Hypothetical types of biochemistry are forms of biochemistry agreed to be scientifically viable but not proven to exist at this time. [2] The kinds of living organisms currently known on Earth all use carbon compounds for basic structural and metabolic functions, water as a solvent, and DNA or RNA to define and control their form. If life exists on other planets or 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, or another solvent in place of water.
The possibility of life-forms being based on "alternative" biochemistries is the topic of an ongoing scientific discussion, informed by what is known about extraterrestrial environments and about the chemical behaviour of various elements and compounds. It is of interest in synthetic biology and is also a common subject in science fiction.
The element silicon has been much discussed as a hypothetical alternative to carbon. Silicon is in the same group as carbon on the periodic table and, like carbon, it is tetravalent. Hypothetical alternatives to water include ammonia, which, like water, is a polar molecule, and cosmically abundant; and non-polar hydrocarbon solvents such as methane and ethane, which are known to exist in liquid form on the surface of Titan.
Type | Basis | Brief description | Remarks |
---|---|---|---|
Alternative-chirality biomolecules | Alternative biochemistry | Mirror image biochemistry | 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 using D amino acids or L sugars are possible, though they would be incompatible with organisms using the opposing chirality molecules. Gram-positive bacteria incorporate D-alanine into their peptidoglycan layer, created through the actions of racemases. [4] |
Ammonia biochemistry | Non-water solvents | Ammonia-based life | Ammonia is relatively abundant in the universe and has chemical similarities to water. The possible role of liquid ammonia as an alternative solvent for life is an idea that goes back at least to 1954, when J. B. S. Haldane raised the topic at a symposium about life's origin. |
Arsenic biochemistry | Alternative biochemistry | Arsenic-based life | Arsenic, which is chemically similar to phosphorus, while poisonous for most life forms on Earth, is incorporated into the biochemistry of some organisms. |
Borane biochemistry (Organoboron chemistry) | Alternative biochemistry | Boranes-based life | Boranes are dangerously explosive in Earth's atmosphere, but would be more stable in a reducing environment. Boron, however, is exceedingly rare in the universe in comparison to its neighbours carbon, nitrogen, and oxygen. On the other hand, structures containing alternating boron and nitrogen atoms share some properties with hydrocarbons. |
Cosmic necklace-based biology | Nonplanetary life | Non-chemical life | In 2020, Luis A. Anchordoqu and Eugene M. Chudnovsky hypothesized that life composed of magnetic semipoles connected by cosmic strings could evolve inside stars. [5] |
Dusty plasma-based biology | Nonplanetary life | Non-chemical life | In 2007, Vadim 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. [6] |
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. |
Heteropoly acid biochemistry | Alternative biochemistry | Heteropoly acid-based life | Various metals can form complex structures with oxygen, such as heteropoly acids. |
Hydrogen fluoride biochemistry | Non-water solvents | Hydrogen fluoride-based life | Hydrogen fluoride has been considered as a possible solvent for life by scientists such as Peter Sneath. |
Hydrogen sulfide biochemistry | Non-water solvents | Hydrogen sulfide-based life | Hydrogen sulfide is a chemical analog of water, but is less polar and a weaker inorganic solvent. |
Methane biochemistry (Azotosome) | Non-water solvents | Methane-based life | Methane (CH4) is relatively abundant in the solar system and the universe, and is known to exist in liquid form on Titan, the largest moon of Saturn. Though highly unlikely, it is considered to be possible for Titan to harbor life. If so, it will most likely be methane-based life. |
Non-green photosynthesizers | Other speculations | Alternate plant life | Physicists have noted that, although photosynthesis on Earth generally involves green plants, a variety of other-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. In particular, retinal is capable of, and has been observed to, perform photosynthesis. [7] Bacteria capable of photosynthesis are known as microbial rhodopsins. A plant or creature that uses retinal photosynthesis is always purple. |
Shadow biosphere | Alternative environment | A hidden life biosphere on Earth | A shadow biosphere is a hypothetical microbial biosphere of Earth that uses radically different biochemical and molecular processes than currently known life. |
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. |
Silicon dioxide biochemistry | Non-water solvents | Silicon dioxide-based life | Gerald Feinberg and Robert Shapiro have suggested that molten silicate rock could serve as a liquid medium for organisms with a chemistry based on silicon, oxygen, and other elements such as aluminium. |
Sulfur biochemistry | Alternative biochemistry | Sulfur-based life | The biological use of sulfur as an alternative to carbon is purely hypothetical, especially because sulfur usually forms only linear chains rather than branched ones. |
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 TNA, which uses threose, HNA, which uses 1,5-anhydrohexitol, GNA, which uses glycol, CeNA, which uses cyclohexene, LNA, which utilizes a form of ribose that contains an extra linkage between its 4' carbon and 2' oxygen, FANA, which uses arabinose but with a single fluorine atom attached to its 2' carbon, and PNA, which uses, in place of sugar and phosphate, N-(2-aminoethyl)-glycine units connected by peptide bonds. [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] |
A shadow biosphere is a hypothetical microbial biosphere of Earth that uses radically different biochemical and molecular processes than currently known life. [11] [12] 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 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 using 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. Amino acids whose chirality is opposite to the norm are found on Earth, and these substances are generally thought to result from decay of organisms of normal chirality. However, physicist Paul Davies speculates that some of them might be products of "anti-chiral" life. [13]
It is questionable, however, whether such a biochemistry would be truly alien. Although it would certainly be an alternative stereochemistry, molecules that are overwhelmingly found in one enantiomer throughout the vast majority of organisms can nonetheless often be found in another enantiomer in different (often basal) organisms such as in comparisons between members of Archaea and other domains,[ 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 pros and cons of using elements other than carbon to form the molecular structures necessary for life, but no one has proposed a theory employing such atoms to form all the necessary structures. However, as Carl Sagan argued, it is very difficult to be certain whether a statement that applies to all life on Earth will turn out to apply to all life throughout the universe. [14] Sagan used the term "carbon chauvinism" for such an assumption. [15] He regarded silicon and germanium as conceivable alternatives to carbon [15] (other plausible elements include but are not limited to palladium and titanium); but, on the other hand, he noted that carbon does seem more chemically versatile and is more abundant in the cosmos. [16] Norman Horowitz devised the experiments to determine whether life might exist on Mars that were carried out by the Viking Lander of 1976, the first U.S. mission to successfully land a probe on the surface of Mars. Horowitz argued that the great versatility of the carbon atom makes it the element most likely to provide solutions, even exotic solutions, to the problems of survival on other planets. [17] 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, because silicon has many chemical similarities to carbon and is in the same group of the periodic table. Like carbon, silicon can create molecules that are sufficiently large to carry biological information. [18]
However, silicon has several drawbacks as a carbon alternative. Carbon is ten times more cosmically abundant than silicon, and its chemistry appears naturally more complex. [19] By 1998, astronomers had identified 84 carbon-containing molecules in the interstellar medium, but only 8 containing silicon, of which half also included carbon. [20] Even though Earth and other terrestrial planets are exceptionally silicon-rich and carbon-poor (silicon is roughly 925 times more abundant in Earth's crust than carbon), terrestrial life bases itself on carbon. It may eschew silicon because silicon compounds are less varied, unstable in the presence of water, or block the flow of heat. [19]
Relative to carbon, silicon has a much larger atomic radius, and forms much weaker covalent bonds to atoms — except oxygen and fluorine, with which it forms very strong bonds. [18] Almost no multiple bonds to silicon are stable, although silicon does exhibit varied coordination number. [21] Silanes, silicon analogues to the alkanes, react rapidly with water, and long-chain silanes spontaneously decompose. [22] Consequently, most terrestrial silicon is "locked up" in silica, and not a wide variety of biogenic precursors. [21]
Silicones, which alternate between silicon and oxygen atoms, are much more stable than silanes, and may even be more stable than the equivalent hydrocarbons in sulfuric acid-rich extraterrestrial environments. [22] Alternatively, the weak bonds in silicon compounds may help maintain a rapid pace of life at cryogenic temperatures. Polysilanols, the silicon homologues to sugars, are among the few compounds soluble in liquid nitrogen. [23] [ unreliable source? ] [21]
All known silicon macromolecules are artificial polymers, and so "monotonous compared with the combinatorial universe of organic macromolecules". [18] [21] Even so, some Earth life uses biogenic silica: diatoms' silicate skeletons. A. G. Cairns-Smith hypothesized that silicate minerals in water played a crucial role in abiogenesis, in that biogenic carbon compounds formed around their crystal structures. [24] [25] Although not observed in nature, carbon–silicon bonds have been added to biochemistry under directed evolution (artificial selection): a cytochrome c protein from Rhodothermus marinus has been engineered to catalyze new carbon–silicon bonds between hydrosilanes and diazo compounds. [26]
Arsenic, which is chemically similar to phosphorus, while poisonous for most life forms on Earth, is incorporated into the biochemistry of some organisms. [29] 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 ). [30] 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 biochemistry in place of phosphorus in the structure of their DNA. [31] A common objection to this scenario is that arsenate esters are so much less stable to hydrolysis than corresponding phosphate esters that arsenic is poorly suited for this function. [32]
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. [33] [34] 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. [34] This claim was heavily criticized almost immediately after publication for the perceived lack of appropriate controls. [35] [36] 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". [37] 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. [38] Alternatively, it was suggested that GFAJ-1 cells grow by recycling phosphate from degraded ribosomes, rather than by replacing it with arsenate. [39]
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, [40] and by the astrobiological committee chaired by John A. Baross. [41] Solvents discussed by the Baross committee include ammonia, [42] sulfuric acid, [43] formamide, [44] hydrocarbons, [44] and (at temperatures much lower than Earth's) liquid nitrogen, or hydrogen in the form of a supercritical fluid. [45]
Water as a solvent limits the forms biochemistry can take. For example, Steven Benner, proposes the polyelectrolyte theory of the gene that claims that for a genetic biopolymer such as, DNA, to function in water, it requires repeated ionic charges. [46] If water is not required for life, these limits on genetic biopolymers are removed.
Carl Sagan once described himself as both a carbon chauvinist and a water chauvinist; [47] however, on another occasion he said that he was a carbon chauvinist but "not that much of a water chauvinist". [48] He speculated on hydrocarbons, [48] : 11 hydrofluoric acid, [49] and ammonia [48] [49] 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 (where geysers have been observed), Europa, Titan, and Ganymede. 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. [50] For instance, water ice has a high albedo, [50] meaning that it reflects a significant quantity 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. [50]
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 (91 atm) 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. [51] 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. [52]
Numerous chemical reactions are possible in an ammonia solution, and liquid ammonia has chemical similarities with water. [51] [53] 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 hydroxyl group (−OH). [53]
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−). [42] Compared to water, however, ammonia is more inclined to accept an H+ ion, and less inclined to donate one; it is a stronger nucleophile. [42] 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. [53] 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=NH) could be used instead. [42]
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. [54] Ammonia is also flammable in oxygen and could not exist sustainably in an environment suitable for aerobic metabolism. [55]
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 between the melting point and boiling point of water, at a pressure designated as normal 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. [55] 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. [51]
A set of conditions where ammonia is liquid at Earth-like temperatures would involve it being at a much higher pressure. For example, at 60 atm ammonia melts at −77 °C (196 K) and boils at 98 °C (371 K). [42]
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. [56]
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. [51] 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. [51] Lakes composed of a mixture of hydrocarbons, including methane and ethane, have been detected on the surface of Titan by the Cassini spacecraft.
There is debate about the effectiveness of methane and other hydrocarbons as a solvent for life compared to water or ammonia. [57] [58] [59] Water is a stronger solvent than the hydrocarbons, enabling easier transport of substances in a cell. [60] However, water is also more chemically reactive and can break down large organic molecules through hydrolysis. [57] A life-form whose solvent was a hydrocarbon would not face the threat of its biomolecules being destroyed in this way. [57] Also, the water molecule's tendency to form strong hydrogen bonds can interfere with internal hydrogen bonding in complex organic molecules. [50] Life with a hydrocarbon solvent could make more use of hydrogen bonds within its biomolecules. [57] Moreover, the strength of hydrogen bonds within biomolecules would be appropriate to a low-temperature biochemistry. [57]
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. [61] 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. [60] [61] [62] 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. [60] 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. [63] He noted that even a non-biological catalyst effective at 95 K would in itself be a startling discovery. [63]
A hypothetical cell membrane termed an azotosome, capable of functioning in liquid methane in Titan conditions was computer-modeled in an article published in February 2015. Composed of acrylonitrile, a small molecule containing carbon, hydrogen, and nitrogen, it is predicted to have stability and flexibility in liquid methane comparable to that of a phospholipid bilayer (the type of cell membrane possessed by all life on Earth) in liquid water. [64] [65] 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. [66] [67] Later studies questioned whether acrylonitrile would be able to self-assemble into azotozomes. [68]
Hydrogen fluoride (HF), like water, is a polar molecule, and due to its polarity it can dissolve many ionic compounds. At atmospheric 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 neighbor molecules, as do water and ammonia. It has been considered as a possible solvent for life by scientists such as Peter Sneath [69] and Carl Sagan. [49]
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. [49] 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. [70]
However, hydrogen fluoride is cosmically rare, unlike water, ammonia, and methane. [71]
Hydrogen sulfide is the closest chemical analog to water, [72] but is less polar and is a weaker inorganic solvent. [73] Hydrogen sulfide is quite plentiful on Jupiter's moon Io and may be in liquid form a short distance below the surface; astrobiologist Dirk Schulze-Makuch has suggested it as a possible solvent for life there. [74] 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 of hydrogen 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 on sulfur 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 as aluminium. [75]
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 the clouds of Venus, in the form of 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. [43]
A proposal has been made that life on Mars may exist and be using a mixture of water and hydrogen peroxide as its solvent. [79] A 61.2% (by mass) mix of water and hydrogen peroxide has a freezing point of −56.5 °C and tends to super-cool rather than crystallize. It is also hygroscopic, an advantage in a water-scarce environment. [80] [81]
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. [76]
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. [82] [83] These studies indicate that blue plants would be unlikely; however yellow or red plants may be relatively common. [83]
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. [84] 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, [84] 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. [85] 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.
The genetic code may have evolved during the transition from the RNA world to a protein world. [86] The Alanine World Hypothesis postulates that the evolution of the genetic code (the so-called GC phase [87] ) started with only four basic amino acids: alanine, glycine, proline and ornithine (now arginine). [88] The evolution of the genetic code ended with 20 proteinogenic amino acids. From a chemical point of view, most of them are Alanine-derivatives particularly suitable for the construction of α-helices and β-sheets – basic secondary structural elements of modern proteins. Direct evidence of this is an experimental procedure in molecular biology known as alanine scanning.
A hypothetical "Proline World" would create a possible alternative life with the genetic code based on the proline chemical scaffold as the protein backbone. Similarly, a "Glycine World" and "Ornithine World" are also conceivable, but nature has chosen none of them. [89] Evolution of life with Proline, Glycine, or Ornithine as the basic structure for protein-like polymers (foldamers) would lead to parallel biological worlds. They would have morphologically radically different body plans and genetics from the living organisms of the known biosphere. [90]
In 2007, Vadim 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. [91] [92] 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 the City University of New York hypothesized that cosmic necklace-based life composed of magnetic monopoles connected by cosmic strings could evolve inside stars. [5] 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 inhabit neutron stars. [93] Physical models in 1973 implied that Drake's creatures would be microscopic.[ citation needed ]
Scientists who have considered possible alternatives to carbon-water biochemistry include:
Astrobiology, is a scientific field within the life and environmental sciences that studies the origins, early evolution, distribution, and future of life in the universe by investigating its deterministic conditions and contingent events. As a discipline, astrobiology is founded on the premise that life may exist beyond Earth.
Extraterrestrial life, alien life, or colloquially aliens, is life which does not originate from Earth. No extraterrestrial life has yet been conclusively detected. Such life might range from simple forms such as prokaryotes to intelligent beings, possibly bringing forth civilizations that might be far more advanced than humans. The Drake equation speculates about the existence of sapient life elsewhere in the universe. The science of extraterrestrial life is known as astrobiology.
The Miller–Urey experiment (or Miller experiment) was an experiment in chemical synthesis carried out in 1952 that simulated the conditions thought at the time to be present in the atmosphere of the early, prebiotic Earth. It is seen as one of the first successful experiments demonstrating the synthesis of organic compounds from inorganic constituents in an origin of life scenario. The experiment used methane (CH4), ammonia (NH3), hydrogen (H2), in ratio 2:2:1, and water (H2O). Applying an electric arc (the latter simulating lightning) resulted in the production of amino acids.
Nitrogen is a chemical element; it has symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at seventh in total abundance in the Milky Way and the Solar System. At standard temperature and pressure, two atoms of the element bond to form N2, a colorless and odorless diatomic gas. N2 forms about 78% of Earth's atmosphere, making it the most abundant chemical species in air. Because of the volatility of nitrogen compounds, nitrogen is relatively rare in the solid parts of the Earth.
Terraforming or terraformation ("Earth-shaping") is the hypothetical process of deliberately modifying the atmosphere, temperature, surface topography or ecology of a planet, moon, or other body to be similar to the environment of Earth to make it habitable for humans to live on.
Tholins are a wide variety of organic compounds formed by solar ultraviolet or cosmic ray irradiation of simple carbon-containing compounds such as carbon dioxide, methane or ethane, often in combination with nitrogen or water. Tholins are disordered polymer-like materials made of repeating chains of linked subunits and complex combinations of functional groups, typically nitriles and hydrocarbons, and their degraded forms such as amines and phenyls. Tholins do not form naturally on modern-day Earth, but they are found in great abundance on the surfaces of icy bodies in the outer Solar System, and as reddish aerosols in the atmospheres of outer Solar System planets and moons.
In chemistry, catenation is the bonding of atoms of the same element into a series, called a chain. A chain or a ring shape may be open if its ends are not bonded to each other, or closed if they are bonded in a ring. The words to catenate and catenation reflect the Latin root catena, "chain".
Formamide is an amide derived from formic acid. It is a colorless liquid which is miscible with water and has an ammonia-like odor. It is chemical feedstock for the manufacture of sulfa drugs and other pharmaceuticals, herbicides and pesticides, and in the manufacture of hydrocyanic acid. It has been used as a softener for paper and fiber. It is a solvent for many ionic compounds. It has also been used as a solvent for resins and plasticizers. Some astrobiologists suggest that it may be an alternative to water as the main solvent in other forms of life.
Planetary habitability is the measure of a planet's or a natural satellite's potential to develop and maintain environments hospitable to life. Life may be generated directly on a planet or satellite endogenously or be transferred to it from another body, through a hypothetical process known as panspermia. Environments do not need to contain life to be considered habitable nor are accepted habitable zones (HZ) the only areas in which life might arise.
An inorganic nonaqueous solvent is a solvent other than water, that is not an organic compound. These solvents are used in chemical research and industry for reactions that cannot occur in aqueous solutions or require a special environment. Inorganic nonaqueous solvents can be classified into two groups, protic solvents and aprotic solvents. Early studies on inorganic nonaqueous solvents evaluated ammonia, hydrogen fluoride, sulfuric acid, as well as more specialized solvents, hydrazine, and selenium oxychloride.
Carbon is a primary component of all known life on Earth, and represents approximately 45–50% of all dry biomass. Carbon compounds occur naturally in great abundance on Earth. Complex biological molecules consist of carbon atoms bonded with other elements, especially oxygen and hydrogen and frequently also nitrogen, phosphorus, and sulfur.
The PAH world hypothesis is a speculative hypothesis that proposes that polycyclic aromatic hydrocarbons (PAHs), known to be abundant in the universe, including in comets, and assumed to be abundant in the primordial soup of the early Earth, played a major role in the origin of life by mediating the synthesis of RNA molecules, leading into the RNA world. However, as yet, the hypothesis is untested.
The chemical element nitrogen is one of the most abundant elements in the universe and can form many compounds. It can take several oxidation states; but the most common oxidation states are -3 and +3. Nitrogen can form nitride and nitrate ions. It also forms a part of nitric acid and nitrate salts. Nitrogen compounds also have an important role in organic chemistry, as nitrogen is part of proteins, amino acids and adenosine triphosphate.
Whether there is life on Titan, the largest moon of Saturn, is currently an open question and a topic of scientific assessment and research. Titan is far colder than Earth, but of all the places in the Solar System, Titan is the only place besides Earth known to have liquids in the form of rivers, lakes, and seas on its surface. Its thick atmosphere is chemically active and rich in carbon compounds. On the surface there are small and large bodies of both liquid methane and ethane, and it is likely that there is a layer of liquid water under its ice shell. Some scientists speculate that these liquid mixes may provide prebiotic chemistry for living cells different from those on Earth.
Abiogenesis is the natural process by which life arises from non-living matter, such as simple organic compounds. The prevailing scientific hypothesis is that the transition from non-living to living entities on Earth was not a single event, but a process of increasing complexity involving the formation of a habitable planet, the prebiotic synthesis of organic molecules, molecular self-replication, self-assembly, autocatalysis, and the emergence of cell membranes. The transition from non-life to life has never been observed experimentally, but many proposals have been made for different stages of the process.
Water is a polar inorganic compound that is at room temperature a tasteless and odorless liquid, which is nearly colorless apart from an inherent hint of blue. It is by far the most studied chemical compound and is described as the "universal solvent" and the "solvent of life". It is the most abundant substance on the surface of Earth and the only common substance to exist as a solid, liquid, and gas on Earth's surface. It is also the third most abundant molecule in the universe.
Planetary oceanography, also called astro-oceanography or exo-oceanography, is the study of oceans on planets and moons other than Earth. Unlike other planetary sciences like astrobiology, astrochemistry, and planetary geology, it only began after the discovery of underground oceans in Saturn's moon Titan and Jupiter's moon Europa. This field remains speculative until further missions reach the oceans beneath the rock or ice layer of the moons. There are many theories about oceans or even ocean worlds of celestial bodies in the Solar System, from oceans made of diamond in Neptune to a gigantic ocean of liquid hydrogen that may exist underneath Jupiter's surface.
Chemical cycling describes systems of repeated circulation of chemicals between other compounds, states and materials, and back to their original state, that occurs in space, and on many objects in space including the Earth. Active chemical cycling is known to occur in stars, many planets and natural satellites.
Formamide-based prebiotic chemistry is a reconstruction of the beginnings of life on Earth, assuming that formamide could accumulate in sufficiently high amounts to serve as the building block and reaction medium for the synthesis of the first biogenic molecules.
The geochemistry of carbon is the study of the transformations involving the element carbon within the systems of the Earth. To a large extent this study is organic geochemistry, but it also includes the very important carbon dioxide. Carbon is transformed by life, and moves between the major phases of the Earth, including the water bodies, atmosphere, and the rocky parts. Carbon is important in the formation of organic mineral deposits, such as coal, petroleum or natural gas. Most carbon is cycled through the atmosphere into living organisms and then respirated back into the atmosphere. However an important part of the carbon cycle involves the trapping of living matter into sediments. The carbon then becomes part of a sedimentary rock when lithification happens. Human technology or natural processes such as weathering, or underground life or water can return the carbon from sedimentary rocks to the atmosphere. From that point it can be transformed in the rock cycle into metamorphic rocks, or melted into igneous rocks. Carbon can return to the surface of the Earth by volcanoes or via uplift in tectonic processes. Carbon is returned to the atmosphere via volcanic gases. Carbon undergoes transformation in the mantle under pressure to diamond and other minerals, and also exists in the Earth's outer core in solution with iron, and may also be present in the inner core.
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