Editing Hair (section)

==Evolution==
Hair has its origins in the common ancestor of mammals, the [[synapsid]]s, about 300 million years ago. It is currently unknown at what stage the synapsids acquired mammalian characteristics such as [[body hair]] and [[mammary gland]]s, as the [[fossil]]s only rarely provide direct evidence for soft tissues. Skin impression of the belly and lower tail of a [[pelycosaur]], possibly ''[[Haptodus]]'' shows the basal synapsid stock bore transverse rows of rectangular [[scute]]s, similar to those of a modern [[crocodile]], so the age of acquirement of hair logically could not have been earlier than ≈299 ma, based on the current understanding of the animal's phylogeny.<ref>{{cite journal|last1=Niedźwiedzki|first1=Grzegorz|last2=Bojanowski|first2=Maciej|title=A Supposed Eupelycosaur Body Impression from the Early Permian of the Intra-Sudetic Basin, Poland|journal=Ichnos|date=July 2012|volume=19|issue=3|pages=150–155|doi=10.1080/10420940.2012.702549|bibcode=2012Ichno..19..150N |s2cid=129567176}}</ref> An exceptionally well-preserved skull of ''[[Estemmenosuchus]]'', a [[therapsid]] from the [[Upper Permian]], shows smooth, hairless skin with what appears to be glandular depressions,<ref>Kardong, K.V. (2002): ''Vertebrates: Comparative anatomy, function, evolution. 3rd Edition.'' McGraw-Hill, New York</ref> though as a semi-aquatic species it might not have been particularly useful to determine the integument of terrestrial species. The oldest undisputed known fossils showing unambiguous imprints of hair are the [[Callovian]] (late middle [[Jurassic]]) ''[[Castorocauda]]'' and several contemporary [[haramiyida]]ns, both [[Mammaliformes|near-mammal]] [[cynodont]]s, giving the age as no later than ≈220 ma based on the modern phylogenetic understanding of these clades.<ref name="JiLuoYuanTabrumCastorocauda">{{Cite journal|author1=Q. Ji|author2=Z-X Luo|author3=C-X Yuan|last4=Tabrum|first4=A. R.|date=February 2006|title=A Swimming Mammaliaform from the Middle Jurassic and Ecomorphological Diversification of Early Mammals|journal=Science|volume=311|issue=5764|pages=1123–7|bibcode=2006Sci...311.1123J|doi=10.1126/science.1123026|pmid=16497926|s2cid=46067702|url=http://doc.rero.ch/record/13437/files/PAL_E249.pdf }} See also the news item at {{cite web|title=Jurassic "Beaver" Found; Rewrites History of Mammals|url=http://news.nationalgeographic.com/news/2006/02/0223_060223_beaver.html|url-status=dead|archive-url=https://web.archive.org/web/20120922055901/http://news.nationalgeographic.com/news/2006/02/0223_060223_beaver.html|archive-date=22 September 2012|access-date=12 August 2012}}</ref><ref>{{cite news|date=9 August 2013|title=Jurassic squirrel's secret is out|newspaper=The Hindu|url=http://www.thehindu.com/opinion/op-ed/jurassic-squirrels-secret-is-out/article5004252.ece|access-date=29 June 2016}}</ref><ref>{{cite journal | last1 = Meng | first1 = Qing-Jin | last2 = Grossnickle | first2 = David M. | last3 = Di | first3 = Liu | last4 = Zhang | first4 = Yu-Guang | last5 = Neander | first5 = April I. | last6 = Ji | first6 = Qiang | last7 = Luo | first7 = Zhe-Xi | year = 2017 | title = New gliding mammaliaforms from the Jurassic | journal = Nature | volume = 548 | issue = 7667 | pages = 291–296 | doi = 10.1038/nature23476 | pmid = 28792929 | bibcode = 2017Natur.548..291M | s2cid = 205259206 }}</ref> More recently, studies on terminal [[Permian]] [[Russia]]n [[coprolites]] may suggest that non-mammalian synapsids from that era had fur.<ref>{{cite journal | doi = 10.1111/let.12156 | title=Microbiota and food residues including possible evidence of pre-mammalian hair in Upper Permian coprolites from Russia | year=2015 | journal=Lethaia | volume=49 | issue=4 | pages=455–477 | last1 = Bajdek | first1 = Piotr}}</ref> If this is the case, these are the oldest hair remnants known, showcasing that fur occurred as far back as the latest [[Paleozoic]].

Some modern mammals have a special gland in front of each [[orbit (anatomy)|orbit]] used to preen the fur, called the [[harderian gland]]. Imprints of this structure are found in the skull of the small early mammals like ''[[Morganucodon]]'', but not in their [[cynodont]] ancestors like ''[[Thrinaxodon]]''.<ref>{{cite book|last1=Lingham-Soliar|first1=Theagarten|title=The vertebrate integument, Vol I|date=2014|publisher=Springer Berlin Heidelberg|location=Berlin, Heidelberg|isbn=978-3-642-53748-6|pages=211–212}}</ref>

The hairs of the fur in modern animals are all connected to nerves, and so the fur also serves as a transmitter for sensory input. Fur could have evolved from sensory hair (whiskers). The signals from this sensory apparatus is interpreted in the [[neocortex]], a section of the brain that expanded markedly in animals like ''Morganucodon'' and ''[[Hadrocodium]]''.<ref>{{cite journal|last1=Rowe|first1=T. B.|last2=Macrini|first2=T. E.|last3=Luo|first3=Z.-X.|title=Fossil Evidence on Origin of the Mammalian Brain|journal=Science|date=19 May 2011|volume=332|issue=6032|pages=955–957|doi=10.1126/science.1203117|pmid=21596988|bibcode=2011Sci...332..955R|s2cid=940501}}</ref> The more advanced therapsids could have had a combination of naked skin, [[whisker]]s, and [[scute]]s. A full [[pelage]] likely did not evolve until the therapsid-mammal transition.<ref name=Fur>{{cite journal| last1=Ruben|first1= J.A. |last2= Jones| first2= T.D. |year=2000|title= Selective Factors Associated with the Origin of Fur and Feathers |journal=Am. Zool.|volume= 40|pages= 585–596 | doi = 10.1093/icb/40.4.585| issue=4 |doi-access= free}}</ref> The more advanced, smaller therapsids could have had a combination of hair and scutes, a combination still found in some modern mammals, such as [[rodents]] and the [[opossum]].<ref>{{cite book|last=Plower|first=R.P.|url=https://archive.org/stream/cu31924001022684/cu31924001022684_djvu.txt|title=An introduction to the study of mammals living and extinct|publisher=Cornell University Library|year=1897|location=New York|page=11|quote=Flat scutes, with the edges in apposition, and not overlaid, clothe both surfaces of the tail of the beaver, rats, and others of the same order, and also of some insectivores and marsupials.|access-date=8 June 2012}}</ref>

The high interspecific variability of the size, color, and microstructure of hair often enables the identification of species based on single hair filaments.<ref name=Teerink2003>{{cite book|author=Teerink, BJ|title=Hair of West European Mammals: Atlas and Identification Key |url= https://books.google.com/books?id=exlhpOZ__BEC| date=2003|publisher=Cambridge University Press| isbn=9780521545778| pages=224}}</ref><ref name="TothM">{{cite book|author=Toth, Maria|url=http://www.hairatlas.hu/|title=Hair and fur atlas of Central European mammals|date=29 December 2017|publisher=Pars Ltd|isbn=978-963-88339-7-6|pages=307|access-date=8 July 2019}}</ref>

[[File:Nacktmull.jpg|thumb|[[Naked mole-rat]] (''Heterocephalus glaber'') in a zoo.]]

{{anchor|Glabrous|Glabrousness}}

In varying degrees most [[mammal]]s have some skin areas without natural hair. On the human body, glabrous [[human skin|skin]] is found on the [[ventral]] portion of the [[finger]]s, [[hand|palms]], [[sole (foot)|soles of feet]] and [[lips]], which are all parts of the body most closely associated with interacting with the world around us,<ref name ="Scholarpedia">{{Cite book|title=Scholarpedia of touch|isbn=978-94-6239-133-8|location=Paris|oclc=932171320|last1=Prescott|first1=Tony|last2=Ahissar|first2=Ehud|last3=Izhikevich|first3=Eugene|date=21 November 2015}}</ref> as are the [[labia minora]] and [[glans penis]].<ref>{{cite book|last1=Linden|first1=David, J.|title=Touch: The Science of Hand, Heart and Mind|date=March 2015|publisher=Viking|isbn=978-0241184035|chapter-url=https://books.google.com/books?id=XjZIBQAAQBAJ&pg=PT36|chapter=Chapter 2}}</ref> There are four main types of [[mechanoreceptor]]s in the glabrous skin of humans: [[lamellar corpuscle|Pacinian corpuscles]], [[tactile corpuscle|Meissner's corpuscles]], [[Merkel nerve ending|Merkel's discs]], and [[bulbous corpuscle|Ruffini corpuscles]].

The [[naked mole-rat]] (''Heterocephalus glaber'') has evolved skin lacking in general, pelagic hair covering, yet has retained long, very sparsely scattered tactile hairs over its body.<ref name ="Scholarpedia"/> Glabrousness is a trait that may be associated with [[neoteny]].<ref>{{Cite journal|last=Rebora|first=Alfredo|date=2010|title=Lucy's pelt: when we became hairless and how we managed to survive|journal=International Journal of Dermatology|language=en|volume=49|issue=1|pages=17–20|doi=10.1111/j.1365-4632.2009.04266.x|pmid=20465604|s2cid=21484729|issn=1365-4632}}</ref>

[[file:Vulpes vulpes sitting.jpg|thumb|The soft, fine hair found on many nonhuman mammals is typically called fur.<!-- moved ref to main text, above: name=Bergman2014 -->]]

===Evolutionary variation===
Primates are relatively hairless compared to other mammals, and [[Hominini]] such as chimpanzees, have less dense hair than would be expected given their body size for a primate.<ref name="Sandel 2013 pp. 145–150">{{cite journal |last=Sandel |first=Aaron A. |date=2013-07-30 |title=Brief communication: Hair density and body mass in mammals and the evolution of human hairlessness |journal=American Journal of Physical Anthropology |volume=152 |issue=1 |pages=145–150 |doi=10.1002/ajpa.22333 |pmid=23900811 |issn=0002-9483|hdl=2027.42/99654 |hdl-access=free }}</ref> Evolutionary biologists suggest that the genus ''[[Homo]]'' arose in [[East Africa]] approximately 2 million years ago.<ref name="Herries Martin Leece Adams 2020 p.">{{cite journal |last1=Herries |first1=Andy I. R. |last2=Martin |first2=Jesse M. |last3=Leece |first3=A. B. |last4=Adams |first4=Justin W. |last5=Boschian |first5=Giovanni |last6=Joannes-Boyau |first6=Renaud |last7=Edwards |first7=Tara R. |last8=Mallett |first8=Tom |last9=Massey |first9=Jason |last10=Murszewski |first10=Ashleigh |last11=Neubauer |first11=Simon |last12=Pickering |first12=Robyn |last13=Strait |first13=David S. |last14=Armstrong |first14=Brian J. |last15=Baker |first15=Stephanie |date=2020-04-03 |title=Contemporaneity of ''Australopithecus'' , ''Paranthropus'' , and early ''Homo erectus'' in South Africa |journal=Science |volume=368 |issue=6486 |page= |doi=10.1126/science.aaw7293 |issn=0036-8075 |last16=Caruana |first16=Matthew V. |last17=Denham |first17=Tim |last18=Hellstrom |first18=John |last19=Moggi-Cecchi |first19=Jacopo |last20=Mokobane |first20=Simon |last21=Penzo-Kajewski |first21=Paul |last22=Rovinsky |first22=Douglass S. |last23=Schwartz |first23=Gary T. |last24=Stammers |first24=Rhiannon C. |last25=Wilson |first25=Coen |last26=Woodhead |first26=Jon |last27=Menter |first27=Colin|pmid=32241925 |doi-access=free }}</ref> Part of this evolution was the development of [[Endurance running hypothesis|endurance running]]<ref name="Ruxton Wilkinson 2011 pp. 169–175">{{cite journal |last1=Ruxton |first1=Graeme D. |last2=Wilkinson |first2=David M. |year=2011 |title=Thermoregulation and endurance running in extinct hominins: Wheeler's models revisited |journal=Journal of Human Evolution |volume=61 |issue=2 |pages=169–175 |doi=10.1016/j.jhevol.2011.02.012 |pmid=21489604 |bibcode=2011JHumE..61..169R |issn=0047-2484}}</ref> and venturing out during the hot times of the day<ref name="Ruxton Wilkinson 2011 pp. 20965–20969">{{cite journal |last1=Ruxton |first1=Graeme D. |last2=Wilkinson |first2=David M. |date=2011-12-12 |title=Avoidance of overheating and selection for both hair loss and bipedality in hominins |journal=Proceedings of the National Academy of Sciences |volume=108 |issue=52 |pages=20965–20969 |doi=10.1073/pnas.1113915108 |pmid=22160694 |pmc=3248486 |bibcode=2011PNAS..10820965R |issn=0027-8424|doi-access=free }}</ref> that required efficient thermoregulation through [[perspiration]]. The loss of heat through [[heat of evaporation]] by means of [[sweat gland]]s is aided by air currents next to the skin surface, which are facilitated by the loss of body hair.<ref name="Jablonski06">{{cite book |author=Jablonski, Nina G. |url=https://books.google.com/books?id=EYi9S3VtIGsC&pg=PP13 |title=Skin: A Natural History |date=1 May 2008 |publisher=University of California Press |isbn=978-0-520-94170-0 |pages=13– |access-date=27 January 2016}}</ref>

Another factor in human evolution that also occurred in the prehistoric past was a preferential selection for [[neoteny]], particularly in females. The idea that adult humans exhibit certain neotenous (juvenile) features, not evinced in the other great apes, is about a century old. [[Louis Bolk]] made a long list of such traits,<ref>{{Cite book|last=Bolk|first=L.|year=1926|title=Das Problem der Menschwerdung|publisher=Fischer|location=Jena|language=de}}</ref> and [[Stephen Jay Gould]] published a short list in ''[[Ontogeny and Phylogeny (book)|Ontogeny and Phylogeny]]''.<ref>short-list of 25 characters reprinted in Gould, Stephen Jay (1977). ''[[Ontogeny and Phylogeny (book)|Ontogeny and phylogeny]]''. Harvard University Press. p. 357. {{ISBN|0674639413}}.</ref> In addition, [[paedomorphic]] characteristics in women are often [[Physical attractiveness#Determinants of female physical attractiveness|acknowledged as desirable by men]] in developed countries.<ref>{{cite journal |last1=Scott|first1= Isabel M.|date=7 October 2014 |title=Human preferences for sexually dimorphic faces may be evolutionarily novel |journal=Proceedings of the National Academy of Sciences of the United States of America|volume=111 |issue=40 |pages= 14388–14393|doi=10.1073/pnas.1409643111 |pmid= 25246593|pmc= 4210032|bibcode=2014PNAS..11114388S|doi-access= free}}</ref> For instance, [[vellus hair]] is a juvenile characteristic. However, while men develop longer, coarser, thicker, and darker [[terminal hair]] through [[sexual differentiation]], women do not, leaving their vellus hair visible.
{{further|Human evolutionary genetics}}

===Texture===

====<span id="curly"></span>Curly hair====
[[File:Yellow curly hair and scalp from body which had long black wig over hair. Parts of wig plait remains. From Egypt, Gurob, probably tomb 23. 18th-19th Dynasty. The Petrie Museum of Egyptian Archaeology, London.jpg|thumb|Yellow curly hair and scalp from body which had long black wig over hair. Parts of wig plait remains. From Egypt, Gurob, probably tomb 23. 18th–19th Dynasty. The Petrie Museum of Egyptian Archaeology, London]]
[[File:David Luiz ConfedCup2013Champions17.jpg|thumb|Man with curly hair ([[David Luiz]], Brazilian footballer)]]
[[File:PSM V52 D323 Global hair texture map.png|thumb|Global hair texture map]]
{{More citations needed section|date=August 2016}}
Jablonski<ref name=Jablonski06/> asserts head hair was evolutionarily advantageous for pre-humans to retain because it protected the scalp as they walked upright in the intense African (equatorial) [[UV light]]. While some might argue that, by this logic, humans should also express hairy shoulders because these body parts would putatively be exposed to similar conditions, the protection of the head, the seat of the brain that enabled humanity to become one of the most successful species on the planet (and which also is very vulnerable at birth) was arguably a more urgent issue (axillary hair in the underarms and groin were also retained as signs of sexual maturity). Sometime during the gradual process by which ''Homo erectus'' began a transition from furry skin to the naked skin expressed by ''Homo sapiens'', hair texture putatively gradually changed from straight hair{{Citation needed|date=September 2010}} (the condition of most mammals, including humanity's closest cousins—chimpanzees) to [[Afro-textured hair]] or 'kinky' (i.e. tightly coiled). This argument assumes that curly hair better impedes the passage of UV light into the body relative to straight hair (thus curly or coiled hair would be particularly advantageous for light-skinned hominids living at the equator).

It is substantiated by Iyengar's findings {{citation needed|date=November 2024}}(1998) that UV light can enter into straight human hair roots (and thus into the body through the skin) via the hair shaft. Specifically, the results of that study suggest that this phenomenon resembles the passage of light through fiber optic tubes (which do not function as effectively when kinked or sharply curved or coiled). In this sense, when hominids (i.e. ''Homo erectus'') were gradually losing their straight body hair and thereby exposing the initially pale skin underneath their fur to the sun, straight hair would have been an adaptive liability. By inverse logic, later, as humans traveled farther from Africa and/or the equator, straight hair may have (initially) evolved to aid the entry of UV light into the body during the transition from dark, UV-protected skin to paler skin.

Jablonski's assertions<ref name=Jablonski06/> suggest that the adjective "woolly" in reference to Afro-hair is a [[misnomer]] in connoting the high heat insulation derivable from the true wool of sheep. Instead, the relatively sparse density of Afro-hair, combined with its springy coils actually results in an airy, almost sponge-like structure that in turn, Jablonski argues,<ref name=Jablonski06/> more likely facilitates an increase in the circulation of cool air onto the scalp. Further, wet Afro-hair does not stick to the neck and scalp unless totally drenched and instead tends to retain its basic springy puffiness because it less easily responds to moisture and sweat than straight hair does. In this sense, the trait may enhance comfort levels in intense equatorial climates more than straight hair (which, on the other hand, tends to naturally fall over the ears and neck to a degree that provides slightly enhanced comfort levels in cold climates relative to tightly coiled hair).

Further, it is notable that the most pervasive expression of this hair texture can be found in sub-Saharan Africa; a region of the world that abundant genetic and paleo-anthropological evidence suggests, was the relatively recent (≈200,000-year-old) point of origin for modern humanity. In fact, although genetic findings (Tishkoff, 2009) suggest that sub-Saharan Africans are the most genetically diverse continental group on Earth, [[Afro-textured hair]] approaches ubiquity in this region. This points to a strong, long-term selective pressure that, in stark contrast to most other regions of the genomes of sub-Saharan groups, left little room for genetic variation at the determining loci. Such a pattern, again, does not seem to support human sexual aesthetics as being the sole or primary cause of this distribution.

[[File:Blackhair10.jpg|thumb|Straight black hair]]

==== The EDAR locus ====
A group of studies have recently shown that genetic patterns at the EDAR locus, a region of the modern human genome that contributes to hair texture variation among most individuals of East Asian descent, support the hypothesis that (East Asian) straight hair likely developed in this branch of the modern human lineage subsequent to the original expression of tightly coiled [[natural afro-hair]].<ref>{{cite journal|pmid=18065779|last1=Fujimoto|first1=A|year=2008|pages=835–43|issue=6|volume=17|last2=Kimura|first2=R|journal=Human Molecular Genetics|last3=Ohashi|first3=J|last4=Omi|first4=K|last5=Yuliwulandari|first5=R|last6=Batubara|first6=L|last7=Mustofa|first7=MS|last8=Samakkarn|first8=U|last9=Settheetham-Ishida|first9=W|last10=Ishida|first10=T.|last11=Morishita|first11=Y.|last12=Furusawa|first12=T.|last13=Nakazawa|first13=M.|last14=Ohtsuka|first14=R.|last15=Tokunaga|first15=K.|title=A scan for genetic determinants of human hair morphology: EDAR is associated with Asian hair thickness|doi=10.1093/hmg/ddm355|display-authors=8|doi-access=free}}</ref><ref>{{cite journal|pmid=18704500|last1=Fujimoto|first1=A|year=2008|pages=179–85|issue=2|volume=124|last2=Ohashi|first2=J|journal=Human Genetics|last3=Nishida|first3=N|last4=Miyagawa|first4=T|last5=Morishita|first5=Y|last6=Tsunoda|first6=T|last7=Kimura|first7=R|last8=Tokunaga|first8=K|title=A replication study confirmed the EDAR gene to be a major contributor to population differentiation regarding head hair thickness in Asia|url=http://www.tulips.tsukuba.ac.jp/dspace/bitstream/2241/103672/1/HG_124-2.pdf|doi=10.1007/s00439-008-0537-1|access-date=14 December 2010|archive-url=https://web.archive.org/web/20110205011119/http://www.tulips.tsukuba.ac.jp/dspace/bitstream/2241/103672/1/HG_124-2.pdf|archive-date=5 February 2011|url-status=dead|hdl=2241/103672|s2cid=20084816|hdl-access=free}}</ref><ref>{{cite journal|last1=Mou|first1=C|last2=Thomason|first2=HA|last3=Willan|first3=PM|last4=Clowes|first4=C|last5=Harris|first5=WE|last6=Drew|first6=CF|last7=Dixon|first7=J|last8=Dixon|first8=MJ|last9=Headon|first9=DJ|year=2008|title=Enhanced ectodysplasin-A receptor (EDAR) signaling alters multiple fiber characteristics to produce the East Asian hair form|url=https://www.pure.ed.ac.uk/ws/files/8264337/Mou_et_al_EDAR_and_Asian_Hair_2008_supplementary_material.pdf|journal=Human Mutation|volume=29|issue=12|pages=1405–11|doi=10.1002/humu.20795|pmid=18561327|hdl=20.500.11820/0b35a959-86c8-44e5-b100-20639dd6bbf1|s2cid=37696013|access-date=30 January 2019|hdl-access=free}}</ref> Specifically, the relevant findings indicate that the EDAR mutation coding for the predominant East Asian 'coarse' or thick, straight hair texture arose within the past ≈65,000 years, which is a time frame that covers from the earliest of the 'Out of Africa' migrations up to now.