Several forms ofbiochemistry are agreed to be scientifically viable, but are not proven to exist at this time.[2] The kinds ofliving organisms known on Earth, as of 2025[update], all usecarbon compounds for basic structural andmetabolic functions,water as asolvent, and deoxyribonucleic acid (DNA) or ribonucleic acid (RNA) to define and control their form. Iflife exists on othercelestial bodies (planets,moons), it may be chemically similar, though it is also possible that there are organisms with quite different chemistries[3] – for instance, involving other classes of carbon compounds, compounds of another element, and/or another solvent in place of water.
The possibility of life-forms being based on "alternative" biochemistries is the topic of an ongoing scientific discussion, informed by what is known about extraterrestrial environments and about the chemical behaviour of various elements and compounds. It is of interest insynthetic biology and is also acommon subject in science fiction.
The elementsilicon has been much discussed as a hypothetical alternative to carbon. Silicon is in the same group as carbon on theperiodic table and, like carbon, it istetravalent. Hypothetical alternatives to water includeammonia, which, like water, is apolar molecule, and cosmically abundant; and non-polarhydrocarbon solvents such asmethane andethane, which are known to exist in liquid form on the surface ofTitan.
| Type | Basis | Brief description | Details |
|---|---|---|---|
| Alternative-chirality biomolecules | Alternative biochemistry | Mirror image biochemistry | Alternative-chirality biomolecules refer to biomolecules with reflectedchirality. In known Earth-based life,amino acids are almost universally of theL form andsugars are of theD form; however, the mirror-image forms could equally form the basis for alternative biochemistries. Synthetic biologists have proposed creating mirror-image versions of existing organisms, using entirely mirror-image biochemistry;[4] these would behave identically to their template organisms except when interacting with existing biomolecules.[5] Mirror-image microorganisms would be resistant to the immune systems of existing organisms. Scientists have stated concern over this risk and discouraged the creation of them.[6][7] |
| Alternative nucleic acids | Alternative biochemistry | Different genetic storage | Xeno nucleic acids (XNA) may possibly be used in place of RNA or DNA. XNA is the general term for a nucleic acid with an altered sugar backbone. Examples of XNA are:[8]
In comparison,Hachimoji DNA changes the base pairs instead of the backbone. These new base pairs are P (2-Aminoimidazo[1,2a][1,3,5]triazin-4(1H)-one), Z (6-Amino-5-nitropyridin-2-one), B (Isoguanine), and S (rS=Isocytosine for RNA, dS=1-Methylcytosine for DNA).[9][10] |
| Ammonia biochemistry | Non-water solvents | Ammonia-based life | Ammonia isrelatively abundant in the universe and has chemical similarities to water. The possible role ofliquid ammonia as an alternative solvent for life is an idea dating back to 1954 at least, whenJ. B. S. Haldane raised the topic at a symposium about life's origin.[11][12] |
| Arsenic biochemistry | Alternative biochemistry | Arsenic-based life | Arsenic, which is chemically similar tophosphorus, while poisonous for mostlife forms on Earth, is incorporated into the biochemistry of some organisms.[citation needed] |
| Borane biochemistry (Organoboron chemistry) | Alternative biochemistry | Borane-based life | Boranes are dangerously explosive in Earth's atmosphere but would be more stable in areducing atmosphere (environment), one with no oxygen or other oxidizing gases, and which may contain actively reductant gases such as hydrogen, carbon monoxide, methane, and hydrogen sulfide. Molecular structures containing alternating boron and nitrogen atoms share some properties with hydrocarbons. However,boron is far rarer in the universe than its neighbours of carbon, nitrogen, and oxygen.[citation needed] |
| Cosmic necklace-based biology | Nonplanetary life | Non-chemical life | In 2020, Luis A. Anchordoqu and Eugene M. Chudnovsky hypothesized that life composed of magnetic semipoles connected bycosmic strings could evolve inside stars.[13] |
| Dusty plasma-based biology | Nonplanetary life | Non-chemical life | In 2007, Vadim N. Tsytovich and colleagues proposed that lifelike behaviours could be exhibited by dust particles suspended in aplasma, under conditions that might exist in space.[14] |
| Extremophiles | Alternative environment | Life in variable environments | It would be biochemically possible to sustain life in environments that are only periodically consistent with life as we know it, such as extremely high or low temperatures, pressures, or pH; or the presence of high levels ofsalt ornuclear radiation.[citation needed] |
| Heteropoly acid biochemistry | Alternative biochemistry | Heteropoly acid-based life | Various metals can form complex structures with oxygen, such asheteropoly acids.[citation needed] |
| Hydrogen fluoride biochemistry | Non-water solvents | Hydrogen fluoride-based life | Hydrogen fluoride has been considered as a possible solvent for life by scientists such as Peter Sneath.[citation needed] |
| Hydrogen sulfide biochemistry | Non-water solvents | Hydrogen sulfide-based life | Hydrogen sulfide is achemical analog of water but is less polar and a weaker inorganic solvent.[citation needed] |
| Methane biochemistry (Azotosome) | Non-water solvents | Methane-based life | Methane isrelatively abundant in the Solar System and the Universe and is known to exist in liquid form onTitan, the largest moon ofSaturn. Though highly unlikely, it is considered to be possible for Titan to harbour life. If so, it will most likely be methane-based life.[citation needed] |
| Non-green photosynthesizers | Other speculations | Alternate plant life | Physicists have noted that, although photosynthesis on Earth generally involves green plants, a variety of other-coloured plants could also support photosynthesis, essential for most life on Earth, and that other colours might be preferred in places that receive a different mix of stellar radiation than Earth. In particular,retinal is capable of, and has been observed to, perform photosynthesis.[15] Bacteria capable of photosynthesis are known asmicrobial rhodopsins. A plant or creature that uses retinal photosynthesis is alwayspurple. |
| Shadow biosphere | Alternative environment | A hidden life biosphere onEarth | A shadow biosphere is a hypotheticalmicrobialbiosphere of Earth that uses radically differentbiochemical andmolecular processes than currently known life. It could exist, for example, deep in the crust or sealed in ancient rocks.[citation needed] |
| Silicon biochemistry (Organosilicon) | Alternative biochemistry | Silicon-based life | Like carbon, silicon can create molecules that are sufficiently large to carry biological information; however, the scope of possible silicon chemistry is far more limited than that of carbon.[citation needed] |
| Silicon dioxide biochemistry | Non-water solvents | Silicon dioxide-based life | Gerald Feinberg andRobert Shapiro have suggested that molten silicate rock could serve as a liquid medium for organisms with a chemistry based on silicon, oxygen, and other elements such asaluminium.[citation needed] |
| Sulfur biochemistry | Alternative biochemistry | Sulfur-based life | The biological use of sulfur as an alternative to carbon is purely hypothetical, especially because sulfur usually forms only linear chains rather than branched ones.[citation needed] |
A shadow biosphere is a hypotheticalmicrobialbiosphere of Earth that uses radically differentbiochemical andmolecular processes than currently known life.[16][17] Although life on Earth is relatively well-studied, the shadow biosphere may still remain unnoticed because the exploration of the microbial world targets primarily the biochemistry of the macro-organisms.
Perhaps the least unusual alternative biochemistry would be one with differingchirality of its biomolecules. In known Earth-based life,amino acids are almost universally of theL form andsugars are of theD form. Molecules usingD amino acids orL sugars may be possible; molecules of such a chirality, however, would be incompatible with organisms using the opposing chirality molecules. Amino acids which chirality is opposite to the norm are found on Earth, and these substances are generally thought to result from decay of organisms of normal chirality. However, physicistPaul Davies speculates that some of them might be products of "anti-chiral" life.[18]
It is questionable, however, whether such a biochemistry would be truly alien. Although it would certainly be an alternativestereochemistry, molecules that are overwhelmingly found in oneenantiomer throughout the vast majority of organisms can nonetheless often be found in another enantiomer in different (oftenbasal) organisms such as in comparisons between members ofArchaea and otherdomains,[citation needed] making it an open topic whether an alternative stereochemistry is truly novel.
On Earth, all known living things have a carbon-based structure and system. Scientists have speculated about the advantages and disadvantages of usingelements 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, asCarl 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.[19] Sagan used the term "carbon chauvinism" for such an assumption.[20] He regardedsilicon andgermanium as conceivable alternatives to carbon[20] (other plausible elements include but are not limited topalladium andtitanium); but, on the other hand, he noted that carbon does seem more chemically versatile and is more abundant in the cosmos.[21]Norman Horowitz devised the experiments to determine whetherlife might exist on Mars that were carried out by theViking Lander of 1976, the first U.S. mission to successfully land a probe on the surface of Mars. Horowitz argued that the great versatility of the carbon atom makes it the element most likely to provide solutions, even exotic solutions, to the problems of survival on other planets.[22] He considered that there was only a remote possibility that non-carbon life forms could exist with genetic information systems capable of self-replication and the ability to evolve and adapt.
The silicon atom has been much discussed as the basis for an alternative biochemical system,[23] because silicon has manychemical similarities to carbon and is inthe same group of the periodic table. Like carbon, silicon can create molecules that are sufficiently large to carry biological information.[24]
However, silicon has several drawbacks as a carbon alternative. Carbon is ten times morecosmically abundant than silicon, and its chemistry appears naturally more complex.[25] By 1998, astronomers had identified 84 carbon-containing molecules in theinterstellar medium, but only 8 containing silicon, of which half also include carbon.[26] Even thoughEarth and otherterrestrial planets are exceptionally silicon-rich and carbon-poor (silicon is roughly 925 timesmore abundant in Earth's crust than carbon), terrestrial life bases itself on carbon. It may avoid silicon because silicon compounds are less varied, unstable in the presence ofwater, or block the flow of heat.[25]
Relative to carbon, silicon has a much largeratomic radius, and forms much weakercovalent bonds to atoms — exceptoxygen andfluorine, with which it forms very strong bonds.[24] Almost nomultiple bonds to silicon are stable, although silicon does exhibit variedcoordination number.[27]Silanes, silicon analogues to thealkanes, react rapidly with water, and long-chain silanes spontaneously decompose.[28] Consequently, most terrestrial silicon is "locked up" insilica, and not a wide variety of biogenic precursors.[27]
Silicones, which alternate between silicon andoxygen atoms, are much more stable than silanes, and may even be more stable than the equivalent hydrocarbons in sulfuric acid-rich extraterrestrial environments.[28] Alternatively, the weak bonds in silicon compounds may help maintain a rapid pace of life atcryogenic temperatures. Polysilanols, the silicon homologues tosugars, are among the few compounds soluble inliquid nitrogen.[29][unreliable source?][27]
All known siliconmacromolecules are artificial polymers, and so "monotonous compared with the combinatorial universe of organic macromolecules".[24][27] Even so, some Earth life usesbiogenic silica:diatoms' silicateskeletons.A. G. Cairns-Smith hypothesized that silicate minerals in waterplayed a crucial role in abiogenesis, in that biogenic carbon compoundsformed around their crystal structures.[30][31] Although not observed in nature, carbon–silicon bonds have been added to biochemistry underdirected evolution (artificial selection): acytochromec protein fromRhodothermus marinus has been engineered to catalyse new carbon–silicon bonds between hydrosilanes anddiazo compounds.[32]
Whilearsenic, which is chemically similar tophosphorus, is poisonous for mostlife forms on Earth, it is incorporated into the biochemistry of some organisms.[35] Somemarine algae incorporate arsenic into complex organic molecules such asarsenosugars andarsenobetaines.Fungi andbacteria can produce volatile methylated arsenic compounds.Arsenate reduction and arsenite oxidation have been observed inmicrobes (Chrysiogenes arsenatis).[36] Additionally, someprokaryotes 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 usedarsenic biochemistry in place of phosphorus in the structure of their DNA.[37] A common objection to this scenario is that arsenate esters are so much less stable tohydrolysis than correspondingphosphate esters that arsenic is poorly suited for this function.[38]
The authors of a 2010geomicrobiology study, supported in part by NASA, have postulated that a bacterium namedGFAJ-1, collected in the sediments ofMono Lake in easternCalifornia, can employ such 'arsenic DNA' when cultured without phosphorus.[39][40] They proposed that the bacterium may employ high levels ofpoly-β-hydroxybutyrate or other means to reduce theeffective concentration of water and stabilize its arsenate esters.[40] This claim was heavily criticized almost immediately after publication for the perceived lack of appropriate controls.[41][42][43] Other authors were unable to reproduce their results and showed that the study had issues with phosphate contamination, suggesting that the low amounts present could sustain extremophile lifeforms.[44] The 2010 paper wasretracted in 2025.[45][46]
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 biochemistSteven Benner,[47] and by the astrobiological committee chaired by John A. Baross.[48] Solvents discussed by the Baross committee includeammonia,[49]sulfuric acid,[50]formamide,[51] hydrocarbons,[51] and (at temperatures much lower than Earth's) liquidnitrogen, or hydrogen in the form of asupercritical fluid.[52]
Water as a solvent limits the forms biochemistry can take. For example, Steven Benner, proposes thepolyelectrolyte theory of the gene that claims that for a geneticbiopolymer such as DNA to function in water, it requires repeatedionic charges.[53] If water is not required for life, these limits on genetic biopolymers are removed.
Carl Sagan once described himself as both acarbon chauvinist and a water chauvinist;[54] however, on another occasion he said that he was a carbon chauvinist but "not that much of a water chauvinist".[55]He speculated on hydrocarbons,[55]: 11 hydrofluoric acid,[56] and ammonia[55][56] as possible alternatives to water.
Some of the properties of water that are important for life processes include:
Water as a compound is cosmically abundant, although much of it is in the form of vapor or ice. Subsurface liquid water is considered likely or possible on several of the outer moons:Enceladus andEuropa (where geysers have been observed),Titan, andGanymede. Earth and Titan are the only worlds currently known to have stable bodies of liquid on their surfaces.
Not all properties of water are necessarily advantageous for life, however.[57] For instance, water ice has a highalbedo,[57] meaning that it reflects a significant quantity of light and heat from the Sun. Duringice ages, as reflective ice builds up over the surface of the water, the effects of global cooling are increased.[57]
There are some properties that make certain compounds and elements much more favourable 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 bedoes the prospective solvent remain liquid, butat what pressure. For example,hydrogen cyanide has a narrow liquid-phase temperature range at 1 atmosphere, but in an atmosphere with the pressure ofVenus, with 92 bars (91 atm) of pressure, it can indeed exist in liquid form over a wide temperature range.
Theammonia 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.[58] The possible role of liquid ammonia as an alternative solvent for life is an idea dating back to 1954 at least, whenJ. B. S. Haldane raised the topic at a symposium about life's origin.[11][12]
Many chemical reactions can occur in an ammonia solution, and liquid ammonia has chemical similarities with water.[58][12] Ammonia dissolves most organic molecules at least as well as water does, and many elemental metals. Haldane indicated that various common water-related organic compounds have ammonia-related analogues; for instance, the ammonia-relatedamine group (−NH2) is analogous to the water-relatedhydroxyl group (−OH).[12]
Ammonia, like water, can either accept or donate an H+ ion. When ammonia accepts an H+, it forms theammonium cation (NH4+), analogous tohydronium (H3O+). When it donates an H+ ion, it forms theamide anion (NH2−), analogous to thehydroxide anion (OH−).[49] Compared to water, however, ammonia is more inclined to accept an H+ ion, and less inclined to donate one; it is a strongernucleophile.[49] Ammonia added to water functions as anArrhenius base: it increases the concentration of the anion hydroxide. Conversely, using asolvent system definition of acidity and basicity, water added to liquid ammonia functions as an acid, because it increases the concentration of the cation ammonium.[12] The carbonyl group (C=O), which is much used in terrestrial biochemistry, would not be stable in ammonia solution, but the analogousimine group (C=NH) could be used instead.[49]
However, ammonia has some problems as a basis for life. Thehydrogen bonds between ammonia molecules are weaker than those in water, causing ammonia'sheat of vaporization to be half that of water, itssurface tension to be a third, and reducing its ability to concentrate non-polar molecules through ahydrophobic 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.[59][12] Ammonia is also flammable in oxygen and could not exist sustainably in an environment suitable foraerobic metabolism.[60]
Abiosphere 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 between the melting point andboiling point of water, at a pressure designated asnormal pressure, between 0 and 100 °C (273 and 373 K). When also held to normal pressure, ammonia's melting and boiling points are −78 °C (195 K) and −33 °C (240 K) respectively. Because chemical reactions generally proceed more slowly at lower temperatures, ammonia-based life existing in this set of conditions might metabolize more slowly and evolve more slowly than life on Earth.[60] On the other hand, lower temperatures could also enable living systems to use chemical species that would be too unstable at Earth temperatures to be useful.[58]
A set of conditions where ammonia is liquid at Earth-like temperatures would involve it being at a much higher pressure. For example, at 60atm ammonia melts at −77 °C (196 K) and boils at 98 °C (371 K).[49]
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-basedhabitability zone. Such conditions could exist, for example, under the surface ofSaturn's largest moonTitan.[61]
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.[58] Hydrocarbons could act as a solvent over a wide range of temperatures but would lackpolarity. Isaac Asimov, thebiochemist and science fiction writer, suggested in 1981 that poly-lipids could form a substitute for proteins in a non-polar solvent such as methane.[58] Lakes composed of a mixture of hydrocarbons, including methane andethane, have been detected on the surface of Titan by theCassini spacecraft.
There is debate about the effectiveness of methane and other hydrocarbons as a solvent for life compared to water or ammonia.[62][63][64] Water is a stronger solvent than the hydrocarbons, enabling easier transport of substances in a cell.[65] However, water is also more chemically reactive and can break down large organic molecules through hydrolysis.[62] A life-form which solvent was a hydrocarbon would not face the threat of its biomolecules being destroyed in this way.[62] Also, the water molecule's tendency to form strong hydrogen bonds can interfere with internal hydrogen bonding in complex organic molecules.[57] Life with a hydrocarbon solvent could make more use of hydrogen bonds within its biomolecules.[62] Moreover, the strength of hydrogen bonds within biomolecules would be appropriate to a low-temperature biochemistry.[62]
AstrobiologistChris 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 andacetylene to methane.[66] Possible evidence for this form oflife on Titan was identified in 2010 by Darrell Strobel ofJohns 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 ifmethanogenic life-forms were present.[65][66][67] 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.[65] 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 surfacecatalyst enabling acetylene to react with hydrogen), or flaws in the current models of material flow.[68] He noted that even a non-biological catalyst effective at 95 K would in itself be a startling discovery.[68]
A hypotheticalcell membrane termed an azotosome, able to function in liquidmethane in Titan conditions was computer-modelled in an article published in February 2015. Composed ofacrylonitrile, a small molecule containing carbon, hydrogen, and nitrogen, it is predicted to have stability and flexibility in liquid methane comparable to that of aphospholipid bilayer (the type of cell membrane possessed by all life on Earth) in liquid water.[69][70] An analysis of data obtained using the Atacama Large Millimeter / submillimeter Array (ALMA), completed in 2017, confirmed substantial amounts of acrylonitrile in Titan's atmosphere.[71][72] Later studies questioned whether acrylonitrile would be able to self-assemble into azotosomes.[73] However, in 2025 a new mechanism was proposed by scientists Christian Mayer and Conor Nixon to overcome the previous barriers to self-assembly of azotosomes in liquid methane, based on 'splashing' of a methane lake surface film by a hydrocarbon raindrop.[74][75]
Hydrogen fluoride (HF), like water, is a polar molecule, and due to its polarity it can dissolve many ionic compounds. Atatmospheric pressure, its melting point is 189.15 K (−84.00 °C), and its boiling point is 292.69 K (19.54 °C); the difference between the two is a little more than 100 K. HF also makes hydrogen bonds with its neighbour molecules, as do water and ammonia. It has been considered as a possible solvent for life by scientists such as Peter Sneath[76] and Carl Sagan.[56]
HF is dangerous to the systems of molecules that Earth-life is made of, but certain other organic compounds, such asparaffin waxes, are stable with it.[56] 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.[77]
However, hydrogen fluoride is cosmically rare, unlike water, ammonia, and methane.[78]
Hydrogen sulfide is the closestchemical analog to water,[79] but is less polar and is a weaker inorganic solvent.[80] Hydrogen sulfide is quite plentiful on Jupiter's moonIo and may be in liquid form a short distance below the surface; astrobiologistDirk Schulze-Makuch has suggested it as a possible solvent for life there.[81] On a planet with hydrogen sulfide oceans, the source of the hydrogen sulfide could come from volcanoes, in which case it could be mixed in with a bit ofhydrogen fluoride, which could help dissolve minerals. Hydrogen sulfide life might use a mixture of carbon monoxide and carbon dioxide as their carbon source. They might produce and live onsulfur monoxide, which is analogous to oxygen (O2). Hydrogen sulfide, like hydrogen cyanide and ammonia, suffers from the small temperature range where it is liquid, though that, like that of hydrogen cyanide and ammonia, increases with increasing pressure.
Silicon dioxide, also known as silica and quartz, is very abundant in the universe and has a large temperature range where it is liquid. However, its melting point is 1,600 to 1,725 °C (2,912 to 3,137 °F), so it would be impossible to make organic compounds in that temperature, because all of them would decompose. Silicates are similar to silicon dioxide and some have lower melting points than silica. Feinberg and Shapiro have suggested that molten silicate rock could serve as a liquid medium for organisms with a chemistry based on silicon, oxygen, and other elements such asaluminium.[82]
Other solvents sometimes proposed:
Sulfuric acid in liquid form is strongly polar. It remains liquid at higher temperatures than water, its liquid range being 10 °C to 337 °C at a pressure of 1 atm, although above 300 °C it slowly decomposes. Sulfuric acid is known to be abundant in theclouds of Venus, in the form ofaerosol droplets. In a biochemistry that used sulfuric acid as a solvent, thealkene 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.[50]
A proposal has been made that life on Mars may exist and be using a mixture of water andhydrogen peroxide as its solvent.[86]A 61.2% (by mass) mix of water and hydrogen peroxide has a freezing point of −56.5 °C and tends tosuper-cool rather than crystallize. It is alsohygroscopic, an advantage in a water-scarce environment.[87][88]
Supercritical carbon dioxide has been proposed as a candidate for alternative biochemistry due to its ability to selectively dissolve organic compounds and assist the functioning of enzymes and because "super-Earth"- or "super-Venus"-type planets with dense high-pressure atmospheres may be common.[83]
Physicists have noted that, although 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 Earth.[89][90]These studies indicate that blue plants would be unlikely; however yellow or red plants may be relatively common.[90]
Many Earth plants and animals undergo major biochemical changes during their life cycles as a response to changing environmental conditions, for example, by having aspore orhibernation state that can be sustained for years or even millennia between more active life stages.[91] 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,[91] 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.[92] 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.
Thegenetic code may have evolved during the transition from theRNA world to aprotein world.[93] Thealanine world hypothesis postulates that the evolution of the genetic code (the so-called GC phase[94]) started with only four basicamino acids:alanine,glycine,proline andornithine (nowarginine).[95] The evolution of the genetic code ended with 20proteinogenic amino acids. From a chemical point of view, most of them are Alanine-derivatives particularly suitable for the construction ofα-helices andβ-sheets – basicsecondary structural elements of modern proteins. Direct evidence of this is an experimental procedure inmolecular biology known asalanine scanning.
A hypotheticalproline world would create a possible alternative life with the genetic code based on the proline chemical scaffold as theprotein backbone. Similarly, aglycine world andornithine world are also conceivable, but nature has chosen none of them.[96] Evolution oflife with Proline, Glycine, or Ornithine as the basic structure for protein-likepolymers (foldamers) would lead to parallel biological worlds. They would have morphologically radically differentbody plans andgenetics from the living organisms of the knownbiosphere.[97]
In 2007, Vadim N. Tsytovich and colleagues proposed that lifelike behaviors could be exhibited by dust particles suspended in aplasma, under conditions that might exist in space.[98][99] Computer models showed that, when the dust became charged, the particles could self-organize into microscopic helical structures, and the authors offer "a rough sketch of a possible model of...helical grain structure reproduction".
In 2020, Luis A. Anchordoqu and Eugene M. Chudnovsky of theCity University of New York hypothesized that cosmic necklace-based life composed of magnetic monopoles connected bycosmic strings could evolve inside stars.[13] This would be achieved by a stretching of cosmic strings due to the star's intense gravity, thus allowing it to take on more complex forms and potentially form structures similar to the RNA and DNA structures found within carbon-based life. As such, it is theoretically possible that such beings could eventually become intelligent and construct a civilization using the power generated by the star's nuclear fusion. Because such use would use up part of the star's energy output, the luminosity would also fall. For this reason, it is thought that such life might exist inside stars observed to be cooling faster or dimmer than current cosmological models predict.
Frank Drake suggested in 1973 that intelligent life could inhabitneutron stars.[100] Physical models in 1973 implied that Drake's creatures would be microscopic.[100]
Scientists who have considered possible alternatives to carbon-water biochemistry include:
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