Death

=== Evolution of aging and mortality ===
{{Main|Evolution of ageing}}

Inquiry into the evolution of aging aims to explain why so many living things and the vast majority of animals weaken and die with age. However, there are exceptions, such as ''[[Hydra (genus)|Hydra]]'' and the jellyfish ''[[Turritopsis dohrnii]]'', which research shows to be [[biological immortality|biologically immortal]].<ref>{{Cite web |last=National Institute on Aging |date=2020 |title=The National Institute on Aging: Strategic Directions for Research, 2020–2025 |url=https://www.nia.nih.gov/about/aging-strategic-directions-research |access-date=February 16, 2023 |website=National Institute on Aging |archive-date=4 June 2020 |archive-url=https://web.archive.org/web/20200604212742/https://www.nia.nih.gov/about/aging-strategic-directions-research |url-status=live }}</ref>

Organisms showing only [[asexual reproduction]], such as bacteria, some [[protist]]s, like the [[euglenoid]]s and many [[amoebozoan]]s, and [[unicellular]] organisms with [[sexual reproduction]], [[Colony (biology)|colonial]] or not, like the [[Volvocales|volvocine]] algae ''[[Pandorina]]'' and ''[[Chlamydomonas]],'' are "immortal" at some extent, dying only due to external hazards, like being eaten or meeting with a fatal accident. In [[multicellular]] organisms and also in [[multinucleate]] [[ciliates]]<ref>Beukeboom, L. & Perrin, N. (2014). ''The Evolution of Sex Determination''. Online Chapter 2: [http://global.oup.com/booksites/content/9780199657148/ The diversity of sexual cycles] {{Webarchive|url=https://web.archive.org/web/20141112034751/http://global.oup.com/booksites/content/9780199657148/ |date=12 November 2014 }}, p. 12. Oxford University Press.</ref> with a [[Weismann barrier|Weismannist development]], that is, with a division of labor between mortal [[somatic cells|somatic (body) cells]] and "immortal" [[germ cell|germ (reproductive) cells]], death becomes an essential part of life, at least for the somatic line.<ref name=Gilbert>{{cite book|last=Gilbert |first=S.F. |year=2003 |title=Developmental biology |edition=7th |place=Sunderland, Mass |publisher=Sinauer Associates |pages=34–35 |isbn=978-0-87893-258-0}}</ref>

The ''[[Volvox]]'' algae are among the simplest organisms to exhibit that division of labor between two completely different cell types, and as a consequence, include the death of somatic line as a regular, genetically regulated part of its [[Biological life cycle|life history]].<ref name=Gilbert /><ref>{{cite journal|doi=10.1007/s00497-010-0158-4 |pmid=21174128 |pmc=3098969 |title=Evolution of reproductive development in the volvocine algae |last=Hallmann |first=A. |journal=Sexual Plant Reproduction |date=June 2011 |volume=24 |issue=2 |pages=97–112}}</ref>