Editing Appendix (anatomy) (section)

==History==
[[Charles Darwin]] suggested that the appendix was mainly used by earlier hominids for digesting fibrous vegetation, then evolved to take on a new purpose over time. The very long cecum of some herbivorous animals, such as in the [[horse]] or the [[koala]], appears to support this hypothesis. The koala's cecum enables it to host bacteria that specifically help to break down cellulose. Human ancestors may have also relied upon this system when they lived on a diet rich in foliage. 

As people began to eat more easily digested foods, they may have become less reliant on cellulose-rich plants for energy. As the cecum became less necessary for digestion, mutations that were previously deleterious (and would have hindered evolutionary progress) were no longer important, so the mutations survived. It is suggested that these [[allele]]s became more frequent and the cecum continued to shrink. After millions of years, the once-necessary cecum degraded to be the appendix of modern humans.<ref name="DescentOfMan">{{Cite book |last=Darwin |first=Charles |author-link=Charles Darwin |title=The Descent of Man, and Selection in Relation to Sex |title-link=The Descent of Man, and Selection in Relation to Sex |publisher=John Murray |year=1871 |location=London |chapter=Jim's Jesus}}</ref>

Dr. Heather F. Smith of [[Midwestern University]] and colleagues explained:

<blockquote>Recently ... improved understanding of gut immunity has merged with current thinking in biological and medical science, pointing to an apparent function of the mammalian cecal appendix as a safe-house for symbiotic gut microbes, preserving the flora during times of gastrointestinal infection in societies without modern medicine. This function is potentially a selective force for the evolution and maintenance of the appendix.
Three morphotypes of cecal-appendices can be described among mammals based primarily on the shape of the cecum: a distinct appendix branching from a rounded or sac-like cecum (as in many primate species), an appendix located at the apex of a long and voluminous cecum (as in the rabbit, greater glider and Cape dune mole rat), and an appendix in the absence of a pronounced cecum (as in the wombat). In addition, long narrow appendix-like structures are found in mammals that either lack an apparent cecum (as in monotremes) or lack a distinct junction between the cecum and appendix-like structure (as in the koala). A cecal appendix has evolved independently at least twice and represents yet another example of convergence in morphology between Australian marsupials and placentals in the rest of the world. Although the appendix has been lost by numerous species, it has also been maintained for more than 80 million years in at least one clade.<ref name=SEAL09>{{cite journal | vauthors = Smith HF, Fisher RE, Everett ML, Thomas AD, Bollinger RR, Parker W | title = Comparative anatomy and phylogenetic distribution of the mammalian cecal appendix | journal = Journal of Evolutionary Biology | volume = 22 | issue = 10 | pages = 1984–99 | date = October 2009 | pmid = 19678866 | doi = 10.1111/j.1420-9101.2009.01809.x | s2cid = 6112969 | doi-access = free }}</ref></blockquote>

In a 2013 paper, the appendix was found to have independently evolved in different animals at least 32 times (and perhaps as many as 38 times) and to have been lost no more than six times throughout history.<ref name = SEAL13>{{cite journal | author1 = Smith H. F. |author2=Parker W. |author3=Kotzé, S. H. |author4=Laurin, M. | year = 2013 | title = Multiple independent appearances of the cecal appendix in mammalian evolution and an investigation of related ecological and anatomical factors | journal = Comptes Rendus Palevol | volume =  12| issue = 6| pages =  339–354| doi = 10.1016/j.crpv.2012.12.001 |bibcode=2013CRPal..12..339S | doi-access = free }}</ref> A more recent study using similar methods on an updated database yielded similar, though less spectacular results, with at least 29 gains and at the most 12 losses (all of which were ambiguous), and this is still significantly asymmetrical.<ref name = SEAL17>{{cite journal | author1 = Smith H. F. |author2=Parker W. |author3=Kotzé, S. H. |author4=Laurin, M. | year = 2017 | title = Morphological evolution of the mammalian cecum and cecal appendix | journal = Comptes Rendus Palevol | volume =  11| issue = 1| pages =  39–57| doi = 10.1016/j.crpv.2016.06.001 |bibcode=2017CRPal..16...39S | doi-access = free }}</ref> 

This suggests that the cecal appendix has a selective advantage in many situations and argues strongly against its vestigial nature. Given that this organ may have a selective advantage in numerous situations, it appears to be associated with greater maximal longevity, for a given body mass.<ref name="Collard et al. 2021">{{cite journal |last1=Collard |first1=Maxime K. |last2=Bardin |first2=Jérémie |last3=Laurin |first3=Michel |last4=Ogier‐Denis |first4=Eric |title=The cecal appendix is correlated with greater maximal longevity in mammals |journal=Journal of Anatomy |date=November 2021 |volume=239 |issue=5 |pages=1157–1169 |doi=10.1111/joa.13501 |url=https://doi.org/10.1111/joa.13501 |language=en |issn=0021-8782|pmc=8546507 }}</ref> For example, in a 2023 study, the protective functions conferred against diarrhea were observed in young primates.<ref>{{Cite journal |last1=Collard |first1=Maxime K. |last2=Bardin |first2=Jérémie |last3=Marquet |first3=Bertille |last4=Laurin |first4=Michel |last5=Ogier-Denis |first5=Éric |date=2023-09-23 |title=Correlation between the presence of a cecal appendix and reduced diarrhea severity in primates: new insights into the presumed function of the appendix |journal=Scientific Reports |language=en |volume=13 |issue=1 |pages=15897 |doi=10.1038/s41598-023-43070-5 |issn=2045-2322 |pmc=10517977 |pmid=37741857|bibcode=2023NatSR..1315897C }}</ref> This complex evolutionary history of the appendix, along with a great heterogeneity in its evolutionary rate in various taxa, suggests that it is a recurrent trait.<ref name = LEAL11>{{cite journal | vauthors = Laurin M, Everett ML, Parker W | title = The cecal appendix: one more immune component with a function disturbed by post-industrial culture | journal = Anatomical Record | volume = 294 | issue = 4 | pages = 567–79 | date = April 2011 | pmid = 21370495 | doi = 10.1002/ar.21357 | s2cid = 3237168 }}</ref>

Such a function may be useful in a culture lacking modern sanitation and healthcare practice, where [[diarrhea]] may be prevalent. Current [[epidemiology|epidemiological]] data on the cause of death in developing countries collected by the [[World Health Organization]] in 2001 show that acute diarrhea is now the fourth leading cause of disease-related death in developing countries (data summarized by the [[Bill & Melinda Gates Foundation|Bill and Melinda Gates Foundation]]). Two of the other leading causes of death are expected to have exerted limited or no selection pressure.<ref>{{Cite web |date=21 August 2009 |orig-date=20 August 2009 |title=Evolution of the Appendix: A Biological 'Remnant' No More |url=http://www.dukehealth.org/health_library/news/evolution_of_the_appendix_a_biological_remnant_no_more |url-status=dead |archive-url=https://web.archive.org/web/20120726030646/http://www.dukehealth.org/health_library/news/evolution_of_the_appendix_a_biological_remnant_no_more |archive-date=2012-07-26 |access-date= |website=Duke Medicine News and Communications}}</ref>