Hypothetical types of biochemistry: Wikis


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The hypothetical types of biochemistry are the different types of speculative biochemistries of alien life forms that differ radically from those known on Earth. It includes biochemistries that use elements other than carbon for its basic structural and physiological functions and/or use solvents other than water. Theories about extraterrestrial life based on these "alternative" biochemistries are common in science fiction.



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 and/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.

Atoms other than carbon

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 molecular machinery necessary for life. Since humans are carbon-based beings and have never encountered life outside the earth’s environment, excluding the possibility of all other elements may be considered carbon chauvinism.


Silicon biochemistry

The most commonly proposed basis for an alternative biochemical system is the silicon atom, since silicon has many chemical properties similar to carbon and is in the same periodic table group, the carbon group.

But silicon has several drawbacks as a carbon alternative. Because silicon atoms are much bigger, having a larger mass and atomic radius, they have difficulty forming double or triple covalent bonds, which are important for a biochemical system. 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.[1] Complex long-chain silicone molecules are still less stable than their carbon counterparts.

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, but it may be possible to have an ammonia-based biochemical system instead of water. The added 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.

Finally, of the varieties of molecules identified in the interstellar medium as of 1998, 84 are based on carbon and 8 are based on silicon.[2] 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.

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.[3] However, 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.

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.

A. G. Cairns-Smith has proposed that the first living organisms to exist on Earth were clay minerals - which were probably based on silicon.[4]

Nitrogen and phosphorus biochemistry

Nitrogen and phosphorus also offer possibilities as the basis for biochemical molecules. Like carbon, phosphorus can form long chain molecules on its own, which would potentially allow it to form complex macromolecules were it not so reactive. However, in combination with nitrogen, it can form much more stable covalent bonds and create a wide range of molecules, including rings (a class of compounds called phosphazenes).

Earth's atmosphere is approximately 78% nitrogen, but this would probably not be of much use to a phosphorus-nitrogen (P-N) life-form since molecular nitrogen (N2) is nearly inert and energetically expensive to "fix" due to its triple bond. (On the other hand, Earth plants such as legumes can fix nitrogen using symbiotic anaerobic bacteria contained in their root nodules.) A nitrogen dioxide (NO2) or ammonia (NH3) atmosphere would be more useful. Nitrogen also forms several oxides, such as nitrogen monoxide, dinitrogen oxide, and dinitrogen tetroxide, and all would be present in a nitrogen-dioxide-rich atmosphere.

  • In a nitrogen dioxide atmosphere, P-N plant analogues could absorb nitrogen dioxide from the air and phosphorus from the ground. The nitrogen dioxide would be reduced, with analogues to sugar being produced in the process, and waste oxygen would be released into the atmosphere. Animals based on phosphorus and nitrogen would consume the plants, use atmospheric oxygen to metabolize the sugar analogues, exhaling nitrogen dioxide and depositing phosphorus, or phosphorus-rich material, as solid waste.
  • In an ammonia atmosphere, P-N plants would absorb ammonia from the air and phosphorus from the ground, then oxidize the ammonia to produce P-N sugars and release hydrogen waste. P-N animals are now the reducers, breathing in hydrogen and converting the P-N sugars to ammonia and phosphorus. This is the opposite pattern of oxidation and reduction from a nitrogen dioxide world, and from the known biochemistry of Earth. It would be analogous to Earth's atmospheric carbon supply being in the form of methane instead of carbon dioxide.

Debate continues, as several aspects of a phosphorus-nitrogen cycle biology would be energy deficient. Also, nitrogen and phosphorus are unlikely to occur in the ratios and quantity required in the universe. Carbon, being preferentially formed during nuclear fusion, is more abundant and is more likely to end up in a preferred location.

Moreover, if an ammoniated atmosphere would be possible and stable at first view (in a reductive environment), it is doubtful that an atmosphere rich in nitrogen dioxide could even exist. Since the nitrogen oxides are all endoenergetic compared with molecular nitrogen and oxygen; and they are oxidizing, and would decompose by stellar radiation and by catalysis on the surface of rocks when they are produced.

Other exotic biochemical elements

  • Arsenic, which is chemically similar to phosphorus, while poisonous for most Earth life, is incorporated into the biochemistry of some organisms.[5] 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).[6] 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 on Earth may have used arsenic in place of phosphorus in the backbone of its DNA.[7]
  • Chlorine is sometimes proposed as a 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.
  • Sulfur is also able to form long-chain molecules, but suffers from the same high reactivity problems that phosphorus and silanes do. The biological use of sulfur as an alternative to carbon is purely theoretical. However, the biological use of sulfur as an alternative to oxygen is widespread—strains of sulfur-reducing bacteria have been discovered in exotic locations on Earth, and also not so exotic locations, such as aging water systems.[8] These bacteria can utilize elemental sulfur instead of oxygen, reducing sulfur to hydrogen sulfide. Examples of this type of metabolism are green sulfur bacteria and purple sulfur bacteria. Examples of micro-organisms that metabolize elemental sulfur can be traced back 3.5 billion years on Earth.[9]

Non-water solvents

In addition to carbon compounds, all currently known terrestrial life also requires water as a solvent. It is sometimes assumed that water is the only suitable chemical to fill this role. 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 a proton 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 is the only compound listed here that is 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.

There are some properties that make certain compounds and elements much more favorable than others as solvents in a successful biosphere. The first of these is a reasonably wide temperature range in which it is liquid. Using ammonia as a baseline, the boiling point must be at least 20% greater than the melting point measured in Kelvin. For example, the boiling point of water is 373.15 K (at 1 atmosphere), while the melting point is 273.15 K. 373.15/273.15=1.37, meaning that the boiling point is about 37% higher than the freezing point, and thus water has an acceptable temperature range. This means that polar solvents, like water (37%), tend to do better than nonpolar solvents, such as methane (23%). However, as shown in the example given, methane does pass the test, albeit by a smaller margin than water. Since boiling points can vary based on the pressure, the question tends not to be does the prospective solvent pass this test, but at what pressure does the prospective solvent pass this test. For example, hydrogen cyanide does not pass this test at 1 atmosphere, with 15%, but in a Venus pressure atmosphere, with 92 bars (9.2 MPa) of pressure, it is very likely that it would.

Another important characteristic of a prospective solvent is that one can reasonably expect to find it in the location that the theoretical life will live. Usually, this is a planet or moon, but other locations such as a planetary nebula or a gas giant are also possible. This requirement has two main consequences. The first of these is that it needs to be composed of common elements. On a planet or moon, the most common elements tend to be carbon, hydrogen, oxygen, nitrogen, silicon, sulfur, and various metals. Solvents made of a combination of these elements tend to have an advantage. This is why hydrogen fluoride, shown below, is not considered a likely solvent. The second offshoot of that is that the prospective solvent needs to be something that can reasonably be expected to be synthesized from these elements in large quantities, using solely natural phenomena. For example, the compounds methane, ammonia, and water, while not only being on the list of common elements, are also widespread, and thus one can probably count on their presence in the relevant temperature ranges. However, while aluminium fits the liquid range requirements (its number is about 200% at 1 atmosphere), and is common (it is about 7% of the composition of rocky bodies), it is almost universally found in the form of its oxide, aluminium oxide, because not even carbon can reduce it chemically. This also means that a prospective solvent cannot be overly reactive, in its given atmosphere, for example sodium because then it would probably not remain in existence.


Ammonia is perhaps the most commonly proposed alternative. Numerous chemical reactions are possible in an ammonia solution, and liquid ammonia has some 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. Given this set of chemical properties it has been theorized that ammonia-based life forms might be possible.

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 three times smaller, and reducing its ability to concentrate non-polar molecules through a hydrophobic effect. For these reasons,some members of the scientific community question how well ammonia could hold prebiotic molecules together to allow the emergence of a self-reproducing system. Ammonia is also combustible and oxidizable and could not exist sustainably in a biosphere that oxidizes it. It would, however, be stable in a reducing environment.

A biosphere based on ammonia would likely exist at temperatures or air pressures that are extremely unusual for terrestrial life. Terrestrial life 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). Such extremely cold temperatures create problems, as they slow biochemical reactions tremendously and may cause biochemical precipitation out of solution due to high melting points. Ammonia could be a liquid at normal temperatures, but at much higher pressures; for example, at 60 atm, ammonia melts at −77 °C (196 K) and boils at 98 °C (371 K).

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.[10]

Hydrogen fluoride

Hydrogen fluoride, 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 little more than 100 °C. HF also makes hydrogen bonds with its neighbor molecules as do water and ammonia. These would make HF a candidate to host life on other planets.

Not much research has been done on liquid HF in regards to its ability to dissolve and react with non-polar molecules. 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 the paraffins are stable with it.

But presence of great amounts of free HF in a planetary scale, like water on Earth, is doubtful, even next to impossible: water would be present in it, forming a solution of hydrofluoric acid that would change all its properties. The hypothetical planet would also surely contain silicates that would react with HF to form inert compounds as silicon fluorides, as soon as HF would be present; thus preventing its concentration in great quantities in an hypothetical planetary environment.

The cosmic abundance of fluorine is low, and it forms chemically inert compounds rapidly in interstellar nebulae, because it is the most reactive element.

Other solvents

Other solvents sometimes proposed include formamide, methanol, hydrogen sulfide and hydrogen chloride. Hydrogen chloride suffers from the low cosmic abundance of chlorine, while hydrogen sulfide suffers from its high reactivity. Moreover, the first two could not be expected to be found in vast quantities on a planetary scale, and would only be part of the internal physiology of organisms.

A mixture of hydrocarbons, such as the methane/ethane lakes detected on Titan by the Cassini spacecraft, could act as a solvent over a wide range of temperatures but would lack polarity.

Isaac Asimov, the biochemist and science fiction writer, suggested that poly-lipids could form a substitute for proteins in a non-polar solvent such as methane or liquid hydrogen.[11]

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 percent (by weight) mix of water and hydrogen peroxide has a freezing point of -56.5 degrees Celsius, and also tends to super-cool rather than crystallize. It is also hygroscopic, an advantage in a water-scarce environment.[12]'[13]

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 solar radiation than that received on Earth.[14] These studies indicate that no photosynthetic plants would be blue-colored, because blue light provides some of the highest photosynthetic yields in the light spectrum (therefore it is important for blue light to be absorbed rather than reflected). The physicists base their conclusions not on chemistry, but on the physical quality of different frequencies of light produced by known types of stars.

The first plants on Earth may have been a sightly different colour, because the Sun was, in the first geological eons, a little less bright, and its light was filtered by passing through an atmosphere with a different composition.

One terrestrial example of energy conversion based on something other than ordinary light involves fungi that convert high energy (compared to visible light) gamma rays into useful energy using the pigment melanin.[15] In most organisms, melanin, a black pigment, instead protects the organisms against ultraviolet and solar radiation.[15] Ordinarily fungi derive their energy from decomposing other biomass, rather than by converting radiation into energy for themselves.

Alternative atmospheres

The gases present in the atmosphere on Earth have varied greatly over its history. Traditional plant photosynthesis has terraformed 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, giving rise to the Cambrian explosion.[16] Precambrian life had to have biochemistries that did not require oxygen.

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 associated with of low oxygen concentrations have been associated with fauna of a smaller scale in the fossil record.[17]

Also, while it is customary to think of plants, which are on one side of the oxygen and nitrogen cycles as being sessile, and animals which are on the other 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, 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

Many Earth plants and animals also 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.

Similarly, frogs in cold climates can survive for extended periods of time with most of their body water in a frozen state[18], while 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.[19] Either type of frog would appear biochemically inactive (i.e. not living) in dormant periods to anything other than the most acute means of sensing this activity.

More exotic speculations

Interstellar dust-based life

In 2007 V. N. Tsytovich and colleagues discovered the possibility of life-like behaviors being exhibited by dust particles suspended in a plasma, similar to conditions in interstellar space.[20][21] 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 speculated on in Fred Hoyle's classic novel The Black Cloud.

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.

Novels, short stories and comics

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. A technical error in a new power plant changes the chirality of a worker. He cannot read because he sees all type as reversed, and he believes his left side is his right. The amusement ends when it is discovered that he is dying from malnutrition because his new biology cannot utilize normal food.

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 metabolise directly 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. Humans from Earth share the DBDG specification with small furry beings called Nidians.

One of the major sentient species in Terry Pratchett's Discworld universe are the "earth"-based (ranging from Detritus to Diamond) Trolls.

Pratchett has also written the science fiction novel The Dark Side of the Sun which features a range of extraordinary life-forms, including a telepathic body of water, creatures called "Sundogs", which are capable of interstellar travel from birth, and a sentient planet: effectively a giant silicon-based computer.

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.[21]

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. Clarke implies that this is what accounts for most supernovae having occurred in the same region of space and warns that the effort will eventually reach Earth.[citation needed]

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. Fluorine is also of low cosmic abundance, so its use in this manner is unlikely.[citation needed]

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. Similar sorts of plasmoid life have sometimes been proposed to exist in other places, such as planetary ionospheres or the interstellar medium, but usually only by fringe theorists (see ball lightning for some additional discussion).[citation needed] 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. 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 imagined perhaps 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, and the Qax, who thrive in any form of convection cells, from swamp gas to the atmospheres of gas giants.

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' which use a collapsed atom chemistry with energetic processes of the same order as nuclear reactions, due to the peculiarities of higher dimensional physics.

In her novel Brain Plague, Joan Slonczewski describes a species of intelligent microrganisms with arsenic based chemistries that live symbiotically with human hosts.

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,[22] and sprout appendages at will, they excel at 'closer-than-melee-range combat, primarily "meme-toxins" against other Amorphs.

A more farcical example comes from the Hitchhiker's Guide to the Galaxy, where the Hooloovoo are a hyperintelligent shade of the colour blue.

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 Horta, a fictional silicon based life-form in the Star Trek universe.

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 save for one who tends all 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.[23]' [24]

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.

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 freezing vacuum of space),[25]' [26] and the Mynock, (pesky bat-like vermin that would attach to spaceship hulls and chew through power conduits to feed off the raw energy),[27]' [28] are said to be silicon-based organisms in 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 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.[29]' [30]


In the movie Alien the science officer Ash notes that the facehugger creature replaces its cells with polarised silicon to give it "prolonged resistance to adverse environmental conditions". Both stages of the alien cycle also have a highly corrosive blood, normally understood to be acid, which would be inconsistent with known terrestrial biochemistry. It should be noted though that the alien's biochemistry was not sufficiently different to prevent it from using humans as a food source.

Other film and television

  • In the movie Titan A.E., an alien race called the Drej was made only of pure energy.
  • In the 1971 movie The Andromeda Strain the Andromeda organism is described as reproducing via the direct conversion of energy into matter.
  • In the movie The Monolith Monsters, 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.
  • 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 are members of the Petrosapien species, which are a form of silicon-based life.
  • In Dragon Ball Z, shortly before the destruction of Planet Namek, Frieza tells Goku that he does not need to breathe. This would allow him to survive the destruction of the planet and suggest that he is not a carbon-based life-form.
  • 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.
  • An episode (the Chrysalids) of the series Space: 1999 presents humanoids on a planet protected by a electric waves system, which are living in a chlorine atmosphere.
  • 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.

Computer and video games

In the Command & Conquer real-time strategy games, both the gameplay and storyline revolve heavily around the introduction to Earth of an extraterrestrial mutagen called Tiberium via meteor, 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 video game series, a race of Covenant aliens named "Grunts" by humans require a breathing apparatus while fighting the humans in an Earth-like atmosphere. According to the novelizations of the video game, the Grunts' apparatus allows them to breathe the methane they need to survive.

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 Metroid Prime Series, Phazon is a highly radioactive, self regenerating mineral with organic properties that is generated by the sentient planet Phaaze.

In Metroid Prime Hunters, Spire is a rock-like, silicon based alien. He is the last Diamont (presumably a play on the word diamond, which is composed of carbon).

In the strategy game: Submarine TITANS 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 dextro-amino acid-based organisms, as opposed to humans, a deoxyribonucleic acid life-form. 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 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, further supporting this hypothesis. There has been no canonical statement on what element the Grox are based on, although Sulfur or Silicon have been suggested by fans.

See also


  1. ^ Gillette, Stephen. World-Building. Writer's Digest Books. 
  2. ^ Lazio, Joseph. "F.10 Why do we assume that other beings must be based on carbon? Why couldn't organisms be based on other substances?". [sci.astro] ET Life (Astronomy Frequently Asked Questions). http://www.faqs.org/faqs/astronomy/faq/part6/section-16.html. Retrieved 2006-07-21. 
  3. ^ "Abundance in Earth's Crust". WebElements.com. http://www.webelements.com/webelements/properties/text/image-flash/abund-crust.html. Retrieved 2007-04-14. 
  4. ^ Cairns-Smith, A. Graham (1985). Seven Clues to the Origin of Life. Cambridge: Cambridge University Press. ISBN 0-521-27522-9. 
  5. ^ Biochemical Periodic Table - Arsenic
  6. ^ Niggemyer, A; Spring S, Stackebrandt E, Rosenzweig RF (December 2001). "Isolation and characterization of a novel As(V)-reducing bacterium: implications for arsenic mobilization and the genus Desulfitobacterium". Appl Environ Microbiol 67 (12): 5568–80. doi:10.1128/AEM.67.12.5568-5580.2001. PMID 11722908. http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=11722908&dopt=Abstract. 
  7. ^ Reilly, Michael (26 April 2008). "Early life could have relied on 'arsenic DNA'". New Scientist 198 (2653): 10. doi:10.1016/S0262-4079(08)61007-6. http://www.newscientist.com/channel/life/mg19826533.600-early-life-could-have-relied-on-arsenic-dna.html. 
  8. ^ http://www.dnr.state.wi.us/org/water/dwg/sulferb.pdf
  9. ^ Early Archaean Microorganisms Preferred Elemental Sulfur, Not Sulfate Science AAAS, by Philippot, et al., (14 September 2007)
  10. ^ [1] As of 2006-03-02, the page appears unavailable.
  11. ^ "Aliens". Atomic Rockets. http://www.projectrho.com/rocket/rocket3aa.html. Retrieved 2006-03-03. 
  12. ^ Houtkooper, Joop M.; Dirk Schulze-Makuch (2007). "The H2O2-H2O Hypothesis: Extremophiles Adapted to Conditions on Mars?" (PDF). EPSC Abstracts (European Planetary Science Congress 2007) 2. EPSC2007-A-00439. http://www.cosis.net/abstracts/EPSC2007/00439/EPSC2007-J-00439.pdf. 
  13. ^ Ellison, Doug (2007-08-24). "Europlanet : Life's a bleach". Planetary.org. http://www.planetary.org/blog/article/00001109/. 
  14. ^ NASA - NASA Predicts Non-Green Plants on Other Planets
  15. ^ a b Dadachova, E; Bryan RA, Huang X, Moadel T, Schweitzer AD, et al. (2007). "Ionizing Radiation Changes the Electronic Properties of Melanin and Enhances the Growth of Melanized Fungi". PLoS ONE 2 (5): e457. doi:10.1371/journal.pone.0000457. http://www.plosone.org/article/fetchArticle.action?articleURI=info:doi/10.1371/journal.pone.0000457. 
  16. ^ The Ediacaran Fauna
  17. ^ Science, "The Rise of Oxygen over the Past 205 Million Years and the Evolution of Large Placental Mammals" (30 September 2005)
  18. ^ Christmas in Yellowstone
  19. ^ Main and Bentley, Ecology, "Water Relations of Australian Burrowing Frogs and Tree Frogs" (1964)
  20. ^ "Physicists Discover Inorganic Dust With Lifelike Qualities". Science Daily. 2007-08-15. http://www.sciencedaily.com/releases/2007/08/070814150630.htm. 
  21. ^ a b Tsytovich, V N; G E Morfill, V E Fortov, N G Gusein-Zade, B A Klumov and S V Vladimirov (14 August 2007). "From plasma crystals and helical structures towards inorganic living matter". New J. Phys. 9 (263): 263. doi:10.1088/1367-2630/9/8/263. http://www.iop.org/EJ/abstract/1367-2630/9/8/263. 
  22. ^ Schlock Mercenary archives - Tuesday, September 5, 2006
  23. ^ SciFi.com StarTrek
  24. ^ Excalbian. Memory Alpha
  25. ^ Space Slug in the Star Wars Databank
  26. ^ Exogorth on Wookieepedia: a Star Wars Wiki
  27. ^ Mynock in the Star Wars Databank
  28. ^ Mynock on Wookieepedia: a Star Wars Wiki
  29. ^ Energy spider on Wookieepedia: a Star Wars Wiki
  30. ^ Energy Spider. The Completely Unofficial Star Wars Encyclopedia

External links


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