Editing Extraterrestrial life (section)

==Biochemical basis==
{{main|Hypothetical types of biochemistry}}
{{see also|Water#Effects on life}}
If extraterrestrial life exists, it could range from simple [[microorganism]]s and [[multicellular organism]]s similar to animals or plants, to complex [[alien intelligence]]s akin to [[human]]s. When scientists talk about extraterrestrial life, they consider all those types. Although it is possible that extraterrestrial life may have other configurations, scientists use the hierarchy of lifeforms from Earth for simplicity, as it is the only one known to exist.<ref>Bennett, p. 3</ref>

The first basic requirement for life is an environment with [[non-equilibrium thermodynamics]], which means that the [[thermodynamic equilibrium]] must be broken by a source of energy. The traditional sources of energy in the cosmos are the stars, such as for life on Earth, which depends on the energy of the sun. However, there are other alternative energy sources, such as [[volcano|volcanoe]]s, [[plate tectonics]], and [[hydrothermal vent]]s. There are ecosystems on Earth in deep areas of the ocean that do not receive sunlight, and take energy from [[black smoker]]s instead.<ref>Aguilera Mochón, p. 42</ref> [[Magnetic field]]s and [[radioactivity]] have also been proposed as sources of energy, although they would be less efficient ones.<ref>Aguilera Mochón, p. 58</ref>

Life on Earth requires water in a liquid state as a [[solvent]] in which biochemical reactions take place. It is highly unlikely that an [[abiogenesis]] process can start within a gaseous or solid medium: the atom speeds, either too fast or too slow, make it difficult for specific ones to meet and start chemical reactions. A liquid medium also allows the transport of nutrients and substances required for metabolism.<ref>Aguilera Mochón, p. 51</ref> Sufficient quantities of carbon and other elements, along with water, might enable the formation of living organisms on [[terrestrial planet]]s with a chemical make-up and temperature range similar to that of Earth.<ref>{{cite journal |last1=Bond |first1=Jade C. |last2=O'Brien |first2=David P. |last3=Lauretta |first3=Dante S. |title=The Compositional Diversity of Extrasolar Terrestrial Planets. I. In Situ Simulations |journal=The Astrophysical Journal |volume=715 |issue=2 |pages=1050–1070 |date=June 2010 |doi=10.1088/0004-637X/715/2/1050 |bibcode=2010ApJ...715.1050B |arxiv=1004.0971|s2cid=118481496 }}</ref><ref>{{cite journal |first=Norman R. |last=Pace |date=20 January 2001 |title=The universal nature of biochemistry |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=98 |issue=3 |pages=805–808 |doi=10.1073/pnas.98.3.805 |pmid=11158550 |bibcode=2001PNAS...98..805P |pmc=33372|doi-access=free }}</ref> Life based on [[ammonia]] rather than water has been suggested as an alternative, though this solvent appears less suitable than water. It is also conceivable that there are forms of life whose solvent is a liquid [[hydrocarbon]], such as [[methane]], [[ethane]] or [[propane]].<ref>{{cite book |chapter-url=http://www.nap.edu/read/11919/chapter/8#74 |chapter=6.2.2: Nonpolar Solvents |title=The Limits of Organic Life in Planetary Systems |publisher=The National Academies Press |author=National Research Council |page=74 |date=2007 |doi=10.17226/11919 |isbn=978-0-309-10484-5}}</ref>

Another unknown aspect of potential extraterrestrial life would be the [[chemical element]]s that would compose it. Life on Earth is largely composed of carbon, but there could be other [[hypothetical types of biochemistry]]. A replacement for carbon would need to be able to create complex molecules, store information required for evolution, and be freely available in the medium. To create [[DNA]], [[RNA]], or a close analog, such an element should be able to bind its atoms with many others, creating complex and stable molecules. It should be able to create at least three covalent bonds: two for making long strings and at least a third to add new links and allow for diverse information. Only nine elements meet this requirement: [[boron]], [[nitrogen]], [[phosphorus]], [[arsenic]], [[antimony]] (three bonds), [[carbon]], [[silicon]], [[germanium]] and [[tin]] (four bonds). As for abundance, carbon, nitrogen, and silicon are the most abundant ones in the universe, far more than the others. On [[Earth's crust]] the most abundant of those elements is silicon, in the [[Hydrosphere]] it is carbon and in the atmosphere, it is carbon and nitrogen. Silicon, however, has disadvantages over carbon. The molecules formed with silicon atoms are less stable, and more vulnerable to acids, oxygen, and light. An ecosystem of silicon-based lifeforms would require very low temperatures, high [[atmospheric pressure]], an atmosphere devoid of oxygen, and a solvent other than water. The low temperatures required would add an extra problem, the difficulty to kickstart a process of abiogenesis to create life in the first place.<ref>Aguilera Mochón, pp. 43–49</ref>  [[Norman Horowitz]], head of the Jet Propulsion Laboratory bioscience section for the Mariner and Viking missions from 1965 to 1976 considered 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 of life on other planets.<ref>Horowitz, N.H. (1986). Utopia and Back and the search for life in the solar system. New York: W.H. Freeman and Company. {{ISBN|0-7167-1766-2}}</ref>  However, he also considered that the conditions found on [[Mars]] were incompatible with carbon based life. 

Even if extraterrestrial life is based on carbon and uses water as a solvent, like Earth life, it may still have a radically different [[biochemistry]].  Life is generally considered to be a product of [[natural selection]].  It has been proposed that to undergo natural selection a living entity must have the capacity to [[DNA replication|replicate]] itself, the capacity to avoid damage/decay, and the capacity to acquire and process resources in support of the first two capacities.<ref>Bernstein, Harris; Byerly, Henry C.; Hopf, Frederick A.; et al. (June 1983). "The Darwinian Dynamic". The Quarterly Review of Biology. 58 (2): 185–207. doi:10.1086/413216. JSTOR 2828805. S2CID 83956410</ref> Life on Earth started with an [[RNA world]] and later evolved to its current form, where some of the [[RNA]] tasks were transferred to [[DNA]] and [[proteins]]. Extraterrestrial life may still be stuck using RNA, or evolve into other configurations. It is unclear if our biochemistry is the most efficient one that could be generated, or which elements would follow a similar pattern.<ref>Aguilera Mochón, pp. 58–59</ref> However, it is likely that, even if cells had a different composition to those from Earth, they would still have a [[cell membrane]]. Life on Earth jumped from [[prokaryote]]s to [[eukaryote]]s and from [[unicellular organism]]s to multicellular organisms through [[evolution]]. So far no alternative process to achieve such a result has been conceived, even if hypothetical. Evolution requires life to be divided into individual organisms, and no alternative organisation has been satisfactorily proposed either. At the basic level, membranes define the limit of a cell, between it and its environment, while remaining partially open to exchange energy and resources with it.<ref>Aguilera Mochón, pp. 42–43</ref>

The evolution from simple cells to eukaryotes, and from them to multicellular lifeforms, is not guaranteed. The [[Cambrian explosion]] took place thousands of millions of years after the origin of life, and its causes are not fully known yet. On the other hand, the jump to multicellularity took place several times, which suggests that it could be a case of [[convergent evolution]], and so likely to take place on other planets as well. Palaeontologist [[Simon Conway Morris]] considers that convergent evolution would lead to kingdoms similar to our plants and animals, and that many features are likely to develop in alien animals as well, such as [[bilateral symmetry]], [[Limb (anatomy)|limbs]], [[Digestion|digestive systems]] and heads with [[sensory organ]]s.<ref name="AM6166"/> Scientists from the University of Oxford analysed it from the perspective of evolutionary theory and wrote in a study in the [[International Journal of Astrobiology]] that aliens may be similar to humans.<ref>{{cite web |url=http://www.ox.ac.uk/news/2017-10-31-aliens-may-be-more-us-we-think |title=Aliens may be more like us than we think |publisher=[[University of Oxford]] |date=31 October 2017}}</ref> The planetary context would also have an influence: a planet with higher [[gravity]] would have smaller animals, and other types of stars can lead to [[Hypothetical types of biochemistry#Non-green photosynthesizers|non-green photosynthesizers]]. The amount of energy available would also affect [[biodiversity]], as an ecosystem sustained by black smokers or hydrothermal vents would have less energy available than those sustained by a star's light and heat, and so its lifeforms would not grow beyond a certain complexity.<ref name="AM6166">Aguilera Mochón, pp. 61–66</ref> There is also research in assessing the capacity of life for developing intelligence. It has been suggested that this capacity arises with the number of potential [[Ecological niche|niches]] a planet contains, and that the complexity of life itself is reflected in the information density of planetary environments, which in turn can be computed from its niches.<ref>{{cite journal |title=Evolutionary exobiology: Towards the qualitative assessment of biological potential on exoplanets |journal=International Journal of Astrobiology |volume=18 |issue=3 |first1=David S. |last1=Stevenson |first2=Sean |last2=Large |date=25 October 2017 |doi=10.1017/S1473550417000349 |pages=204–208|s2cid=125275411 }}</ref>

=== Harsh environmental conditions on Earth harboring life ===
It is common knowledge that the conditions on other planets in the solar system, in addition to the many galaxies outside of the [[Milky Way galaxy]], are very harsh and seem to be too extreme to harbor any life.<ref>{{Cite web |title=Atmosphere - Planets, Composition, Pressure {{!}} Britannica |url=https://www.britannica.com/science/atmosphere/The-atmospheres-of-other-planets |access-date=2024-04-17 |website=www.britannica.com |language=en}}</ref> The environmental conditions on these planets can have intense [[UV radiation]] paired with extreme temperatures, lack of water,<ref>{{Cite journal |last1=Amils |first1=Ricardo |last2=González-Toril |first2=Elena |last3=Fernández-Remolar |first3=David |last4=Gómez |first4=Felipe |last5=Aguilera |first5=Ángeles |last6=Rodríguez |first6=Nuria |last7=Malki |first7=Mustafá |last8=García-Moyano |first8=Antonio |last9=Fairén |first9=Alberto G. |last10=de la Fuente |first10=Vicenta |last11=Luis Sanz |first11=José |date=February 2007 |title=Extreme environments as Mars terrestrial analogs: The Rio Tinto case |url=https://linkinghub.elsevier.com/retrieve/pii/S0032063306001826 |journal=Planetary and Space Science |language=en |volume=55 |issue=3 |pages=370–381 |doi=10.1016/j.pss.2006.02.006|bibcode=2007P&SS...55..370A }}</ref> and much more that can lead to conditions that don't seem to favor the creation or maintenance of extraterrestrial life. However, there has been much historical evidence that some of the earliest and most basic forms of life on Earth originated in some extreme environments<ref>{{Cite journal |last1=Daniel |first1=Isabelle |last2=Oger |first2=Philippe |last3=Winter |first3=Roland |date=2006 |title=Origins of life and biochemistry under high-pressure conditions |url=https://xlink.rsc.org/?DOI=b517766a |journal=Chemical Society Reviews |language=en |volume=35 |issue=10 |pages=858–875 |doi=10.1039/b517766a |pmid=17003893 |issn=0306-0012}}</ref> that seem unlikely to have harbored life at least at one point in Earth's history. Fossil evidence as well as many historical theories backed up by years of research and studies have marked environments like [[hydrothermal vent]]s or acidic hot springs as some of the first places that life could have originated on Earth.<ref>{{Cite journal |last1=Dong |first1=Hailiang |last2=Yu |first2=Bingsong |date=2007-09-01 |title=Geomicrobiological processes in extreme environments: A review |journal=Episodes |language=en |volume=30 |issue=3 |pages=202–216 |doi=10.18814/epiiugs/2007/v30i3/003 |issn=0705-3797|doi-access=free }}</ref> These environments can be considered extreme when compared to the typical ecosystems that the majority of life on Earth now inhabit, as hydrothermal vents are scorching hot due to the [[magma]] escaping from the [[Earth's mantle]] and meeting the much colder oceanic water. Even in today's world, there can be a diverse population of bacteria found inhabiting the area surrounding these hydrothermal vents<ref name=":1">{{Cite journal |last1=Georgieva |first1=Magdalena N. |last2=Little |first2=Crispin T.S. |last3=Maslennikov |first3=Valeriy V. |last4=Glover |first4=Adrian G. |last5=Ayupova |first5=Nuriya R. |last6=Herrington |first6=Richard J. |date=June 2021 |title=The history of life at hydrothermal vents |journal=Earth-Science Reviews |language=en |volume=217 |pages=103602 |doi=10.1016/j.earscirev.2021.103602|bibcode=2021ESRv..21703602G |doi-access=free }}</ref> which can suggest that some form of life can be supported even in the harshest of environments like the other planets in the solar system.

The aspects of these harsh environments that make them ideal for the origin of life on Earth, as well as the possibility of creation of life on other planets, is the [[chemical reaction]]s forming spontaneously. For example, the [[hydrothermal vent]]s found on the ocean floor are known to support many [[Chemosynthesis|chemosynthetic]] processes<ref name=":0" /> which allow organisms to utilize energy through reduced chemical compounds that fix carbon.<ref name=":1" /> In return, these reactions will allow for organisms to live in relatively low oxygenated environments while maintaining enough energy to support themselves. The early Earth environment was reducing<ref>{{Cite journal |last1=Zahnle |first1=Kevin J. |last2=Lupu |first2=Roxana |last3=Catling |first3=David C. |last4=Wogan |first4=Nick |date=2020-06-01 |title=Creation and Evolution of Impact-generated Reduced Atmospheres of Early Earth |journal=The Planetary Science Journal |volume=1 |issue=1 |pages=11 |doi=10.3847/PSJ/ab7e2c |doi-access=free |arxiv=2001.00095 |bibcode=2020PSJ.....1...11Z |issn=2632-3338}}</ref> and therefore, these carbon fixing compounds were necessary for the survival and possible [[origin of life on Earth]]. With the little amount of information that scientists have found regarding the atmosphere on other planets in the [[Milky Way galaxy]] and beyond, the atmospheres are most likely reducing or with very low oxygen levels,<ref>{{Cite journal |last1=Atreya |first1=S.K |last2=Mahaffy |first2=P.R |last3=Niemann |first3=H.B |last4=Wong |first4=M.H |last5=Owen |first5=T.C |date=February 2003 |title=Composition and origin of the atmosphere of Jupiter—an update, and implications for the extrasolar giant planets |url=https://linkinghub.elsevier.com/retrieve/pii/S0032063302001447 |journal=Planetary and Space Science |language=en |volume=51 |issue=2 |pages=105–112 |doi=10.1016/S0032-0633(02)00144-7|bibcode=2003P&SS...51..105A }}</ref> especially when compared with Earth's atmosphere. If there were the necessary elements and ions on these planets, the same carbon fixing, reduced chemical compounds occurring around hydrothermal vents could also occur on these planets' surfaces and possibly result in the origin of extraterrestrial life.