Biochemical hypotheses are hypotheses that scientists have accepted as scientifically feasible but have not yet been shown to exist. All known living species on Earth utilize carbon molecules to perform their essential structural and metabolic tasks. Water serves as a solvent, and DNA or RNA is used to define and govern their shape. However, it is feasible that the chemistry of life on other planets and moons is substantially different — for example, using other kinds of carbon compounds or compounds of another element, or substituting another solvent for water.
We may be able to develop solvents and energy sources on Titan that aren't found in our biosphere, allowing for the emergence of life forms that aren't related to anything on Earth.
Considering what we know about alien settings and the chemical behavior of various elements and compounds, scientists are debating whether or not life forms could be founded on "alternative" biochemistries. It's a hot topic in synthetic biology and science fiction, too.
As a possible replacement for carbon, silicon has been widely discussed. The periodic chart places silicon in the same family as carbon, and both elements are tetravalent. Hydrocarbon solvents like methane and ethane exist as liquids on Titan's surface. They could serve as an alternative to water because they are non-polar polar molecules like ammonia, which is also a polar molecule.
As a result of the Arecibo message (1974), we learned more about Earth's introductory chemistry.
Microbial life on Earth is thought to exist in an entirely different biochemical and molecular context from what is now understood. The shadow biosphere may go unreported because research into the microbial world focuses primarily on macro-organism biology.
Biochemistry with different chirality of its biomolecules is maybe the least unique alternative biochemistry out there. Most of Earth's amino acids and carbohydrates are of the L and D forms. It is feasible to make molecules with D amino acids or L sugars, but these molecules would be incompatible with organisms that use molecules of the opposite chirality. The decay of creatures with normal chirality is hypothesized to be the source of amino acids with an anti-chiral chirality, which can be found on Earth. According to Paul Davies, a theoretical physicist, some of these may result from "anti-chiral" life.
Whether or not such chemistry would be considered alien is debatable. Because molecules commonly found in one enantiomer in the vast majority of organisms are also frequently seen in another enantiomer in different (often basal) organisms, such as when comparing Archaea and other domain members, it is unclear whether an alternative stereochemistry is genuinely novel.
On our planet, every living thing has a carbon-based skeleton and mechanism. No one has come up with a theory that would allow scientists to use atoms other than carbon to construct the molecular structures necessary for life. Still, they have hypothesized about the advantages and disadvantages of doing so. Carl Sagan stated that it is impossible to know if a statement that applies to all life on Earth will also apply to all life in the universe. The phrase "carbon chauvinism" was used by Sagan to describe this belief. However, he remarked that carbon appears more chemically flexible and more common in the universe than other probable elements, such as silicon and germanium (palladium and titanium). The Viking Lander, the first U.S. mission to safely land an independent probe on Mars' surface in 1976, carried out experiments designed by Norman Horowitz to investigate whether life might exist on Mars. Based on its remarkable adaptability, the carbon atom, according to Horowitz, is the one element most likely to offer novel options for resolving the challenges of extraterrestrial life. Non-carbon living forms with self-replicating genetic information systems and the capacity to adapt and evolve were believed by him to be a remote possibility.
Because silicon shares many chemical properties with carbon and is in the same periodic table group, the carbon group, it has been widely explored as the basis for an alternative biological system. Like carbon, Silicon can produce molecules large enough to carry biological information.
The disadvantages of silicon as a replacement for carbon, on the other hand, are numerous. When it comes to forming chemical connections with different atoms, silicon cannot do so. However, this weakness makes silicon less prone to forming bonds with all kinds of contaminants than carbon does. Oxygen, nitrogen, phosphorus, sulfur, iron, magnesium, and zinc are some elements that form organic functional groups with carbon. There are only a handful types of other atoms that can interact with silicon. It has been described as "monotonous compared to the combinatorial cosmos of organic macromolecules" when it comes into contact with other elements. As a result of their greater mass and atomic radius, silicon atoms have a more difficult time establishing double bonds (the double-bonded carbon is part of the carbonyl group, a fundamental motif of carbon-based bio-organic chemistry).
In contrast to alkane hydrocarbons, silanes are very reactive with water, and long-chain silanes disintegrate upon contact with water. Direct silicon-to-silicon connections are unstable; silicone molecules, on the other hand, use polymers of silicon and oxygen atoms alternately to form their bonds. There has been speculation that silicone-based compounds would hold up better than similar hydrocarbons in sulfuric acid-rich environments, such as those found in some extraterrestrial locales.
There are 84 kinds of molecules based on carbon in the interstellar medium, compared to only eight types based on silicon. Among the eight compounds, four of them contain carbon as well. Carbon and silicon are around 10 to 1 in abundance in the universe. A greater diversity of complex carbon compounds in the galaxy may mean that silicon-based biologies have less of a basis for building, at least under the conditions on planets' surfaces. Earth's crust is 925 times richer in silicon than in carbon, even though terrestrial life is mostly carbon-based (the ratio of silicon to carbon in Earth's crust is around 925:1). This could be evidence that silicon is unsuitable for biochemistry on Earth-like planets since carbon is employed instead of silicon. This may be because silicon is less versatile than carbon in creating compounds. The compounds silicon makes are unstable, and silicon acts as a heat-blocking barrier.
Aside from diatom skeletons, biogenic silica is utilized by several organisms on Earth. According to a clay hypothesis by A. G. Cairns-Smith, silicate minerals in the water repeated their crystal structures and interacted with carbon compounds. They served as the progenitors of life-based carbon compounds.
Directed evolution has been used to introduce carbon-silicon bonds to biochemistry, which are not found in nature (artificial selection). Directed evolution was used to create a cytochrome c protein from Rhodothermus Marinus that catalyzes the production of new carbon-silicon bonds between hydrosilanes and diazo compounds. The protein contains heme.
It is possible that silicon compounds could be biologically beneficial at temperatures or pressures different from those found on the surface of a terrestrial planet. This suggests that polysilanols, the silicon compounds equivalent to sugars, could function in biochemistry at very low temperatures.
Other biochemistries based on unusual elements are also available (e.g., Organoboron chemistry)
Borane is dangerously explosive in Earth's atmosphere but would be more stable in a lower-oxygen, lower-temperature environment. In contrast to carbon, boron is much less likely to be a building block of life due to its low cosmic abundance.
Various metals with oxygen can build exceedingly complex and thermally stable structures that rival organic compounds; heteropoly acids are a family of chemicals. Some metal oxides, like carbon, can form nanotubes and diamond-like crystals, making them similar to carbon (such as cubic zirconia). The Earth's crust has more titanium, aluminum, magnesium, and iron than carbon. Even in extreme settings (such as high temperatures), metal-oxide-based life may exist. Tungsten polyoxometalates can self-assemble into cell-like spheres, according to the Cronin group at Glasgow University. To create a porous membrane, the metal oxide content of the spheres can be altered to create holes that allow specific chemicals to enter and exit the sphere based on their size.
Like phosphorus and silanes, sulfur can also form long-chain compounds but has the same high reactivity issues. Sulfur's potential in biological systems as a carbon substitute is speculative, partly because sulfur often only forms linear chains, not branched ones. Although molecular oxygen has been used in biological systems for over 3 billion years, sulfur's function as an electron acceptor is widespread and dates back 3.5 billion years. Sulfur-reducing bacteria can reduce sulfur to hydrogen sulfide by using elemental sulfur instead of oxygen.)
Arsenic, chemically identical to phosphorus, gets absorbed into the biochemistry of some organisms, although arsenic is toxic to most life forms on Earth. Arsenosugars and arsenobetaines are two examples of arsenic-containing organic compounds found in marine algae. Fungi and bacteria can produce volatile methylated arsenic compounds. Microbes have been reported to reduce arsenate and oxidize arsenite (Chrysiogenes arsenatis). Arsenate can be used as a terminal electron acceptor by some prokaryotes during anaerobic growth, whereas arsenite can be used as an electron donor by others.
Arsenic biochemistry has been suggested as a possible alternative to phosphorus in the DNA structure of the earliest life forms on Earth. Arsenic is unsuitable for this role because arsenate esters are less resilient to hydrolysis than their corresponding phosphate esters.
When cultivated without phosphorus, a bacterium dubbed GFAJ-1 found in the sediments of Mono Lake in eastern California may be able to use this "arsenic DNA," according to the authors of a 2010 geomicrobiology study funded in part by NASA. A bacterium may use large quantities of poly-hydroxybutyrate or other techniques to lower water concentration and stabilize its arsenate esters, according to the study authors. Almost soon after its publication, this assertion was roundly condemned for what critics saw as a lack of adequate controls. Carl Zimmer, a science writer, solicited opinions from many experts: "I contacted a dozen experts in the field... NASA's scientists have, almost universally, failed to present their case." Since no one else could duplicate the study's findings, it appears that the study was flawed due to high levels of phosphate contamination. GFAJ-1 cells may increase by reusing phosphate from degraded ribosomes rather than by replacing it with arsenate, according to another theory.
All known terrestrial life relies on water as a solvent in addition to carbon molecules. As a result, there have been debates over whether water is the sole liquid capable of playing that job. The scientist Steven Benner and the astrobiological committee directed by John A. Baross have taken seriously the concept that life on other planets could be based on a solvent other than water. In addition to the solvents mentioned earlier, those under consideration by the Baross committee include ammonia, sulfuric acid, formamide, hydrocarbons, and liquid nitrogen or hydrogen in a supercritical fluid (at temperatures far lower than those seen on Earth).
It has been suggested that Carl Sagan is at one time both carbon and at another, a water chauvinist; yet, he has also remarked that at the latter, he is carbon, but "not that much a water." As an alternative to drinking water, hydrocarbons, hydrofluoric acid, and ammonia were considered by him.
For example, some of the water's qualities that are critical to life processes are as follows:
However, not all of water's qualities are beneficial to life. When water ice melts in the Arctic, its high albedo reflects significant amounts of sunlight and heat to space. Reflective ice covers the ocean's surface during the ice ages, increasing the impact of global cooling.
In a healthy biosphere, some chemicals and elements are more suited to serving as solvents due to their unique qualities. This means that the solvent must be able to maintain liquid equilibrium throughout a wide range of temperatures. Rather than focusing on whether or not the solvent will remain liquid, the question is more about what pressure the solvent will remain liquid at. The liquid phase temperature range of hydrogen cyanide, for example, is restricted to 1 atmosphere. Still, it can persist in liquid form over a wide temperature range in an atmosphere with the pressure of Venus, with 92 bars (91 atm) of pressure.
Hydrogen and nitrogen are two of the most prevalent elements in the universe, and the ammonia molecule, like the water molecule, combines these two elements. At a symposium on the origin of life, J. B. S. Haldane highlighted the possibility of liquid ammonia serving as an alternate solvent for life.
In an ammonia solution, numerous chemical reactions can occur, and liquid ammonia shares many chemical properties with water. It can dissolve numerous organic molecules as well as water, and it also can dissolve a wide range of inorganic and metallic elements. Ammonia-related amine groups (NH2) and water-related hydroxyl groups (OH) are comparable in Haldane's view, as are many other water-related organic molecules.
H+ ions can be exchanged between ammonia and water. Ammonia generates the ammonium cation (NH4+) when it receives an H+, equivalent to hydronium (H3O+). In this case, the amide anion (NH2) is formed, comparable to the hydroxide anion (OH). While water prefers to donate an H+ ion, ammonia prefers to take one; it is a stronger nucleophile. The addition of ammonia to water acts as an Arrhenius base, increasing the anion hydroxide concentration. When water is added to liquid ammonia, it acts as an acid because it raises the concentration of the cation ammonium, defined as basicity. Although it is common in terrestrial biochemistry to employ the carbonyl group (C=O), the equivalent imine group (C=NH) can be used instead because it is ammonia solution stable.
There are, however, significant drawbacks to using ammonia as a foundation for life. Ammonia's heat of vaporization is half that of water, its surface tension is a third lower, and its capacity to concentrate non-polar molecules through a hydrophobic effect is reduced because of weaker hydrogen bonds between ammonia molecules. Whether ammonia might hold primordial molecules together effectively enough to allow for the birth of a self-reproducing system has been debated by Gerald Feinberg and Robert Shapiro. Ammonia is also explosive when exposed to oxygen, and therefore it could not persist in a setting conducive to aerobic metabolism on a long-term basis.
Ammonia-based biospheres are expected to exist at temperatures and pressures much above the range of life's normal operating conditions on Earth. Water's melting and boiling points, at a pressure called "normal pressure," and a temperature range of 0°C (273 K) to 100°C are typical operating ranges for life on Earth (373 K). Assuming normal pressure is maintained, ammonia's melting and boiling temperatures are 78°C (195 K) and 33°C (240 K). Chemical processes take longer to complete at lower temperatures, and therefore ammonia-based life could evolve more slowly than life on Earth under these conditions. Life systems could use chemical compounds that would be too unstable at Earth's temperatures if temperatures were lower.
Ammonia can also be a liquid at Earth-like temperatures if it's under a lot more pressure than it usually would be. For instance, ammonia melts at 77 °C (196 K) and boils at 98 °C when compressed to 60 atm (bar) (371 K).
Even at temperatures considerably below the freezing point of pure water, ammonia and ammonia–water mixes are still liquid, making them potentially suitable for planets and moons outside the water-based habitability zone. For example, Titan, Saturn's biggest moon, might have such conditions.
As a hydrocarbon, methane (CH4) is made up of hydrogen and carbon, the two most abundant elements in the universe and as abundant in the universe as ammonia. Hydrocarbons can be used as a solvent throughout an extensive temperature range. However, they lack polarity. In 1981, Isaac Asimov, a biologist and science fiction writer, proposed that poly-lipids may be used as a protein substitute in a non-polar solvent like methane. The Cassini spacecraft has discovered lakes of hydrocarbons, including methane and ethane, on the surface of Titan.
Methane and other hydrocarbons have been likened to water and ammonia as a solvent for life. In cells, water is a better solvent than hydrocarbons, facilitating the movement of chemicals. Hydrolysis, on the other hand, is a chemical reaction that breaks down big organic molecules. The biomolecules of a living organism whose solvent was a hydrocarbon would not be in danger of being destroyed in this manner. Internal hydrogen bonds in organic molecules can be disrupted by the water molecule's propensity to create strong hydrogen bonds. If life existed in a hydrocarbon solvent, its biomolecules could make better use of hydrogen bonding. Low-temperature biochemistry would benefit from the strong hydrogen bonds in biomolecules.
Astrobiologist Chris McKay has proposed on thermodynamic grounds that if life does exist on Titan's surface, it is likely to exploit the more complex hydrocarbons as an energy source by reacting them with hydrogen resulting in the reduction of the more volatile hydrocarbons to methane. When Johns Hopkins University's Darrell Strobel discovered in 2010 a higher concentration of molecular hydrogen in Titan's upper atmospheric layers than in its lower ones, he hypothesized that the molecular hydrogen would have diffused down at a rate of about 1025 molecules per second and then vanished altogether near Titan's surface. Chris McKay's predictions of the effects of the presence of methanogenic life-forms, as observed by Strobel, were confirmed by Strobel's observations. Chris McKay's interpretation of acetylene levels on Titan's surface as consistent with the notion of creatures converting acetylene to methane was supported by another investigation in the same year. Aside from the biological idea, McKay noted other options, such as unexplained physical or chemical processes (e.g., a non-living surface catalyst enabling acetylene to react with hydrogen), or errors in the present material flow models, are more possible. According to him, a non-biological catalyst capable of operating at 95 K would be a groundbreaking finding.
A computer model of an azotosome, a hypothetical cell membrane capable of functioning in liquid methane under Titan conditions, was published in February 2015. An acrylonitrile bilayer (a phospholipid bilayer is the sort of cell membrane found in all life on Earth) is projected to have the same stability and flexibility in liquid methane as a phospholipid bilayer in liquid water. Acrylonitrile was found in significant quantities in Titan's atmosphere by analyzing data from the Atacama Large Millimeter / submillimeter Array (ALMA), completed in 2017. It was later debated whether acrylonitrile could form azotozomes on its own.
Due to its similarity to water, hydrogen fluoride (HF) can be used to dissolve a wide range of ionic compounds. Temperature differences of more than 100 K separate its melting point (84°C) from its boiling point (19.54°C) (both at atmospheric pressure). As with water and ammonia, hydrogen bonds can be formed between HF and its neighbors. Several scientists, including Peter Sneath and Carl Sagan, have speculated that it could be a solvent for life.
HF threatens Earth's molecular systems, while other organic substances, such as paraffin waxes, can withstand its presence. Liquid hydrogen fluoride, like water and ammonia, has acid-base chemistry. When liquid HF is mixed with nitric acid, it acts as a base according to a solvent system's definition of acidity and basicity.
Fluoride hydrogen is rare compared to other common gases like water, nitric acid, and methane, but it exists.
Unlike water, hydrogen sulfide is less polar and less effective as an inorganic solvent. The astrobiologist Dirk Schulze-Makuch hypothesizes that hydrogen sulfide, which is abundant on Jupiter's moon Io, could serve as a solvent for life on the moon. The hydrogen sulfide oceans on a planet with volcanoes may be mixed up with a little amount of hydrogen fluoride, which could aid in the dissolution of minerals. Carbon monoxide and carbon dioxide may be used as a carbon source by hydrogen sulfide life. Because sulfur monoxide is comparable to oxygen, they may manufacture and use it as a source of energy (O2). With increasing pressure, the temperature range where hydrogen sulfide is liquid expands, just like that of hydrogen ammonia and cyanide.
Silica dioxide, often known as silica and quartz, is one of the most numerous substances in the universe and has a wide temperature range where it can be dissolved. Organic compounds cannot be made at that temperature since the melting point is between 1,600 and 1,725 °C (2,912 and 3,137 °F). Some silicates have lower melting points than silica, making them a good substitute for silica. Regarding organisms that use silicon, oxygen, and other elements like aluminum, Feinberg and Shapiro have proposed the possibility of liquid silicate rock.
Other solvents have also been suggested, such as:
According to a new theory, life on Mars may be using a mixture of water and hydrogen peroxide as a solvent. When water and hydrogen peroxide are mixed at a 61.2 percent (by mass) ratio, it freezes at 56.5 °C rather than crystallizing. Water-starved environments benefit from their ability to absorb moisture.
The capacity of supercritical carbon dioxide to selectively dissolve organic compounds and let enzymes function has been presented as an option for alternative biochemistry because planets of the "super-Earth" or "super-Venus" type with dense high-pressure atmospheres may be frequent.
There has been some debate among physicists as to whether plants of various hues could support photosynthesis, which is necessary for most life on Earth and may even be favored in regions with a different mix of star radiation than Earth. There is little evidence to support the existence of blue plants, but there is some evidence that yellow or red plants are more prevalent.
Because of the long-term or even millennia-long persistence of spore or hibernation states, many Earth-dwelling plants and animals experience significant metabolic changes throughout their life cycles in response to shifting environmental conditions. Hence, biochemically, even in situations that are only occasionally compatible with life as we know it, it is conceivable to sustain life.
While frogs can survive for long periods in freezing temperatures, desert frogs in Australia can become inactive and dehydrated in dry seasons, losing up to 75% of their fluids. Still, they can quickly rehydrate in wet periods when they can do so. The latest state of any species would make it appear as if it were not alive to anyone who didn't have a sensitive way of detecting low metabolism levels.
The genetic code may have evolved in the shift from RNA to protein. For the Alanine World Hypothesis, only four amino acids were necessary to begin the evolution of the genetic code: alanine, glycine, proline, and ornithine (now arginine). With the addition of 20 protein-producing amino acids, genetic code evolution ended. Alanine-derivatives, which can be used to build helices and sheets, are most commonly found in current proteins and are particularly useful for this purpose. Alanine scanning, a molecular biology experiment, provides concrete evidence of this.
As the protein backbone, a "Proline World" might develop an alternate life form with a genetic code based on proline's molecular architecture. A "Glycine World" and an "Ornithine World" are both possible, but nature hasn't decided which one to use. Foldamers, which are protein-like polymers, might lead to parallel biological universes if Proline, Glycine, or Ornithine were used as the building block for their formation. They would have morphologically significantly different body designs and DNA from the living species of the known biosphere.
In 2007, Vadim N. Tsytovich and colleagues postulated that dust particles trapped in plasma might exhibit lifelike behaviors under conditions that may exist in space. According to computer models, the particles in dust could self-organize into minuscule helical structures, and the authors present "a preliminary sketch of a hypothetical model of...helical grain structure reproduction. "
In 2020, researchers at the City University of New York proposed that magnetic semipoles linked by cosmic strings could form a necklace-like life inside stars.
Frank Drake's theory suggests intelligent life might exist in neutron stars as early as 1973, and Drake's creatures appeared to be minuscule in physical mockups from 1973. Robert L Forward used Drake's idea for Dragon's Egg as a thesis when he authored the science fiction novel in 1980.