unlike carbon, lacks the ability to form chemical bonds with diverse types of atoms as is necessary for the chemical versatility required for metabolism. Elements creating organic functional groups with carbon include hydrogen, oxygen, nitrogen, phosphorus, sulfur, and metals such as iron, magnesium, and zinc. Silicon, on the other hand, interacts with very few other types of atoms. Moreover, where it does interact with other atoms, silicon creates molecules that have been described as "monotonous compared with the combinatorial universe of organic macromolecules". This is because silicon atoms are much bigger, having a larger mass and atomic radius, and so have difficulty forming double bonds (the double bonded carbon is part of the carbonyl group, a fundamental motif of bio-organic chemistry).
Silanes, which are chemical compounds of hydrogen and silicon that are analogous to the alkane hydrocarbons, are highly reactive with water, and long-chain silanes spontaneously decompose. Molecules incorporating polymers of alternating silicon and oxygen atoms instead of direct bonds between silicon, known collectively as silicones, are much more stable. It has been suggested that silicone-based chemicals would be more stable than equivalent hydrocarbons in a sulfuric-acid-rich environment, as is found in some extraterrestrial locations. Complex long-chain silicone molecules are still less stable than their carbon counterparts, though.
Another obstacle is that silicon dioxide (a common ingredient of many sands), the analog of carbon dioxide, is a non-soluble solid at the temperature range where water is liquid, making it difficult for silicon to be introduced into water-based biochemical systems even if the necessary range of biochemical molecules could be constructed out of it. Another problem with silicon dioxide is that it would be the product of aerobic respiration. If a silicon-based life form were to breathe using oxygen, as life on Earth does, it would possibly produce silicon dioxide as a by-product of this, assuming that the only difference between the two types of life is silicon in place of carbon. This implies that the exhaled product, silicon dioxide, would be a solid, thus filling the respiratory organs of the organism with sand. This however would be solved if the organism lives in temperatures of several hundred to thousand degrees, where the silicon dioxide becomes a liquid. Oxygen-breathing silicon life, if it exists, is therefore most likely to exist in environments with very high temperatures or pressure.
Finally, of the varieties of molecules identified in the interstellar medium as of 1998[update], 84 are based on carbon while only 8 are based on silicon. Moreover, of those 8 compounds, four also include carbon within them. The cosmic abundance of carbon to silicon is roughly 10 to 1. This may suggest a greater variety of complex carbon compounds throughout the cosmos, providing less of a foundation upon which to build silicon-based biologies, at least under the conditions prevalent on the surface of planets. Somewhat in support, in September 2012, NASA scientists reported that polycyclic aromatic hydrocarbons (PAHs), subjected to interstellar medium (ISM) conditions, are transformed, through hydrogenation, oxygenation and hydroxylation, to more complex organics - "a step along the path toward amino acids and nucleotides, the raw materials of proteins and DNA, respectively". (Further, as a result of these transformations, the PAHs lose their spectroscopic signature which could be one of the reasons "for the lack of PAH detection in interstellar ice grains, particularly the outer regions of cold, dense clouds or the upper molecular layers of protoplanetary disks.")
Also, even though Earth and other terrestrial planets are exceptionally silicon-rich and carbon-poor (the relative abundance of silicon to carbon in the Earth's crust is roughly 925:1), terrestrial life is carbon-based. The fact that carbon, though rare, has proven to be much more successful as a life base than the much more abundant silicon, may be evidence that silicon is poorly suited for biochemistry on Earth-like planets. For example: silicon is less versatile than carbon in forming compounds; the compounds formed by silicon are unstable and it blocks the flow of heat. Even so, biogenic silica is used by some Earth life, such as the silicate skeletal structure of diatoms. This suggests that extraterrestrial life forms may have silicon-based structure molecules and carbon-based proteins for metabolic purposes, therefore enabling the ability to feed on a common resource on a terrestrial planet like Earth for building up the silicon-based part of their body.
Silicon compounds may possibly be biologically useful under temperatures or pressures different from the surface of a terrestrial planet, either in conjunction with or in a role less directly analogous to carbon.
In cinematic and literary science fiction, a moment when man-made machines cross from nonliving to living, it is often posited, this new form would be the first example of non-carbon-based life. Since the advent of the microprocessor in the late 1960s, these machines are often classed as computers (or computer-guided robots) and filed under "silicon-based life", even though the silicon backing matrix of these processors is not nearly as fundamental to their operation as carbon is for "wet life".
Other exotic element-based biochemistries 
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- Boron's chemistry is possibly even more variable than that of carbon, since it has the ability to form polyhedral clusters and three-center two-electron bonds. Boranes are dangerously explosive in Earth's atmosphere, but would be more stable in a reducing environment. However, boron's low cosmic abundance makes it less likely as a base for life than carbon.
- Various metals, together with oxygen, can form very complex and thermally stable structures rivaling those of organic compounds; the heteropoly acids are one such family. Some metal oxides are also similar to carbon in their ability to form both nanotube structures and diamond-like crystals (such as cubic zirconia). Titanium, aluminium, magnesium, and iron are all more abundant in the Earth's crust than carbon. Metal-oxide-based life could therefore be a possibility under certain conditions, including those (such as high temperatures) at which carbon-based life would be unlikely.
- Sulfur is also able to form long-chain molecules, but suffers from the same high-reactivity problems as phosphorus and silanes. The biological use of sulfur as an alternative to carbon is purely theoretical, especially because sulfur usually forms only linear chains rather than branched ones. (The biological use of sulfur as an electron acceptor is widespread and can be traced back 3.5 billion years on Earth, thus predating the use of molecular oxygen. Sulfur-reducing bacteria can utilize elemental sulfur instead of oxygen, reducing sulfur to hydrogen sulfide.)
Chlorine as an alternative to oxygen 
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A number of alternatives to molecular oxygen as a terminal electron acceptor are known from anaerobic life forms on Earth. However, it has been proposed that chlorine might serve as a more general biological alternative to oxygen, either in carbon-based biologies or hypothetical non-carbon-based ones. But chlorine is much less abundant than oxygen in the universe, and so planets with a sufficiently chlorine-rich atmosphere are likely to be rare, if they exist at all. Chlorine will, instead, likely be bound up as salts and other inert compounds.
Arsenic as an alternative to phosphorus 
Arsenic, which is chemically similar to phosphorus, while poisonous for most life forms on Earth, is incorporated into the biochemistry of some organisms. Some marine algae incorporate arsenic into complex organic molecules such as arsenosugars and arsenobetaines. Fungi and bacteria can produce volatile methylated arsenic compounds. Arsenate reduction and arsenite oxidation have been observed in microbes (Chrysiogenes arsenatis). Additionally, some prokaryotes can use arsenate as a terminal electron acceptor during anaerobic growth and some can utilize arsenite as an electron donor to generate energy.
It has been speculated that the earliest life forms on Earth may have used arsenic in place of phosphorus in the backbone of their DNA. A common objection to this scenario is that arsenate esters are so much less stable to hydrolysis than corresponding phosphate esters that arsenic would not be suitable for this function.
The authors of a 2010 geomicrobiology study, supported in part by NASA, have postulated that a bacterium, named GFAJ-1, collected in the sediments of Mono Lake in eastern California, can employ such 'arsenic DNA' when cultured without phosphorus. They proposed that the bacterium may employ high levels of poly-β-hydroxybutyrate or other means to reduce the effective concentration of water and stabilize its arsenate esters. This claim was heavily criticized almost immediately after publication for the perceived lack of appropriate controls Science writer Carl Zimmer contacted several scientists for an assessment: "I reached out to a dozen experts ... Almost unanimously, they think the NASA scientists have failed to make their case". Other authors were unable to reproduce their results and showed that the NASA scientists had issues with phosphate contamination (3 microM), which could sustain extremophile lifeforms.
Selenium or tellurium as an alternative to protein sulfur 
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Some organisms are already known to feature selenoproteins, in which sulfur is replaced by selenium. Some fungi also can produce telluro-methionine and telluro-cysteine. Selenomethionine is commonly included in media used to culture transformed bacteria for the purposes of X-ray diffraction studies of a protein target.
Non-water solvents 
In addition to carbon compounds, all currently known terrestrial life also requires water as a solvent. This has led to discussions about whether water is the only liquid capable of filling that role. The idea that an extraterrestrial life-form might be based on a solvent other than water has been taken seriously in recent scientific literature by the biochemist Steven Benner, and by the astrobiological committee chaired by John A. Baross. Solvents discussed by the Baross committee include ammonia, sulfuric acid, formamide, hydrocarbons, and (at temperatures much lower than Earth's) liquid nitrogen, or hydrogen in the form of a supercritical fluid.
Carl Sagan once described himself as both a carbon chauvinist and a water chauvinist; however on another occasion he said he was a carbon chauvinist but "not that much of a water chauvinist". He considered hydrocarbons, hydrofluoric acid, and ammonia  as possible alternatives to water.
Some of the properties of water that are important for life processes include a large temperature range over which it is liquid, a high heat capacity (useful for temperature regulation), a large heat of vaporization, and the ability to dissolve a wide variety of compounds. Water is also amphoteric, meaning it can donate and accept an H+ ion, allowing it to act as an acid or a base. This property is crucial in many organic and biochemical reactions, where water serves as a solvent, a reactant, or a product. There are other chemicals with similar properties that have sometimes been proposed as alternatives. Additionally, water has the unusual property of being less dense as a solid (ice) than as a liquid. This is why bodies of water freeze over but do not freeze solid (from the bottom up). If ice were denser than liquid water (as is true for nearly all other compounds), then large bodies of liquid would slowly freeze solid, which would not be conducive to the formation of life.
Not all properties of water are necessarily advantageous for life, however. For instance, water ice has a high albedo, meaning that it reflects a lot of light and heat from the Sun. During ice ages, as reflective ice builds up over the surface of the water, the effects of global cooling are increased.
There are some properties that make certain compounds and elements much more favorable than others as solvents in a successful biosphere. The solvent must be able to exist in liquid equilibrium over a range of temperatures the planetary object would normally encounter. Because boiling points vary with the pressure, the question tends not to be does the prospective solvent remain liquid, but at what pressure. For example, hydrogen cyanide has a narrow liquid phase temperature range at 1 atmosphere, but in an atmosphere with the pressure of Venus, with 92 bars (9.2 MPa) of pressure, it can indeed exist in liquid form over a wide temperature range.
The ammonia molecule (NH3), like the water molecule, is abundant in the universe, being a compound of hydrogen (the simplest and most common element) with another very common element, nitrogen. The possible role of liquid ammonia as an alternative solvent for life is an idea that goes back at least to 1954, when J.B.S. Haldane raised the topic at a symposium about life's origin.
Numerous chemical reactions are possible in an ammonia solution, and liquid ammonia has chemical similarities with water. Ammonia can dissolve most organic molecules at least as well as water does and, in addition, it is capable of dissolving many elemental metals. Haldane made the point that various common water-related organic compounds have ammonia-related analogs; for instance the ammonia-related amine group (-NH2) is analogous to the water-related alcohol group (-OH).
Ammonia, like water, can either accept or donate an H+ ion. When ammonia accepts an H+, it forms the ammonium cation (NH4+), analogous to hydronium (H3O+). When it donates an H+ ion, it forms the amide anion (NH2−), analogous to the hydroxide anion (OH−). Compared to water, however, ammonia is more inclined to accept an H+ ion, and less inclined to donate one; it is a stronger nucleophile. Ammonia added to water functions as Arrhenius base: it increases the concentration of the anion hydroxide. Conversely, using a solvent system definition of acidity and basicity, water added to liquid ammonia functions as an acid, because it increases the concentration of the cation ammonium. The carbonyl group (C=O), which is much used in terrestrial biochemistry, would not be stable in ammonia solution, but the analogous imine group (C=N) could be used instead.
However, ammonia has some problems as a basis for life. The hydrogen bonds between ammonia molecules are weaker than those in water, causing ammonia's heat of vaporization to be half that of water, its surface tension to be a third, and reducing its ability to concentrate non-polar molecules through a hydrophobic effect. Gerald Feinberg and Robert Shapiro have questioned whether ammonia could hold prebiotic molecules together well enough to allow the emergence of a self-reproducing system. Ammonia is also flammable in oxygen, and could not exist sustainably in an environment suitable for aerobic metabolism.
A biosphere based on ammonia would likely exist at temperatures or air pressures that are extremely unusual in relation to life on Earth. Life on Earth usually exists within the melting point and boiling point of water at normal pressure, between 0 °C (273 K) and 100 °C (373 K); at normal pressure ammonia's melting and boiling points are between −78 °C (195 K) and −33 °C (240 K). Chemical reactions generally proceed more slowly at a lower temperature, therefore liquid-ammonia life, if it exists, might metabolize more slowly and evolve more slowly than life on Earth. On the other hand, lower temperatures could also enable living systems to use chemical species which at Earth temperatures would be too unstable to be useful.
Ammonia and ammonia–water mixtures remain liquid at temperatures far below the freezing point of pure water, so such biochemistries might be well suited to planets and moons orbiting outside the water-based habitability zone. Such conditions could exist, for example, under the surface of Saturn's largest moon Titan.
Methane and other hydrocarbons 
Methane (CH4) is a simple hydrocarbon: that is, a compound of two of the most common elements in the cosmos, hydrogen and carbon. It has a cosmic abundance comparable with ammonia. Hydrocarbons could act as a solvent over a wide range of temperatures, but would lack polarity. Isaac Asimov, the biochemist and science fiction writer, suggested in 1981 that poly-lipids could form a substitute for proteins in a non-polar solvent such as methane. Lakes composed of a mixture of hydrocarbons, including methane and ethane, have been detected on Titan by the Cassini spacecraft.
There is debate about the effectiveness of methane and other hydrocarbons as a medium for life compared to water or ammonia. Water is a stronger solvent than the hydrocarbons, enabling easier transport of substances in a cell. However, water is also more chemically reactive, and can break down large organic molecules through hydrolysis. A life-form whose solvent was a hydrocarbon would not face the threat of its biomolecules being destroyed in this way. Also, the water molecule's tendency to form strong hydrogen bonds can interfere with internal hydrogen bonding in complex organic molecules. Life with a hydrocarbon solvent could make more use of hydrogen bonds within its biomolecules. Moreover, the strength of hydrogen bonds within biomolecules would be appropriate to a low temperature biochemistry.
Astrobiologist Chris McKay has argued, on thermodynamic grounds, that if life does exist on Titan's surface, using hydrocarbons as a solvent, it is likely also to use the more complex hydrocarbons as an energy source by reacting them with hydrogen, reducing ethane and acetylene to methane. Possible evidence for this form of life on Titan was identified in 2010 by Darrell Strobel of Johns Hopkins University; a greater abundance of molecular hydrogen in the upper atmospheric layers of Titan compared to the lower layers, arguing for a downward diffusion at a rate of roughly 1025 molecules per second and disappearance of hydrogen near Titan's surface. As Strobel noted, his findings were in line with the effects Chris McKay had predicted if methanogenic life-forms were present. The same year, another study showed low levels of acetylene on Titan's surface, which were interpreted by Chris McKay as consistent with the hypothesis of organisms reducing acetylene to methane. While restating the biological hypothesis, McKay cautioned that other explanations for the hydrogen and acetylene findings are to be considered more likely: the possibilities of yet unidentified physical or chemical processes (e.g., a non-living surface catalyst enabling acetylene to react with hydrogen), or flaws in the current models of material flow. He noted that even a non-biological catalyst, effective at 95 Kelvin, would in itself be a startling discovery.
(While Mars is not known to have liquid methane, methane gas in its atmosphere is of astrobiological interest as a substance that might be produced by living organisms. See Life on Mars (planet).)
Hydrogen fluoride 
Hydrogen fluoride (HF), like water, is a polar molecule, and due to its polarity it can dissolve many ionic compounds. Its melting point is −84 °C and its boiling point is 19.54 °C (at atmospheric pressure); the difference between the two is a little more than 100 K. HF also makes hydrogen bonds with its neighbor molecules, as do water and ammonia. It has been considered as a possible solvent for life by scientists such as Peter Sneath and Carl Sagan.
The biota in an HF ocean could use the fluorine as an electron acceptor to photosynthesize energy. HF is dangerous to the systems of molecules that Earth-life is made of, but certain other organic compounds, such as paraffin waxes, are stable with it. Like water and ammonia, liquid hydrogen fluoride supports an acid-base chemistry. Using a solvent system definition of acidity and basicity, nitric acid functions as a base when it is added to liquid HF.
However, hydrogen fluoride, unlike water, ammonia and methane, is cosmically rare.
Other solvents or cosolvents 
Other solvents sometimes proposed:
- Simple hydrogen compounds: hydrogen sulfide, hydrogen chloride,
- More complex compounds: sulfuric acid, formamide, methanol,
- Very-low-temperature fluids: liquid nitrogen, hydrogen in the form of a supercritical fluid.
Hydrogen sulfide is the closest chemical analog to water, but is less polar and a weaker inorganic solvent. Hydrogen sulfide and hydrogen chloride are cosmically rarer than water and ammonia. Nonetheless, hydrogen sulfide is quite plentiful on Jupiter's moon Io, and may be in liquid form a short distance below the surface; and astrobiologist Dirk Schulze-Makuch has suggested the compound as a possible solvent for life there.
Sulfuric acid in liquid form is strongly polar. It is known to be abundant in the clouds of Venus, in the form of as aerosol droplets. In a biochemistry that used sulfuric acid as a solvent, the alkene group (C=C), with two carbon atoms joined by a double bond, could function analogously to the carbonyl group (C=O) in water-based biochemistry.
A proposal has been made that life on Mars may exist and be using a mixture of water and hydrogen peroxide as its solvent. A 61.2% (by weight) mix of water and hydrogen peroxide has a freezing point of −56.5 °C, and also tends to super-cool rather than crystallize. It is also hygroscopic, an advantage in a water-scarce environment.
Other types of speculations 
Non-green photosynthesizers 
Physicists have noted that, while photosynthesis on Earth generally involves green plants, a variety of other colored plants could also support photosynthesis, essential for most life on Earth, and that other colors might be preferred in places that receive a different mix of stellar radiation than that received on Earth. These studies indicate that, while blue photosynthetic plants would be unlikely (because absorbed blue light provides some of the highest photosynthetic yields in the light spectrum), yellow or red plants are plausible. These conclusions are, in part, based on the luminosity spectra of different types of stars, the transmission characteristics of hypothetical planetary atmospheres, and the absorption spectra of various photosynthetic pigments from organisms on Earth.
Alternative atmospheres 
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The gases present in the atmosphere on Earth have varied greatly over its history. Traditional plant photosynthesis transformed the atmosphere by sequestering carbon from carbon dioxide, increasing the proportion of molecular oxygen, and by participating in the nitrogen cycle. Modern oxygen breathing animals would have been biochemically impossible until earlier photosynthetic life transformed Earth's atmosphere. The first dramatic rise in atmospheric oxygen on Earth, to about a tenth of its present-day value, occurred approximately 2.5 billion years ago, and that level did not change significantly until the Cambrian era approximately 600 million years ago.
Changes in the gas mixture in the atmosphere, even in an atmosphere made up predominantly of the same molecules of Earth's atmosphere, impacts the biochemistry and morphology of life. For example, periods of high oxygen concentrations (determined from ice core samples) have been associated with fauna of a larger scale in the fossil record, while periods of low oxygen concentrations have been associated with fauna of a smaller scale in the fossil record.
Also, while it is customary to think of plants on one side of the oxygen and nitrogen cycles as being sessile, and of animals on the other side as being motile, this is not a biological imperative. There are animals which are sessile for all or most of their lives (such as corals), and there are plants (such as tumbleweeds, and venus fly traps) that exhibit more mobility than is customarily associated with plants. On a slowly rotating planet, for example, it might be adaptive for photosynthesis to be performed by "plants" that can move to remain in the light, like Earth's sunflowers; while non-photosynthetic "animals", much like Earth's fungi, might have a lesser need to move from place to place on their own. This would be a mirror image of Earth's ecology.
Variable environments 
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Many Earth plants and animals undergo major biochemical changes during their life cycles as a response to changing environmental conditions, for example, by having a spore or hibernation state that can be sustained for years or even millennia between more active life stages. Thus, it would be biochemically possible to sustain life in environments that are only periodically consistent with life as we know it.
For example, frogs in cold climates can survive for extended periods of time with most of their body water in a frozen state, whereas desert frogs in Australia can become inactive and dehydrate in dry periods, losing up to 75% of their fluids, yet return to life by rapidly rehydrating in wet periods. Either type of frog would appear biochemically inactive (i.e. not living) during dormant periods to anyone lacking a sensitive means of detecting low levels of metabolism.
Nonplanetary life 
Dust and plasma-based 
In 2007, V. N. Tsytovich and colleagues proposed that lifelike behaviors could be exhibited by dust particles suspended in a plasma, under conditions that might exist in space. Computer models showed that, when the dust became charged, the particles could self-organize into microscopic helical structures capable of replicating themselves, interacting with other neighboring structures, and evolving into more stable forms. Similar forms of life were described in Fred Hoyle's classic novel The Black Cloud.
Usage in fiction 
In the realm of science fiction, there have occasionally been forms of life proposed that, while often highly speculative and unsupported by rigorous theoretical examination, are nevertheless interesting and in some cases even plausible.
In Arthur C. Clarke's short story "Technical Error", there is an example of differing chirality. This is not a case of alien life, rather it is an accident. The concept of reversed chirality also figured prominently in the plot of James Blish's Star Trek novel Spock Must Die!, where a transporter experiment gone awry ends up creating a duplicate Spock who turns out to be a perfect mirror-image of the original all the way down to the atomic level.
An example of silicon based life forms takes place in the Alan Dean Foster novel Sentenced to Prism in which the protagonist, Evan Orgell, is trapped on a planet whose entire ecosystem is mostly silicon-based.
Perhaps the most extreme example in science fiction is James White's Sector General: a series of novels and short stories about multienvironment hospital for the strangest life-forms imaginable, some of them breathing methane, chlorine, water and sometimes also oxygen. Some of the species directly metabolise hard radiation and their environment doesn't differ much from the atmosphere of a star, while others live in near absolute zero temperatures. All life forms are classified according to their metabolism, internal and external features, and more extreme abilities (telepathy, empathy, hive mind, etc.) with four letter codes.
One of the major sentient species in Terry Pratchett's Discworld universe are the "Earth"-based (ranging from Detritus to Diamond) Trolls. Fred Hoyle's classic novel The Black Cloud features a life form consisting of a vast cloud of interstellar dust, the individual particles of which interact via electromagnetic signalling analogous to how the individual cells of multicellular terrestrial life interact.
Outside of science-fiction, life in interstellar dust has been proposed as part of the panspermia hypothesis. The low temperatures and densities of interstellar clouds would seem to imply that life processes would operate much more slowly there than on Earth. Inorganic dust-based life has been speculated upon based on recent computer simulations. Similarly, Arthur C. Clarke's "Crusade" revolves around a planetwide life-form based on silicon and superfluid helium located in deep intergalactic space, processing its thoughts slowly by human standards, that sends probes to look for similar life in nearby galaxies. It concludes that it needs to make planets more habitable for similar life-forms, and sends out other probes to foment supernovae to do so.
Robert L. Forward's Camelot 30K describes an ecosystem on the surface of Kuiper belt objects that is based on a fluorocarbon chemistry with OF2 as the principal solvent instead of H2O. The organisms in this ecology keep warm by secreting a pellet of uranium-235 inside themselves and then moderating its nuclear fission using a boron-rich carapace around it. Kuiper belt objects are known to be rich in organic compounds, such as tholins, so some form of life existing on their surfaces is not entirely implausible–though perhaps not going so far as to develop natural internal nuclear reactors, as have Forward's. In Forward's Rocheworld series, an Earth-like biochemistry is proposed that uses a mixture of water and ammonia as its solvent. In Dragon's Egg and Starquake, Forward proposes life on the surface of a neutron star utilizing "nuclear chemistry" in the degenerate matter crust. Since such life utilised strong nuclear forces instead of electromagnetic interactions, it was posited that life might function millions of times faster than typical on Earth.
Gregory Benford and David Brin's Heart of the Comet features a comet with a conventional carbon-and-water-based ecosystem that becomes active near the perihelion when the Sun warms it. Brin's own novel Sundiver is an example of science fiction proposing a form of life existing within the plasma atmosphere of a star using complex self-sustaining magnetic fields. Gregory Benford had a form of plasma-based life exist in the accretion disk of a primordial black hole in his novel Eater. The suggestion that life could even occur within the plasma of a star has been picked up by other science fiction writers, as in David Brin's Uplift Saga or Frederik Pohl's novel The World at the End of Time. The idea is that places where reactions occur–even an incredible environment as a star–presents a possible medium for some chain of events that could produce a system able to replicate.
The Outsiders in Larry Niven's Known Space universe are cryogenic creatures based on liquid helium. They derive thermoelectric energy from a temperature gradient by basking half their body in sunlight, keeping the other half in shadow and exposed to interstellar vacuum.
Stephen Baxter has, perhaps, imagined some of the most unusual exotic life-forms in his Xeelee series of novels and stories, including supersymmetric photino-based life that congregate in the gravity wells of stars, entities composed of quantum wave functions, and the Qax, who thrive in any form of convection cells, from swamp gas to the atmospheres of gas giants. In his book Manifold: Space, he also proposes natural robots, life forms made of iron, called the Gaijin, evolving from creatures in oceans of iron carbonyl.
In his novel Diaspora, Greg Egan posits entire virtual universes implemented on Turing Machines encoded by Wang Tiles in gargantuan polysaccharide 'carpets.' The sentient ocean that covers much of the surface of Solaris in Stanislaw Lem's eponymous novel also seems, from much of the fictional research quoted and discussed in the book, to be based on some element other than carbon. In the same novel, Egan describes lifeforms in the 6-D 'macrosphere' that use a collapsed atom chemistry with energetic processes of the same order as nuclear reactions, due to the peculiarities of higher dimensional physics.
Sergeant Schlock is one of the lead characters in the webcomic Schlock Mercenary. His species, Carbosilicate Amorphs, evolved from self-repairing distributed data storage devices and, as such, redundantly distribute their 'brain' throughout their body. They are highly resistant to hard vacuum, explosive decompression, projectile weapons, chemical-based explosives, and dismemberment. Their only specialty organ is their eyes, which they harvest as fruit from the Ghanj-Rho eye-tree on their home planet. While the Amorphs have the ability to move fast, quietly, and sprout appendages at will, they excel at 'closer-than-melee-range combat, primarily "meme-toxins" against other Amorphs.
Alien warriors recruited by the god Klael in David Eddings' "Tamuli" trilogy are noted by their human opponents to breathe marsh-gas (methane). Within Eddings' universe, this limits their capacity for exertion in an oxygen atmosphere, and also determines the tactics used to fight them and eventually to destroy them in their encampments.
Star Trek 
A well-known example of a non–carbon-based life-form in science fiction is the Horta in the original Star Trek episode "Devil in the Dark". A highly intelligent silicon-based creature made almost entirely of pure rock, it tunnels through rock as easily as humans move through air. The entire species dies out every 50,000 years except for one who tends the eggs, which take the form of silicon nodules scattered throughout the caverns and tunnels of its home planet, Janus VI. The inadvertent destruction of many of these eggs by a human mining colony led the mother Horta to respond by killing the colonists and sabotaging their equipment; it was only through a Vulcan mind meld that the race's benevolence and intelligence were discovered and peaceful relations established.
Star Trek would later offer other corporeal life-forms with an alternative biochemistry. The Tholians of "The Tholian Web" are depicted and described, in that episode and later in the Star Trek: Enterprise episode "In a Mirror, Darkly" as being primarily of mineral-based composition and thriving only in superheated conditions. Another episode from TOS's third season, "The Savage Curtain", depicted another rock creature called an Excalbian, which is believed in fanon to also have been silicon-based.' 
In Star Trek: The Next Generation, the Crystalline Entity appeared in two episodes, "Datalore" and "Silicon Avatar". This was an enormous spacefaring crystal lattice that had taken thousands of lives in its quest for energy. It may have been unaware of this, however, but it was destroyed before communications could be established at a level sufficient to ascertain it. In another episode, "Home Soil", intelligent crystals that formed a "microbrain" were discovered during a terraforming mission, and they described the humans they encountered as "ugly bags of mostly water."
"The Disease", an episode of Star Trek: Voyager featured some artificially engineered silicon-based parasites, and an Enterprise episode, "Observer Effect", also presented a lethal silicon-based virus. In another Voyager episode, "Hope and Fear", a xenon-based life-form was mentioned. In the Enterprise episode "The Communicator", an alien species is encountered whose blood chemistry, while not explicitly stated, is sufficiently different from terrestrial organisms that it is not red and iron is toxic to it. Various Star Trek series also had episodes featuring photonic lifeforms.
Star Wars 
In the Star Wars movie The Empire Strikes Back, two life-forms were encountered by the characters that were non-carbon based entities. Although details of their physiology were not mentioned on screen, the space slug, (a giant worm-like creature that lived on asteroids in the vacuum of space), and the Mynock, (pesky bat-like vermin that would attach to spaceship hulls and chew through power conduits to feed off the raw energy), are said to be silicon-based organisms in Star Wars Expanded Universe sources. Also from The Empire Strikes Back, the bounty hunter Zuckuss is a member of the Gand race, an ammonia-based life-form. However, it is worth noting that the Gand are divided into two subspecies, only one of which breathes at all, the other drawing all their required sustenance from food intake and producing speech by essentially modulated flatulence.
Appearing only in the Star Wars Expanded Universe is the spice spider of Kessel, a creature made of glitterstim spice and silicon that spun crystalline webs harvested by miners as glitterstim spice, an illegal psychoactive narcotic. The spider used the webs to catch bogeys, tiny energy creatures that it consumed for energy.
Other film and television 
- In the movie The Monolith Monsters (1957), a silicon meteor reproduces, draining silicates from everything it touches. It needs water to start its cycle and contains molecular structures typical of many kinds of rocks, mixed together. A geologist says that its structure is nearly impossible. The meteor is killed by salt water, which can stop the cycle.
- On the Island of Terror (1966), silicon based life-forms are created accidentally by scientists while researching a cure for cancer. The "Silicates" are not pretty, they feed on bones (human or otherwise) and they can reproduce by mitosis. Not much to say about the scientific side though, since the survivors ignore how the creatures are exactly made of. They eventually use a very 60's way to defeat them.
- In Firewalker, a second-season episode of The X-Files, a silicon-based plant that infects humans parasitically through its spores is discovered living deep in a volcano.
- Also from The X-Files, the first-season episode "Ice" deals with an ammonia-based vermiform parasite.
- A key plot point in the comedy Evolution involves nitrogen-based life forms, and using selenium-based shampoo to poison them (with the bonus of a product placement for Head & Shoulders).
- In the Stargate SG-1 fourth season episode "Scorched Earth", a Human society known as the Enkarans are threatened on their new homeworld by an alien ship that is terraforming the planet to be suitable for the sulfur-based Gadmeer species.
- In the Stargate Atlantis fifth season episode "Remnants", a device is found whose purpose was to seed a planet with silicon-based life.
- In Ben 10, both the Omnitrix alien Diamondhead and the alien bounty hunter Tetrax Shard are members of the Petrosapien species, which are a form of silicon-based life. Other silicon-based lifeforms on the show include the Omnitrix aliens Chromostone (who is crystalline), and Echo Echo and Upgrade (who are both biomechanical). Other member's of Upgrade's species have appeared, including the shape-shifting "Ship," a pet of Ben's girlfriend, Julie.
- Indiana Jones and the Kingdom of the Crystal Skull (2008) introduces thirteen "extra dimensional beings" with crystal skeletons, who founded a city that became the basis of the El Dorado myth. Though their flesh has died and rotted away, their minds still live on within their skeletons, which communicate telepathically.
- The episode "The Stones of Blood", of the 16th season of Doctor Who, the Fourth Doctor encounters the Ogri, a silicon-based life form, and in the same sub-plot, the Megara, who are made entirely out of an unknown substance, possibly energy, and they uphold the word of the law, and execute all who break the law with a beam of energy.
- in Doctor Who's first series after its revival, episodes 4-5 and 11 featured the alien Slitheen family of the planet Raxicoricofallapatorius, which were said to be Calcium-based life forms, causing them to be blown up in contact with vinegar. The Slitheen family also reappeared in several episodes in The Sarah Jane Adventures series, as well as cameo appearances in later Doctor who Episodes.
- In the anime movie Mobile Suit Gundam 00 the Movie: A Wakening of the Trailblazer, there is a race of techno-organic sentient lifeform named ELS (Extraterrestrial Living-Metal Shape-Shifters).
Computer and video games 
In the Command & Conquer real-time strategy games, both the gameplay and storyline revolve heavily around the introduction to Earth, via meteor, of the extraterrestrial mutagen Tiberium, which displays strikingly lifelike behaviours, such as self-replication, evolution, and homeostasis, without undergoing anything like common carbon-based metabolic cycles, and which appears to be colonising the Earth, converting it into an environment unsuited to carbon-based biology. Earth creatures (such as animals, plants and even humans) exposed to Tiberium can either be killed because of the radiation or be transformed into Tiberium-based life-forms, to whom Tiberium radiation is curative rather than toxic. It is later revealed that Tiberium was introduced to Earth by the Scrin, an extremely advanced race of Tiberium-based aliens bent on mining the planet after the Tiberium deposits have reached maturity.
In the Halo franchise, the weak, low-ranking Grunts of the Covenant originate on a frozen exoplanet named Balaho, where methane is a primary constituent of the atmosphere, prevents the planet becoming even more frigid than it already is due to its distance from its parent star, and thus, Grunts have evolved to utilize the gas for respiration. Grunts are also shown to be able to use benzene as a recreational drug.
In the Master of Orion series of space strategy games, there exists an extraterrestrial race called Silicoids, whose appearance (and presumably composition) is similar to crystalline mineral structures. The game posits that this grants them immunity to the effects of hostile environments and pollution and they require no sustenance, at the expense of impeding their reproductive rate and their ability to interact with other intelligent species.
In the Submarine TITANS strategy game, the alien race in the game are called "the Silicons" because they are silicon-based life forms.
In the Star Control series, the Chenjesu, are hyperintelligent, peaceful silicon-based life-forms that were the backbone of the Alliance of Free Stars. Their crystalline biology apparently gives them the ability to send and receive hyperwave transmissions. Also, there are the Slylandro, who are gas beings residing in the upper atmosphere of a gas giant. As well, there are evidences of another silicon-based race, the Taalo who are described by the xenophobic Ur-Quan as the only race to have not awakened their territorial instincts. The Taalo were also immune to mind control.
In the game of Xenosaga, artificial life forms known as Realians have been created using silicon-based chemistry. They resemble humans in every aspect, except they are considered to be lower than humans on the social ladder.
In Mass Effect the alien Turians and Quarians, are both based on dextro-amino acids, as opposed to all the other sentient species of the galaxy based on levo-amino acids. There are also the Volus, an ammonia based species that must wear pressure suits to survive in environments suited to the other races.
In Spore, the Grox refer to the player and to other alien empires as "slow thinking carbon-based lifeforms" and "carbon wads", implying that the Grox (which are at least partly machine life) are not carbon-based. Also, the Grox can only exist on barren planets which cannot support other life, and when a planet is terraformed the Grox inhabiting it die immediately. The Grox seem to gather sustenance from the radiation from the galactic core, as the Grox colonies are larger the closer they are to the galactic core.
In Muv Luv, the BETA which calls itself the "higher/superior existence" says they were created by a silicon-based being simply called "The Creator". As such, they don't consider any non-silicon-based creature to be alive, not even themselves. Its reasoning was that only silicon-based beings occur naturally and have the ability to reproduce and disperse. When the human main character, Takeru, argues that humans also have the ability to reproduce and disperse, the higher existence says carbon too easily mingles with other elements and therefore it would be impossible for a carbon-based existence to have evolved on its own. Thus, humans must be other biological machines created by a life form just as the BETA are.
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Further reading 
- W. Bains (2004). "Many Chemistries Could Be Used to Build Living Systems". Astrobiology 4 (2): 137–167. Bibcode:2004AsBio...4..137B. doi:10.1089/153110704323175124. PMID 15253836.