human vestigiality

Charles Darwin listed a number of putative human vestigial features, which he termed rudimentary, in The Descent of Man (1871). These included the muscles of the ear; wisdom teeth; the appendix; the tail bone; body hair; and the semilunar fold in the corner of the eye. Darwin also commented on the sporadic nature of many vestigial features, particularly musculature. Making reference to the work of the anatomist William Turner, Darwin highlighted a number of sporadic muscles that he identified as vestigial remnants of the panniculus carnosus, particularly the sternalis muscle.Darwin C, The Descent of Man, and Selection in Relation to Sex, London: John Murray, 1890, p.13.[https://archive.org/details/ajp4619.0001.001.umich.edu]Turner W, On the musculus sternalis, Proc. Royal Soc. Edinburgh session 1866–1867, p.65.[https://archive.org/stream/proceedingso618661869roya#page/64/mode/2up/]

In 1893, Robert Wiedersheim published The Structure of Man, a book on human anatomy and its relevance to evolutionary history. This book contains a list of 86 human organs he considered vestigial, which he called "wholly or in part functionless, some appearing in the Embryo alone, others present during Life constantly or inconstantly. For the greater part Organs which may be rightly termed Vestigial."Wiedersheim, R. (1893) The Structure of Man: An Index to His Past History. Second Edition. Translated by H. and M. Bernard. London: Macmillan and Co. 1895. [https://archive.org/details/structureofmanin00wiediala] His list of supposedly vestigial organs included many of the examples on this page as well as others then mistakenly believed to be purely vestigial, such as the pineal gland, the thymus gland, and the pituitary gland. Some of these organs that had lost their obvious, original functions later turned out to have retained functions that had gone unrecognized before the discovery of hormones or many of the functions and tissues of the immune system.{{cite encyclopedia|last=Muller |first= G. B. |year=2002 |title=Vestigial Organs and Structures |encyclopedia=Encyclopedia of Evolution |editor-first=Mark |editor-last=Pagel |location=New York |publisher=Oxford University Press |pages=1131–1133}}{{cite news | last=Koerth-Baker | first=Maggie | title=Vestigial Organs Not So Useless After All | date=30 July 2009 | work=National Geographic | url=http://news.nationalgeographic.com/news/2009/07/090730-spleen-vestigial-organs.html | archive-url=https://web.archive.org/web/20090804032748/http://news.nationalgeographic.com/news/2009/07/090730-spleen-vestigial-organs.html | url-status=dead | archive-date=4 August 2009 | access-date=27 July 2013}} Examples included:

• the role of the pineal in the regulation of the circadian rhythm (neither the function nor even the existence of melatonin was yet known);
• discovery of the role of the thymus in the immune system lay many decades in the future; it remained a mystery until the mid-20th century;
• the pituitary and hypothalamus, with their many and varied hormones, were far from understood, let alone the complexity of their interrelationships.

Historically, there was a trend not only to dismiss the appendix as being uselessly vestigial, but an anatomical hazard liable to dangerous inflammation. As late as the mid-20th century, many reputable authorities conceded it no beneficial function.{{cite book|last1=Wells |first1=H. G. |author1-link=H. G. Wells |last2=Huxley |first2=J. |author2-link=Julian Huxley |last3=Wells |first3=G. P. |author3-link=G. P. Wells |title=The Science of Life |publisher= Cassells |year=1929|title-link=The Science of Life }} This was a view supported, or perhaps inspired, by Darwin himself in the 1874 edition of his book The Descent of Man, and Selection in Relation to Sex. The organ's patent liability to appendicitis and poorly understood role left it open to blame for a number of possibly unrelated conditions. For example, in 1916, a surgeon claimed that removal of the appendix had cured several cases of trifacial neuralgia and other nerve pain about the head and face, even though he said the evidence for appendicitis in those patients was inconclusive.Rosenthal, M. I.: Journal of the American Medical Association, Volume 67, Issues 15–26, 1916. p. 1326 The discovery of hormones and hormonal principles, notably by Bayliss and Starling, argued against these views, but in the early 20th century, a great deal of fundamental research remained to be done on the functions of large parts of the digestive tract. In 1916, an author found it necessary to argue against the idea that the colon had no important function and that "the ultimate disappearance of the appendix is a coordinate action and not necessarily associated with such frequent inflammations as we are witnessing in the human".W. Colin MacKenzie. "A Contribution to the Biology of the Vermiform Appendix". Medical Record, Volume 89, p. 342, 1916

There had been a long history of doubt about such dismissive views. Around 1920, the surgeon Kenelm Hutchinson Digby documented previous observations, going back more than 30 years, that suggested lymphatic tissues, such as the tonsils and appendix, might have substantial immunological functions.

File:Ileum-Caecum-Colon of rabbit IMG 5771.JPG]]

File:Gray536.png

The appendix was once believed to be a vestige of a redundant organ that in ancestral species had digestive functions, much as it still does in extant species in which intestinal flora hydrolyze cellulose and similar indigestible plant materials.Darwin, Charles (1871). The Descent of Man, and Selection in Relation to Sex. John Murray: London. This view has changed in recent decades,{{cite journal|vauthors=Kooij IA, Sahami S, Meijer SL, Buskens CJ, Te Velde AA|date=October 2016|title=The immunology of the vermiform appendix: a review of the literature|journal=Clinical and Experimental Immunology|volume=186|issue=1|pages=1–9|doi=10.1111/cei.12821|pmc=5011360|pmid=27271818}} with research suggesting that the appendix may serve an important purpose. In particular, it may serve as a reservoir for beneficial gut bacteria, possibly to allow the bacteria to reestablish in the colon during recovery from diarrhea or other illnesses.{{cite journal |last1=Yap |first1=Daniel Ren Yi |last2=Lui |first2=Rashid N |last3=Samol |first3=Jens |last4=Ngeow |first4=Joanne |last5=Sung |first5=Joseph JY |last6=Wong |first6=Sunny H |title=Beyond a vestigial organ: effects of the appendix on gut microbiome and colorectal cancer |journal=Journal of Gastroenterology and Hepatology |date=February 2024 |volume=39 |issue=5 |pages=826–835 |doi=10.1111/jgh.16497 |doi-access=free |pmid=38303116 }}

Some herbivorous animals, such as rabbits, have a terminal vermiform appendix and cecum that apparently bear patches of tissue with immune functions and that may also be important in maintaining the composition of intestinal flora. It does not seem to have much digestive function, if any, and is not present in all herbivores, even those with large caeca.{{cite book |author1=Stevens, C. Edward |author2=Hume, Ian | title = Comparative Physiology of the Vertebrate Digestive System | publisher = Cambridge University Press | location = Cambridge | year = 2004 | isbn = 978-0-521-61714-7 }} As shown in the accompanying pictures, the human appendix typically is about comparable to that of the rabbit's in size, though the caecum is reduced to a single bulge where the ileum empties into the colon. Some carnivorous animals have appendices too, but few have more than vestigial caeca.Peter Robert Cheeke, Ellen S. Dierenfeld, Comparative Animal Nutrition and Metabolism. Publisher: CABI; 2010 {{ISBN|978-1-84593-631-0}} In line with the possibility that vestigial organs develop new functions, some research suggests that the appendix may guard against the loss of symbiotic bacteria that aid in digestion, though that is unlikely to be a novel function, given the presence of vermiform appendices in many herbivores.{{cite web|url=http://www.nbcnews.com/id/21153898|archive-url=https://web.archive.org/web/20130307074605/http://www.nbcnews.com/id/21153898|url-status=dead|archive-date=7 March 2013|title=Appendix may be useful after all – Health – Health care – More health news – NBC News|publisher=NBC News}}{{cite journal |last1=Randal Bollinger |first1=R. |last2=Barbas |first2=Andrew S. |last3=Bush |first3=Errol L. |last4=Lin |first4=Shu S. |last5=Parker |first5=William |title=Biofilms in the large bowel suggest an apparent function of the human vermiform appendix |journal=Journal of Theoretical Biology |volume=249 |issue=4 |pages=826–31 |year=2007 |pmid=17936308 |doi=10.1016/j.jtbi.2007.08.032|bibcode=2007JThBi.249..826R }} Intestinal bacterial populations entrenched in the appendix may support quick reestablishment of the flora of the large intestine after an illness, poisoning, or after an antibiotic treatment depletes or otherwise causes harmful changes to the bacterial population of the colon.Charles Q. Choi, "The Appendix: Useful and in Fact Promising", Live Science, 2009, [http://www.livescience.com/10571-appendix-fact-promising.html Appendix has useful function]

A 2013 study refutes the idea of an inverse relationship between cecum size and appendix size and presence. It is widely present in Euarchontoglires (a superorder of mammals that includes rodents, lagomorphs and primates) and has also evolved independently in the diprotodont marsupials and monotremes, and is highly diverse in size and shape, which could suggest it is not vestigial. Researchers deduce that the appendix has the ability to protect good bacteria in the gut: when the gut is affected by diarrhea or another illness that cleans out the intestines, the good bacteria in the appendix can repopulate the digestive system and keep the person healthy.{{cite journal |doi=10.1111/j.1420-9101.2009.01809.x |pmid=19678866 |title=Comparative anatomy and phylogenetic distribution of the mammalian cecal appendix |journal=Journal of Evolutionary Biology |volume=22 |issue=10 |pages=1984–99 |year=2009 |last1=Smith |first1=H. F. |last2=Fisher |first2=R. E. |last3=Everett |first3=M. L. |last4=Thomas |first4=A. D. |last5=Randal Bollinger |first5=R. |last6=Parker |first6=W. |doi-access=free }}

The coccyx, or tailbone, is the remnant of a lost tail.{{Citation|title=Spectrum of human tails: A report of six cases |author=Biswanath Mukhopadhyay, Ram M. Shukla, Madhumita Mukhopadhyay, Kartik C. Mandal, Pankaj Haldar, & Abhijit Benare |journal=Journal of Indian Association of Pediatric Surgeons |year=2012 |volume=17 |issue=1 |pages=23–25 |doi=10.4103/0971-9261.91082 |pmid=22279360 |pmc=3263034 |doi-access=free }} All mammals have a tail at some point in their development; in humans, it is present for a period of 4 weeks, during stages 14 to 22 of human embryogenesis.{{cite journal |doi=10.1016/S0940-9602(11)80496-6 |pmid=8059973 |title=Morphological and immunohistochemical characteristics of axial structures in the transitory human tail |journal=Annals of Anatomy - Anatomischer Anzeiger |volume=176 |issue=3 |pages=277–86 |year=1994 |last1=Saraga-Babić |first1=Mirna |last2=Lehtonen |first2=Eero |last3=Švajger |first3=Anton |last4=Wartiovaara |first4=Jorma }} This tail is most prominent in human embryos 31–35 days old.{{cite journal |doi=10.1002/aja.1001520108 |pmid=677043 |title=Evidence of a role for cell death in the disappearance of the embryonic human tail |journal=American Journal of Anatomy |volume=152 |issue=1 |pages=111–29 |year=1978 |last1=Fallon |first1=John F. |last2=Simandl |first2=B. Kay }} The tailbone, at the end of the spine, has lost its original function in assisting balance and mobility, though it still serves some secondary functions, such as being an attachment point for muscles, which explains why it has not degraded further.

In rare cases, congenital defect results in a short tail-like structure being present at birth. Twenty-three cases of human babies born with such a structure have been reported in the medical literature since 1884.{{cite journal |doi=10.1016/S0046-8177(84)80079-9 |pmid=6373560 |title=Human tails and pseudotails |journal=Human Pathology |volume=15 |issue=5 |pages=449–53 |year=1984 |last1=Dao |first1=Anh H. |last2=Netsky |first2=Martin G. }}{{cite journal |doi=10.1097/00000637-198804000-00009 |pmid=3284435 |title=Detailing the Human Tail |journal=Annals of Plastic Surgery |volume=20 |issue=4 |pages=340–4 |year=1988 |last1=Dubrow |first1=Terry J. |last2=Wackym |first2=Phillip Ashley |last3=Lesavoy |first3=Malcolm A. }} In these cases, the spine and skull were determined to be entirely normal. The only abnormality was that of a tail approximately 12 centimeters long. These tails, though of no deleterious effect, were almost always surgically removed.{{cite journal |doi=10.3171/jns.1985.63.3.0461 |pmid=3894599 |title=The human tail: A benign stigma |journal=Journal of Neurosurgery |volume=63 |issue=3 |pages=461–2 |year=1985 |last1=Spiegelmann |first1=Roberto |last2=Schinder |first2=Edgardo |last3=Mintz |first3=Mordejai |last4=Blakstein |first4=Alexander }}

Wisdom teeth are vestigial third molars that human ancestors used to help in grinding down plant tissue. The common postulation is that their skulls had larger jaws with more teeth, which were possibly used to help chew down foliage to compensate for a lack of ability to efficiently digest the cellulose that makes up a plant cell wall. As human diets changed, smaller jaws were naturally selected, but the third molars, or "wisdom teeth", still commonly develop in human mouths.Johnson, Dr. George B. [http://www.txtwriter.com/backgrounders/Evolution/EVpage12.html "Evidence for Evolution"] {{webarchive|url=https://web.archive.org/web/20080310041049/http://txtwriter.com/Backgrounders/Evolution/EVpage12.html |date=10 March 2008 }}. (Page 12) Txtwriter Inc. 8 June 2006.

Agenesis (failure to develop) of wisdom teeth in human populations ranges from zero in Tasmanian Aboriginals to nearly 100% in indigenous Mexicans.{{cite journal |last1=Rozkovcová |first1=E |last2=Marková |first2=M |last3=Dolejsí |first3=J |title=Studies on agenesis of third molars amongst populations of different origin |journal=Sbornik Lekarsky |volume=100 |issue=2 |pages=71–84 |year=1999 |pmid=11220165}} The difference is related to the PAX9 gene (and perhaps other genes).{{cite journal |doi=10.1073/pnas.0509562103 |pmid=16585527 |pmc=1458632 |title=Natural selection and molecular evolution in primate PAX9 gene, a major determinant of tooth development |journal=Proceedings of the National Academy of Sciences |volume=103 |issue=15 |pages=5676–81 |year=2006 |last1=Pereira |first1=T. V. |last2=Salzano |first2=F. M. |last3=Mostowska |first3=A. |last4=Trzeciak |first4=W. H. |last5=Ruiz-Linares |first5=A. |last6=Chies |first6=J. A. B. |last7=Saavedra |first7=C. |last8=Nagamachi |first8=C. |last9=Hurtado |first9=A. M. |last10=Hill |first10=K. |last11=Castro-De-Guerra |first11=D. |last12=Silva-Junior |first12=W. A. |last13=Bortolini |first13=M.-C. |bibcode=2006PNAS..103.5676P |doi-access=free }}

In some animals, the vomeronasal organ (VNO) is part of a second, completely separate sense of smell, known as the accessory olfactory system. Many studies have been performed to find if there is an actual presence of a VNO in adult human beings. Trotier et al.{{cite journal |doi=10.1093/chemse/25.4.369 |pmid=10944499 |title=The Vomeronasal Cavity in Adult Humans |journal=Chemical Senses |volume=25 |issue=4 |pages=369–80 |year=2000 |last1=Trotier |first1=D. |last2=Eloit |first2=C |last3=Wassef |first3=M |last4=Talmain |first4=G |last5=Bensimon |first5=J. L. |last6=Døving |first6=K. B. |last7=Ferrand |first7=J |doi-access=free }} estimate that around 92% of their subjects who had not had septal surgery had at least one intact VNO. Kjaer and Fisher Hansen, on the other hand,{{cite journal |doi=10.1111/j.1600-0722.1996.tb00043.x |pmid=8653495 |title=The human vomeronasal organ: Prenatal developmental stages and distribution of luteinizing hormone-releasing hormone |journal=European Journal of Oral Sciences |volume=104 |issue=1 |pages=34–40 |year=1996 |last1=Kjær |first1=Inger |last2=Hansen |first2=Birgit Fischer }} found that the VNO structure disappeared during fetal development as it does for some primates.{{cite journal |doi=10.1002/ar.1152 |pmid=11519019 |title=Reappraisal of the vomeronasal system of catarrhine primates: Ontogeny, morphology, functionality, and persisting questions |journal=The Anatomical Record |volume=265 |issue=4 |pages=176–192 |year=2001 |last1=Smith |first1=Timothy D. |last2=Siegel |first2=Michael I. |last3=Bhatnagar |first3=Kunwar P. |doi-access=free }} Smith and Bhatnagar (2000){{cite journal |doi=10.1046/j.1469-7580.2000.19730421.x |pmid=11117628 |pmc=1468143 |title=The human vomeronasal organ. Part II: Prenatal development |journal=Journal of Anatomy |volume=197 |issue=3 |pages=421–436 |year=2000 |last1=Smith |first1=Timothy D. |last2=Bhatnagar |first2=Kunwar P. }} asserted that Kjaer and Fisher Hansen simply missed the structure in older fetuses. Won (2000) found evidence of a VNO in 13 of his 22 cadavers (59.1%) and in 22 of his 78 living patients (28.2%).{{cite journal |last1=Won |first1=J |last2=Mair |first2=E. A. |last3=Bolger |first3=W. E. |last4=Conran |first4=R. M. |title=The vomeronasal organ: an objective anatomic analysis of its prevalence |journal=Ear, Nose, & Throat Journal |volume=79 |issue=8 |pages=600–605 |year=2000 |pmid=10969469|doi=10.1177/014556130007900814 |doi-access=free }} Given these findings, some scientists have argued that there is a VNO in adult human beings.{{cite journal |last1=Johnson |first1=A. |last2=Josephson |first2=R. |last3=Hawke |first3=M. |title=Clinical and histological evidence for the presence of the vomeronasal (Jacobson's) organ in adult humans |journal=The Journal of Otolaryngology |volume=14 |issue=2 |pages=71–79 |year=1985 |pmid=4068105}}{{cite journal |first1=René |last1=Foltán |first2=Jiří |title=Behavioral changes of patients after orthognathic surgery develop on the basis of the loss of vomeronasal organ: a hypothesis |journal=Head & Face Medicine |volume=5 |pages=5 |year=2009 |last2=Šedý |pmid=19161592 |pmc=2653472 |doi=10.1186/1746-160X-5-5 |doi-access=free }} Most have sought to identify the opening of the vomeronasal organ in humans, rather than identify the tubular epithelial structure itself.{{cite journal |last1=Bhatnagar |first1=Kunwar P. |last2=Smith |first2=Timothy D. |title=The human vomeronasal organ. III. Postnatal development from infancy to the ninth decade |journal=Journal of Anatomy |volume=199 |issue=Pt 3 |pages=289–302 |year=2001 |pmid=11554506 |pmc=1468331 |doi=10.1046/j.1469-7580.2001.19930289.x }} Thus it has been argued that such studies, employing macroscopic observational methods, have sometimes missed or even misidentified the vomeronasal organ.{{cite journal |last1=Bhatnagar |first1=Kunwar P. |last2=Kennedy |first2=Ray C. |last3=Baron |first3=Georg |last4=Greenberg |first4=Richard A. |title=Number of mitral cells and the bulb volume in the aging human olfactory bulb: A quantitative morphological study |journal=The Anatomical Record |volume=218 |issue=1 |pages=73–87 |year=1987 |pmid=3605663 |doi=10.1002/ar.1092180112 |s2cid=25630359 }}

Among studies that use microanatomical methods, there is no reported evidence that human beings have active sensory neurons like those in other animals' working vomeronasal systems.{{Cite book|last1=Witt |first1=M. |last2=Hummel |first2=T. |title=Vomeronasal Versus Olfactory Epithelium: Is There a Cellular Basis for Human Vomeronasal Perception? |volume=248 |pages=209–259 |year=2006 |pmid=16487792 |doi=10.1016/S0074-7696(06)48004-9 |series=International Review of Cytology |isbn=978-0-12-364652-1}} Furthermore, no evidence suggests there are nerve and axon connections between any existing sensory receptor cells in the adult human VNO and the brain.{{cite journal |vauthors=Wysocki CJ, Preti G |title=Facts, fallacies, fears, and frustrations with human pheromones |journal=The Anatomical Record Part A: Discoveries in Molecular, Cellular, and Evolutionary Biology |volume=281 |issue=1 |pages=1201–1211 |date=November 2004 |pmid=15470677 |doi= 10.1002/ar.a.20125|doi-access=free }} Likewise, there is no evidence of any accessory olfactory bulb in adult human beings, and the key genes involved in other mammals' VNO function have become pseudogenes in human beings. Therefore, while the presence of a structure in adult human beings is debated, a review of the scientific literature by Tristram Wyatt concluded, "most in the field ... are sceptical about the likelihood of a functional VNO in adult human beings on current evidence."{{cite book |last=Wyatt |first=Tristram D. |year=2003 |title=Pheromones and Animal Behaviour: Communication by Smell and Taste |url=https://archive.org/details/pheromonesanimal0000wyat |url-access=registration |location=Cambridge |publisher=Cambridge University Press |isbn=978-0-521-48526-5 |page=[https://archive.org/details/pheromonesanimal0000wyat/page/295 295]}}

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| footer = Left: Muscles of the human ear.
Right: The non-vestigial auricular muscle in the donkey can help it to move its ears like antennae.}}

The ears of a macaque monkey and most other monkeys have far more developed muscles than those of humans, and therefore have the capability to move their ears to better hear potential threats.Prof. A. Macalister, Annals and Magazine of Natural History, vol. vii., 1871, p. 342. Humans and other primates such as the orangutan and chimpanzee however have ear muscles that are minimally developed and non-functional, yet still large enough to be identifiable. A muscle attached to the ear that cannot move the ear, for whatever reason, can no longer be said to have any biological function. In humans there is variability in these muscles, such that some people are able to move their ears in various directions, and it can be possible for others to gain such movement by repeated trials.{{cite journal|last1=Bair|first1=J. H.|title=Development of voluntary control|journal=Psychological Review|year=1901|volume=8|issue=5|pages=474–510|doi=10.1037/h0074157|hdl=2027/mdp.39015070189314|url=https://zenodo.org/record/1429153}} In such primates, the inability to move the ear is compensated mainly by the ability to turn the head on a horizontal plane, an ability which is not common to most monkeys—a function once provided by one structure is now replaced by another.Mr. St. George Mivart, Elementary Anatomy, 1873, p. 396.

The outer structure of the ear also shows some vestigial features, such as the node or point on the helix of the ear known as Darwin's tubercle which is found in around 10% of the population.

The plica semilunaris is a small fold of tissue on the inside corner of the eye. It is the vestigial remnant of the nictitating membrane, i.e., third eyelid, an organ that is fully functional in some other species of mammals.Owen, R. 1866–1868. Comparative Anatomy and Physiology of Vertebrates. London.{{page needed|date=June 2017}} Its associated muscles are also vestigial. Only one species of primate, the Calabar angwantibo, is known to have a functioning nictitating membrane.{{cite journal |doi=10.1002/ajpa.1330250307 |pmid=5971502 |title=The skin of primates. XXXIII. The skin of the angwantibo (Arctocebus calabarensis) |journal=American Journal of Physical Anthropology |volume=25 |issue=3 |pages=277–90 |year=1966 |last1=Montagna |first1=W. |last2=Machida |first2=H. |last3=Perkins |first3=E. M. }}

The orbitalis muscle is a vestigial or rudimentary nonstriated muscle (smooth muscle) of the eye that crosses from the infraorbital groove and sphenomaxillary fissure and is intimately united with the periosteum of the orbit. It was described by Johannes Peter Müller and is often called Müller's muscle. The muscle forms an important part of the lateral orbital wall in some animals, but in humans it is not known to have any significant function.{{cite journal |pmid=653491 |year=1978 |last1=Toerien |first1=M. J. |title=The orbital muscle of Müller |journal=South African Medical Journal |volume=53 |issue=4 |pages=139–41 |last2=Gous |first2=A. E. }}Dutton, J.J., Atlas of Clinical and Surgical Orbital Anatomy, 2nd Edition, Elsevier, 2011. p.116-117.

{{Main|List of homologues of the human reproductive system}}

In the internal genitalia of each human sex, there are some residual organs of mesonephric and paramesonephric ducts during embryonic development:

• Gartner's duct
• Epoophoron
• Vesicular appendages of epoophoron
• Paroophoron

Human vestigial structures also include leftover embryological remnants that once served a function during development, such as the belly button, and analogous structures between biological sexes. For example, men are also born with two nipples, which are not known to serve a function compared to women."Breast Anatomy and Embryology". Essentials of Plastic Surgery (2015): 355–361 In regards to genitourinary development, both internal and external genitalia of male and female fetuses have the ability to fully or partially form their analogous phenotype of the opposite biological sex if exposed to a lack/overabundance of androgens or the SRY gene during fetal development.{{cite journal |doi=10.1016/S0022-3476(06)80172-1 |pmid=7965425 |title=True hermaphroditism: Genetic variants and clinical management |journal=The Journal of Pediatrics |volume=125 |issue=5 |pages=738–744 |year=1994 |last1=Hadjiathanasiou |first1=C.G. |last2=Brauner |first2=R. |last3=Lortat-Jacob |first3=S. |last4=Nivot |first4=S. |last5=Jaubert |first5=F. |last6=Fellous |first6=M. |last7=Nihoul-Fékété |first7=C. |last8=Rappaport |first8=R. }}{{cite journal |doi=10.4274/jcrpe.2495 |pmid=26761946 |pmc=5096479 |title=Homozygous Ala65Pro Mutation with V89L Polymorphism in SRD5A2 Deficiency |journal=Journal of Clinical Research in Pediatric Endocrinology |volume=8 |issue=2 |pages=218–223 |year=2016 |last1=Eren |first1=Erdal |last2=Edgünlü |first2=Tuba |last3=Asut |first3=Emre |last4=Karakaş Çelik |first4=Sevim }} Examples of vestigial remnants of genitourinary development include the hymen, which is a membrane that surrounds or partially covers the external vaginal opening that derives from the sinus tubercle during fetal development and is homologous to the male seminal colliculus.{{cite book |doi=10.1007/174_2010_128 |chapter=Embryology of the Female Reproductive Tract |title=Imaging of Gynecological Disorders in Infants and Children |pages=21–30 |series=Medical Radiology |year=2010 |last1=Healey |first1=Andrew |isbn=978-3-540-85601-6 }} Some researchers{{who|date=January 2013}} have hypothesized that the persistence of the hymen may be to provide temporary protection from infection, as it separates the vaginal lumen from the urogenital sinus cavity during development.{{cite journal |doi=10.1016/j.jpag.2008.03.009 |pmid=19646660 |title=Hymen Sparing Surgery for Imperforate Hymen: Case Reports and Review of Literature |journal=Journal of Pediatric and Adolescent Gynecology |volume=22 |issue=4 |pages=e61–64 |year=2009 |last1=Basaran |first1=Mustafa |last2=Usal |first2=Deniz |last3=Aydemir |first3=Cumhur }}

A number of muscles in the human body are thought to be vestigial, either by virtue of being greatly reduced in size compared to homologous muscles in other species, by having become principally tendonous, or by being highly variable in their frequency within or between populations.

The occipitalis minor is a muscle in the back of the head which normally joins to the auricular muscles of the ear. This muscle is very sporadic in frequency—always present in Malays, present in 56% of Africans, 50% of Japanese, and 36% of Europeans, and nonexistent in the Khoekhoe people of southwestern Africa and in Melanesians.{{cite journal |jstor=30079154 |first1=Alexander |last1=Macalister |year=1875 | title =Additional Observations on Muscular Anomalies in Human Anatomy. (Third Series) With a Catalogue of the Principal Muscular Variations Hitherto Published |journal=The Transactions of the Royal Irish Academy |volume=25 |pages=1–134 }} Other small muscles in the head associated with the occipital region and the post-auricular muscle complex are often variable in their frequency.{{cite journal |doi=10.1001/archfaci.6.5.342 |pmid=15381582 |title=Variability of the Postauricular Muscle Complex |journal=Archives of Facial Plastic Surgery |volume=6 |issue=5 |pages=342–7 |year=2004 |last1=Guerra |first1=Aldo Benjamin |last2=Metzinger |first2=Stephen Eric |last3=Metzinger |first3=Rebecca Crawford |last4=Xie |first4=Chen |last5=Xie |first5=Yue |last6=Rigby |first6=Peter Lister |last7=Naugle |first7=Thomas |doi-access=free }}

The platysma, a quadrangular (four sides) muscle in a sheet-like configuration, is a vestigial remnant of the panniculus carnosus of animals. In horses, it is the muscle that allows it to flick a fly off its back.{{cn|date=November 2023}}

In many animals, the upper lip and sinus area is associated with whiskers or vibrissae which serve a sensory function. In humans, these whiskers do not exist but there are still sporadic cases where elements of the associated vibrissal capsular muscles or sinus hair muscles can be found. Based on histological studies of the upper lips of 20 cadavers, Tamatsu et al. found that structures resembling such muscles were present in 35% (7/20) of their specimens.{{cite journal |doi=10.1002/ca.20497 |pmid=17458869 |title=Vestiges of vibrissal capsular muscles exist in the human upper lip |journal=Clinical Anatomy |volume=20 |issue=6 |pages=628–31 |year=2007 |last1=Tamatsu |first1=Yuichi |last2=Tsukahara |first2=Kazue |last3=Hotta |first3=Mitsuyuki |last4=Shimada |first4=Kazuyuki |s2cid=21055062 }}

The palmaris longus muscle is seen as a small tendon between the flexor carpi radialis and the flexor carpi ulnaris, although it is not always present. The muscle is absent in about 14% of the population, however this varies greatly with ethnicity. It is believed that this muscle actively participated in the arboreal locomotion of primates, but currently has no function, because it does not provide more grip strength.{{cite journal |doi=10.1155/2014/178906 |pmid=24860810 |pmc=4016873 |title=Morphometric and Statistical Analysis of the Palmaris Longus Muscle in Human and Non-Human Primates |journal=BioMed Research International |volume=2014 |pages=1–6 |year=2014 |last1=Aversi-Ferreira |first1=Roqueline A. G. M. F. |last2=Bretas |first2=Rafael Vieira |last3=Maior |first3=Rafael Souto |last4=Davaasuren |first4=Munkhzul |last5=Paraguassú-Chaves |first5=Carlos Alberto |last6=Nishijo |first6=Hisao |last7=Aversi-Ferreira |first7=Tales Alexandre |doi-access=free }} One study has shown the prevalence of palmaris longus agenesis in 500 Indian patients to be 17.2% (8% bilateral and 9.2% unilateral).{{cite journal |doi=10.1111/j.1447-073X.2007.00199.x |pmid=18402087 |title=Clinical relevance of palmaris longus agenesis: Common anatomical aberration |journal=Anatomical Science International |volume=83 |issue=1 |pages=45–8 |year=2008 |last1=Kapoor |first1=Sudhir K. |last2=Tiwari |first2=Akshay |last3=Kumar |first3=Abhishek |last4=Bhatia |first4=Rajesh |last5=Tantuway |first5=Vinay |last6=Kapoor |first6=Saurabh |s2cid=25324691 }} The palmaris is a popular source of tendon material for grafts and this has prompted studies which have shown the absence of the palmaris does not have any appreciable effect on grip strength.{{cite journal |doi=10.1016/j.jhsb.2005.03.011 |pmid=15935531 |title=Does the absence of the palmaris longus affect grip and pinch strength? |journal=The Journal of Hand Surgery: Journal of the British Society for Surgery of the Hand |volume=30 |issue=4 |pages=406–8 |year=2005 |last1=Sebastin |first1=S |last2=Lim |first2=A |last3=Bee |first3=W |last4=Wong |first4=T |last5=Methil |first5=B |s2cid=35394120 }}

The levator claviculae muscle in the posterior triangle of the neck is a supernumerary muscle present in only 2–3% of all people{{cite journal |date=April 1999 |last1=Rubinstein |first1= David |last2=Escott |first2=Edward J. |last3=Hendrick |first3=Laura L. |title=The prevalence and CT appearance of the levator claviculae muscle: a normal variant not to be mistaken for an abnormality |journal= AJNR Am J Neuroradiol |volume=20 |issue=4 |pages=583–6 |pmid=10319965 |pmc=7056035 |url=http://www.ajnr.org/cgi/reprint/20/4/583.pdf}} but nearly always present in most mammalian species, including gibbons and orangutans.{{cite journal |last1=Loukas |first1=M. |last2=Sullivan |first2=A. |last3=Tubbs |first3=R.S. |last4=Shoja |first4=M.M. |title=Levator claviculae: a case report and review of the literature |journal=Folia Morphol. |year=2008 |volume=67 |issue=4 |pages=307–310 |pmid=19085875 |url=http://www.fm.viamedica.pl/zamow_art_pdf.phtml?id=46&indeks_art=781}}

The pyramidalis muscle of the abdomen is a small and triangular muscle, anterior to the rectus abdominis, and contained in the rectus sheath. It is absent in 20% of humans and when absent, the lower end of the rectus then becomes proportionately increased in size. Anatomical studies suggest that the forces generated by the pyramidalis muscles are relatively small.{{cite journal |doi=10.1111/j.1447-073X.2007.00226.x |pmid=19159363 |pmc=3531545 |title=Architecture and fiber type of the pyramidalis muscle |journal=Anatomical Science International |volume=83 |issue=4 |pages=294–7 |year=2008 |last1=Lovering |first1=Richard M. |last2=Anderson |first2=Larry D. }}

The latissimus dorsi muscle of the back has several sporadic variations. One particular variant is the existence of the dorsoepitrochlearis or latissimocondyloideus muscle which is a muscle passing from the tendon of the latissimus dorsi to the long head of the triceps brachii. It is notable due to its well developed character in other apes and monkeys, where it is an important climbing muscle, namely the dorsoepitrochlearis brachii.{{cite journal |doi=10.1002/ca.20799 |pmid=19373904 |title=Development and clinical significance of the musculus dorsoepitrochlearis in men |journal=Clinical Anatomy |volume=22 |issue=4 |pages=481–8 |year=2009 |last1=P. |first1=Haninec |last2=R. |first2=Tomáš |last3=R. |first3=Kaiser |last4=R. |first4=Čihák |s2cid=221547558 }}Edwards, William E., The Musculoskeletal Anatomy of the Thorax and Brachium of an Adult Female Chimpanzee,6571st Aeromedical Research Laboratory, New Mexico, 1965. http://www.dtic.mil/dtic/tr/fulltext/u2/462433.pdf {{Webarchive|url=https://web.archive.org/web/20170211002121/http://www.dtic.mil/dtic/tr/fulltext/u2/462433.pdf |date=11 February 2017 }} This muscle is found in ≈5% of humans.{{Cite web|url=http://www.anatomyatlases.org/AnatomicVariants/MuscularSystem/Text/L/06Latissimus.shtml|title=Anatomy Atlases: Illustrated Encyclopedia of Human Anatomic Variation: Opus I: Muscular System: Alphabetical Listing of Muscles: L:Latissimus Dorsi}}

The plantaris muscle is composed of a thin muscle belly and a long thin tendon. The muscle belly is approximately {{convert|5|-|10|cm|inch|abbr=off|sigfig=1}} long, and is absent in 7–10% of the human population. It has some weak functionality in moving the knee and ankle but is generally considered redundant and is often used as a source of tendon for grafts. The long, thin tendon of the plantaris is humorously called "the freshman's nerve", as it is often mistaken for a nerve by new medical students.

Another example of human vestigiality occurs in the tongue, specifically the chondroglossus muscle. In a morphological study of 100 Japanese cadavers, it was found that 86% of fibers identified were solid and bundled in the appropriate way to facilitate speech and mastication. The other 14% of fibers were short, thin and sparse – nearly useless, and thus concluded to be of vestigial origin.{{cite journal |doi=10.1016/S0940-9602(02)80087-5 |pmid=12392330 |title=Morphological study of the human chondroglossus muscle in Japanese |journal=Annals of Anatomy - Anatomischer Anzeiger |volume=184 |issue=5 |pages=493–9 |year=2002 |last1=Ogata |first1=Shigemitsu |last2=Mine |first2=Kazuharu |last3=Tamatsu |first3=Yuichi |last4=Shimada |first4=Kazuyuki }}

Extra nipples or breasts sometimes appear along the mammary lines of humans, appearing as a remnant of mammalian ancestors who possessed more than two nipples or breasts.{{cite journal | last1 = Kajava | first1 = Y | year = 1915 | title = The proportions of supernumerary nipples in the Finnish population | journal = Duodecim | volume = 1 | pages = 143–70 }}{{cite journal |doi=10.4103/0971-6866.108051 |pmid=23716953 |pmc=3656534 |title=Seven nipples in a male: World′s second case report |journal=Indian Journal of Human Genetics |volume=18 |issue=3 |pages=373–5 |year=2012 |last1=Goyal |first1=Tarang |last2=Bakshi |first2=SK |last3=Varshney |first3=Anupam |doi-access=free }} One 2021 report demonstrated that all healthy young men and women who participated in an anatomic study of the front surface of the body exhibited 8 pairs of focal fat mounds running along the embryological mammary ridges from axillae to the upper inner thighs. These were always located in the same relative anatomic sites – analogous to the loci of breasts in other placental mammals – and often had nipple-like moles or extra hairs located atop the mounds. Therefore, focal fatty prominences on the fronts of human torsos likely represent chains of vestigial breasts composed of primordial breast fat.{{Cite journal |last1=Teplica |first1=David |last2=Kovich |first2=Grant |last3=Srock |first3=Jamey |last4=Whitaker |first4=Robert |last5=Jeffers |first5=Eileen |last6=Wagstaff |first6=David A. |date=2021-10-14 |title=Newly Identified Gross Human Anatomy: Eight Paired Vestigial Breast Mounds Run along the Embryological Mammary Ridges in Lean Adults |journal=Plastic and Reconstructive Surgery - Global Open |language=en |volume=9 |issue=10 |pages=e3863 |doi=10.1097/GOX.0000000000003863 |issn=2169-7574 |pmc=8517303 |pmid=34667697}}

{{See also | Vestigial response#In humans}}

Humans also bear some vestigial behaviors and reflexes.{{Citation|title=Remnants of evolution |author=Laura Spinney |journal=New Scientist |year=2008 |volume=198 |issue=2656 |pages=42–45 |doi=10.1016/S0262-4079(08)61231-2}}

File:Gaensehaut.jpg are an example of a vestigial human reaction to stress.]]

The formation of goose bumps in humans under stress is a vestigial reflex; a possible function in the distant evolutionary ancestors of humanity was to raise the body's hair, making the ancestor appear larger and scaring off predators.Darwin, Charles. (1872) The Expression of the Emotions in Man and Animals John Murray, London.{{Page needed|date=March 2011}} Raising the hair is also used to trap an extra layer of air, keeping an animal warm. Due to the diminished amount of hair in humans, the reflex formation of goose bumps when cold is also vestigial.

The palmar grasp reflex is thought to be a vestigial behavior in human infants. When placing a finger or object to the palm of an infant, it will securely grasp it. This grasp is found to be rather strong.{{cite book |title=Psychology |edition=fifth |author=Peter Gray |year=2007 |page=[https://archive.org/details/psychology0000gray/page/66 66] |publisher=Worth Publishers |isbn=978-0-7167-0617-5 |url=https://archive.org/details/psychology0000gray/page/66 }} Some infants—37% according to a 1932 study—are able to support their own weight from a rod,[https://books.google.com/books?id=OzL2Bgvf9Q0C&pg=PA122&dq=%22chancy+and+mcgraw+(31)+investigated+the+reflex+in+the+125+infants%22%22 Behavior Development in Infants] (via Google Books) by Evelyn Dewey, citing a study "Reflexes and other motor activities in newborn infants: a report of 125 cases as a preliminary study of infant behavior" published in the Bull. Neurol. Inst. New York, 1932, Vol. 2, pp. 1–56. although there is no way they can cling to their mother. The grasp is also evident in the feet. When a baby is sitting down, its prehensile feet assume a curled-in posture, similar to that observed in an adult chimp.{{cite book |title=Why Evolution is True |author=Jerry Coyne |year=2009 |pages=[https://archive.org/details/whyevolutionistr00coyn/page/85 85–86] |publisher=Penguin Group |isbn=978-0-670-02053-9 |url=https://archive.org/details/whyevolutionistr00coyn/page/85 }}{{cite book |title=Archetype: A Natural History of the Self |author=Anthony Stevens |year=1982 |page=87 |publisher=Routledge & Kegan Paul |isbn=978-0-7100-0980-7}} An ancestral primate would have had sufficient body hair to which an infant could cling, unlike modern humans, thus allowing its mother to escape from danger, such as climbing up a tree in the presence of a predator without having to occupy her hands holding her baby.

It has been proposed that the hiccup is an evolutionary remnant of earlier amphibian respiration.{{cite journal |doi=10.1002/bies.10224 |pmid=12539245 |title=A phylogenetic hypothesis for the origin of hiccough |journal=BioEssays |volume=25 |issue=2 |pages=182–8 |year=2003 |last1=Straus |first1=C. |last2=Vasilakos |first2=K. |last3=Wilson |first3=R. J. A. |last4=Oshima |first4=T. |last5=Zelter |first5=M. |last6=Derenne |first6=J-Ph. |last7=Similowski |first7=T. |last8=Whitelaw |first8=W. A. }} Amphibians such as tadpoles gulp air and water across their gills via a rather simple motor reflex akin to mammalian hiccuping. The motor pathways that enable hiccuping form early during fetal development, before the motor pathways that enable normal lung ventilation form. Additionally, hiccups and amphibian gulping are inhibited by elevated CO{{Sub2}} and may be stopped by GABA_B receptor agonists, illustrating a possible shared physiology and evolutionary heritage. These proposals may explain why premature infants spend 2.5% of their time hiccuping, possibly gulping like amphibians, as their lungs are not yet fully formed. Fetal intrauterine hiccups are of two types. The physiological type occurs before 28 weeks after conception and tend to last five to ten minutes. These hiccups are part of fetal development and are associated with the myelination of the phrenic nerve, which primarily controls the thoracic diaphragm. The phylogeny hypothesis explains how the hiccup reflex might have evolved, and if there is not an explanation, it may explain hiccups as an evolutionary remnant, held-over from our amphibious ancestors.

This hypothesis has been questioned because of the existence of the afferent loop of the reflex, the fact that it does not explain the reason for glottic closure, and because the very short contraction of the hiccup is unlikely to have a significant strengthening effect on the slow-twitch muscles of respiration.{{Citation needed|reason=Reliable source needed for most of this paragraph. Also, what is the second type of fetal intrauterine hiccups?|date=May 2015}}

{{Main|Pseudogene}}

There are many pseudogenes present in the human genome. One example of this is L-gulonolactone oxidase, a gene that is functional in most other mammals and produces an enzyme that synthesizes vitamin C.{{cite journal |doi=10.1016/S0304-4165(99)00123-3 |pmid=10572964 |title=Random nucleotide substitutions in primate nonfunctional gene for l-gulono-γ-lactone oxidase, the missing enzyme in l-ascorbic acid biosynthesis |journal=Biochimica et Biophysica Acta (BBA) - General Subjects |volume=1472 |issue=1–2 |pages=408–11 |year=1999 |last1=Ohta |first1=Yuriko |last2=Nishikimi |first2=Morimitsu }} In humans and other members of the suborder Haplorrhini, a mutation disabled the gene and made it unable to produce the enzyme. However, the remains of the gene are still present in the human genome.{{cite journal |vauthors=Nishikimi M, Fukuyama R, Minoshima S, Shimizu N, Yagi K |title=Cloning and chromosomal mapping of the human nonfunctional gene for L-gulono-gamma-lactone oxidase, the enzyme for L-ascorbic acid biosynthesis missing in man |journal=J. Biol. Chem. |volume=269 |issue=18 |pages=13685–8 |date=6 May 1994 |doi=10.1016/S0021-9258(17)36884-9 |pmid=8175804 |doi-access=free }}

• Color blindness
• Deprecation
• Myopia

{{Reflist}}

{{Wikisource|Descent of Man/Chapter I}}

• {{cite book|last=Shubin|first=Neil|author-link=Neil Shubin|title=Your Inner Fish: A Journey into the 3.5-Billion-Year History of the Human Body|year=2009|publisher=Vintage Books|location=New York|isbn=978-0-307-27745-9|url=https://books.google.com/books?id=c008kdNwR1cC}}

Category:Evolutionary biology concepts

Category:Human anatomy

Category:Human evolution

Category:Human physiology