Immortality

===Biological immortality===
[[File:Telomere caps.gif|thumb|right|Human [[chromosome]]s (grey) capped by telomeres (white)]]
{{Main|Biological immortality}}

Biological immortality is an absence of aging. Specifically it is the absence of a sustained increase in [[rate of mortality]] as a function of chronological age. A cell or organism that does not experience aging, or ceases to age at some point, is biologically immortal.<ref>{{cite web|url=http://www.immortality.foundation/aging|title=What is Aging?|access-date=6 November 2020}}</ref>

[[Biologist]]s have chosen the word "immortal" to designate cells that are not limited by the [[Hayflick limit]], where cells no longer divide because of [[DNA repair|DNA damage]] or shortened [[telomere]]s. The first and still most widely used immortal cell line is [[HeLa]], developed from cells taken from the malignant cervical tumor of [[Henrietta Lacks]] without her consent in 1951. Prior to the 1961 work of [[Leonard Hayflick]], there was the erroneous belief fostered by [[Alexis Carrel]] that all normal [[Somatic (biology)|somatic]] cells are immortal. By preventing cells from reaching senescence one can achieve biological immortality; telomeres, a "cap" at the end of DNA, are thought to be the cause of cell aging. Every time a cell divides the telomere becomes a bit shorter; when it is finally worn down, the cell is unable to split and dies. [[Telomerase]] is an enzyme which rebuilds the telomeres in stem cells and cancer cells, allowing them to replicate an infinite number of times.<ref name="LinKahWai">{{cite journal |author=Lin Kah Wai |author-link=Lin Kah Wai |title=Telomeres, Telomerase, and Tumorigenesis – A Review |journal=MedGenMed |date=18 April 2004 |volume=6 |issue=3 |page=19 |pmid=15520642 |pmc=1435592}}</ref> No definitive work has yet demonstrated that telomerase can be used in human somatic cells to prevent healthy tissues from aging. On the other hand, scientists hope to be able to grow organs with the help of stem cells, allowing organ transplants without the risk of rejection, another step in extending human life expectancy. These technologies are the subject of ongoing research, and are not yet realized.<ref>{{cite news|url=https://www.nytimes.com/2017/01/26/science/chimera-stemcells-organs.html|title=New Prospects for Growing Human Replacement Organs in Animals|website=[[The New York Times]]|date=26 January 2017 |access-date=3 March 2018|last1=Wade |first1=Nicholas }}</ref>

====Biologically immortal species====
{{see also|List of longest-living organisms}}

Life defined as biologically immortal is still susceptible to causes of death besides aging, including disease and trauma, as defined above. Notable immortal species include:
* ''Bacteria'' – Bacteria reproduce through [[Fission (biology)|binary fission]]. A parent bacterium splits itself into two identical daughter cells which eventually then split themselves in half. This process repeats, thus making the bacterium essentially immortal. A 2005 [[PLoS Biology]] paper<ref>{{cite web|url= http://hal.archives-ouvertes.fr/docs/00/08/01/54/PDF/stewart_Plos.pdf|title= Aging and Death in an Organism That Reproduces by Morphologically Symmetric Division}}</ref> suggests that after each division the daughter cells can be identified as the older and the younger, and the older is slightly smaller, weaker, and more likely to die than the younger.<ref>{{cite web |url=https://www.newscientist.com/channel/health/mg18524855.800-bacteria-death-reduces-human-hopes-of-immortality.html |title=Bacteria Death Reduces Human Hopes of Immortality |date=5 February 2005 |website=New Scientist magazine, issue 2485| page= 19 |access-date=2 April 2007}}</ref>
* ''[[Turritopsis dohrnii]]'', a jellyfish (phylum [[Cnidaria]], class [[Hydrozoa]], order [[Anthoathecata]]), after becoming a sexually mature adult, can transform itself back into a [[polyp (zoology)|polyp]] using the cell conversion process of [[transdifferentiation]].<ref name="CheatingDeath">{{cite web|url=http://10e.devbio.com/article.php?ch=2&id=6|title=Cheating Death: The Immortal Life Cycle of Turritopsis|last=Gilbert|first=Scott F.|date=2006|access-date=14 June 2009|url-status=dead|archive-url=https://web.archive.org/web/20151121052612/http://10e.devbio.com/article.php?ch=2&id=6|archive-date=21 November 2015}}</ref> ''Turritopsis dohrnii'' repeats this cycle, meaning that it may have an [[indefinite lifespan]].<ref name="CheatingDeath" /> Its immortal adaptation has allowed it to spread from its original habitat in the Caribbean to "all over the world".<ref name="Telegraph-immortal-jellyfish">{{cite news |url=https://www.telegraph.co.uk/earth/wildlife/4357829/Immortal-jellyfish-swarming-across-the-world.html |archive-url=https://web.archive.org/web/20090130115250/http://www.telegraph.co.uk/earth/wildlife/4357829/Immortal-jellyfish-swarming-across-the-world.html |url-status=dead |archive-date=30 January 2009 |title={{-'}}Immortal' jellyfish swarming across the world |date=30 January 2009 |publisher=[[Telegraph Media Group]] |access-date= 14 June 2009 |location=London}}</ref><ref>{{cite web|url=http://news.nationalgeographic.com/news/2009/01/090130-immortal-jellyfish-swarm.html|archive-url=https://web.archive.org/web/20090202063305/http://news.nationalgeographic.com/news/2009/01/090130-immortal-jellyfish-swarm.html|url-status=dead|archive-date=2 February 2009|title="Immortal" Jellyfish Swarm World's Oceans|website=news.nationalgeographic.com|date=29 January 2009|access-date=19 August 2017}}</ref>
* ''[[Hydra (genus)|Hydra]]'' is a [[genus]] belonging to the phylum [[Cnidaria]], the class [[Hydrozoa]] and the order [[Anthomedusae]]. They are simple fresh-water [[predatory]] animals possessing [[symmetry (biology)#Radial symmetry|radial symmetry]].<ref>Gilberson, Lance, ''Zoology Lab Manual'', 4th edition. Primis Custom Publishing. 1999.</ref><ref>{{Cite news|url=https://www.livescience.com/53178-hydra-may-live-forever.html|title=Hail the Hydra, an Animal That May Be Immortal|work=Live Science|access-date=19 August 2017}}</ref>

====Evolution of aging====
{{Main|Evolution of aging}}<!--that's how they spell aging in England ... and in that article-->
As the existence of biologically immortal species demonstrates, there is no [[second law of thermodynamics|thermodynamic]] necessity for senescence: a defining feature of life is that it takes in [[Gibbs free energy|free energy]] from the environment and unloads its [[entropy]] as waste. Living systems can even build themselves up from seed, and routinely repair themselves. Aging is therefore presumed to be a byproduct of [[evolution]], but why mortality should be selected for remains a subject of research and debate. Programmed cell death and the telomere "end replication problem" are found even in the earliest and simplest of organisms.<ref>Clark, W.R. 1999. ''A Means to an End: The biological basis of aging and death.'' New York: Oxford University Press. {{cite web |url=http://wrclarkbooks.com/means_to_an_end.html |title=A Means to an End - Why do we age? Is aging inevitable? Questions and Answers are available here by downloading a sample chapter of WR Clark's book. The WRClark Site also features a chat room for you to ask your questions about aging |access-date=25 May 2008 |url-status=dead |archive-url=https://web.archive.org/web/20080511212036/http://www.wrclarkbooks.com/means_to_an_end.html |archive-date=11 May 2008 }} About telomeres and programmed cell death.</ref> This may be a tradeoff between selecting for cancer and selecting for aging.<ref>Harrison, ''Principles of Internal Medicine'', Ch. 69, "Cancer cell biology and angiogenesis", Robert G. Fenton and Dan L. Longo, p. 454.</ref>

Modern theories on the evolution of aging include the following:
* Mutation accumulation is a theory formulated by [[Peter Medawar]] in 1952 to explain how evolution would select for aging. Essentially, aging is never selected against, as organisms have offspring before the mortal mutations surface in an individual.
* [[Pleiotropy|Antagonistic pleiotropy]] is a theory proposed as an alternative by [[George C. Williams (biologist)|George C. Williams]], a critic of Medawar, in 1957. In antagonistic pleiotropy, genes carry effects that are both beneficial and detrimental. In essence this refers to genes that offer benefits early in life, but exact a cost later on, i.e. decline and death.<ref>Williams, G.C. 1957. Pleiotropy, natural selection and the evolution of senescence. ''Evolution'', '''11''':398–411. {{cite web|url=http://www.telomere.org/Downloads/Williams_searchable.pdf |title=Archived copy |access-date=23 July 2006 |url-status=dead |archive-url=https://web.archive.org/web/20060713071204/http://www.telomere.org/Downloads/Williams_searchable.pdf |archive-date=13 July 2006 }} Paper in which Williams describes his theory of antagonistic pleiotropy.</ref>
* The disposable soma theory was proposed in 1977 by [[Tom Kirkwood|Thomas Kirkwood]], which states that an individual body must allocate energy for metabolism, reproduction, and maintenance, and must compromise when there is food scarcity. Compromise in allocating energy to the repair function is what causes the body gradually to deteriorate with age, according to Kirkwood.<ref>Kirkwood, T.B.L. 1977. Evolution of aging. ''Nature'', '''270''': 301–304. [http://www.nature.com/nature/journal/v270/n5635/abs/270301a0.html] Origin of the disposable soma theory.</ref>

====Immortality of the germline====

Individual organisms ordinarily age and die, while the germlines which connect successive generations are potentially immortal. The basis for this difference is a fundamental problem in biology.   The Russian biologist and historian [[Zhores A. Medvedev]]<ref name="Medvedev1981">{{cite journal |last=Medvedev |first=Zhores A. |title=On the immortality of the germ line: Genetic and biochemical mechanisms. A review |journal=Mechanisms of Ageing and Development |volume=17 |issue=4 |year=1981 |pages=331–359 |issn=0047-6374 |doi=10.1016/0047-6374(81)90052-X|pmid=6173551 |s2cid=35719466 }}</ref> considered that the accuracy of [[genome]] replicative and other synthetic systems alone cannot explain the immortality of [[germline]]s. Rather Medvedev thought that known features of the biochemistry and genetics of [[sexual reproduction]] indicate the presence of unique information maintenance and restoration processes at the different stages of [[gametogenesis]]. In particular, Medvedev considered that the most important opportunities for information maintenance of [[germ cell]]s are created by [[genetic recombination|recombination during meiosis]] and [[DNA repair]]; he saw these as processes within the germ cells that were capable of restoring the integrity of [[DNA]] and [[chromosome]]s from the types of damage that cause irreversible aging in [[somatic cell]]s.