This article is about the part of the face. For other uses, seeChin (disambiguation).
Photo showing the chin of a human skull
Thechin is the forward pointed part of the anterior mandible (mental region) below the lowerlip. A fully developed human skull has a chin of between 0.7 cm and 1.1 cm.
The presence of a well-developed chin is considered to be one of the morphological characteristics ofHomo sapiens thatdifferentiates them from other human ancestors such as the closely relatedNeanderthals.[1][2] Early human ancestors have variedsymphysial morphology, but none of them have a well-developed chin. The origin of the chin is traditionally associated with the anterior–posterior breadth shortening of thedental arch or tooth row; however, its general mechanical or functional advantage during feeding, developmental origin, and link with human speech, physiology, and social influence are highly debated.
Louis Robinson (1913)[3] suggests that the demand to resistmasticatory stresses triggered bone thickening in the mental region of themandible and ultimately formed a prominent chin. Moreover, David J. Daegling (1993)[4] explains the chin as a functional adaptation to resist masticatory stress that causes vertical bending stresses in thecoronal plane. Others have argued that the prominent chin is adapted to resisting wishboning forces,[5] dorso-ventral shear forces, and generally a mechanical advantage to resist lateral transverse bending and vertical bending in the coronal plane.[6] On the contrary, others[7] have suggested that the presence of the chin is not related to mastication. The presence of thick bone in the relatively small mandible may indicate better force resistance capacity. However, the question stands of whether the chin is anadaptive or nonadaptive structure.
Recent works on themorphological changes of the mandible during development[8][9][10] have shown that the human chin, or at least the inverted-T shaped mental region, develops during theprenatal period, but the chin does not become prominent until the earlypostnatal period. This later modification happens bybone remodeling processes (bone resorption andbone deposition).[11] Coquerelle et al.[9][10] show that the anteriorly positionedcervical column of the spine and forward displacement of thehyoid bone limit the anterior–posterior breadth in the oral cavity for the tongue,laryngeal, andsuprahyoid musculatures. Accordingly, this leads the upper parts of the mandible (alveolar process) to retract posteriorly, following the posterior movement of the upper tooth row, while the lower part of the symphysis remained protruded to create more space, thereby creating the inverted-T shaped mental relief during early ages and the prominent chin later. The alveolar region (upper or superior part of the symphysis) is sculpted by bone resorption, but the chin (lower or inferior part) is depository in its nature.[11] These coordinated bone growth and modeling processes mold the vertical symphysis present at birth into the prominent shape of the chin.
Recent research on the development of the chin[12] suggests that the evolution of this unique characteristic was formed not by mechanical forces such aschewing but by evolutionary adaptations involving reduction in size and change in shape of the face. Holton et al. claim that this adaptation occurred as the face became smaller compared to that of other ancient humans.
Robert Franciscus takes a more anthropological viewpoint: he believes that the chin was formed as a consequence of the change in lifestyle humans underwent approximately 80,000 years ago. As humans'hunter-gatherer societies grew into larger social networks, territorial disputes decreased because the new social structure promoted building alliances in order to exchange goods and belief systems. Franciscus believes that this change in the human environment reduced hormone levels, especially in men, resulting in the natural evolution of the chin.[13]
Overall, human beings are unique in the sense that they are the only species amongprimates who have chins. In the paperThe Enduring Puzzle of the Human Chin, evolutionary anthropologists James Pampush and David Daegling discuss various theories that have been raised to solve the puzzle of the chin. They conclude that "each of the proposals we have discussed falter either empirically or theoretically; some fail, to a degree, on both accounts… This should serve as motivation, not discouragement, for researchers to continue investigating this modern human peculiarity… perhaps understanding the chin will reveal some unexpected insight into what it means to be human."[14]
The terms cleft chin,[15] chin cleft,[15][16] dimple chin,[17][18] or chin dimple[15] refer to adimple on the chin. It is aY-shaped fissure on the chin with an underlying bony peculiarity.[19] Specifically, the chin fissure follows the fissure in the lowerjaw bone that resulted from the incomplete fusion of the left and right halves of the jaw bone, or muscle, during the embryonal and fetal development. It can also develop during the latermandibular symphysis, due to growth of the mental protuberance during puberty, or as a result ofacromegaly. In some cases, onemental tubercle may grow more than another, which can cause facial asymmetry.[15]
A cleft chin is aninherited trait in humans and can be influenced by many factors. The cleft chin is also a classic example of variablepenetrance[20] with environmental factors or amodifier gene possibly affecting thephenotypical expression of the actualgenotype. Cleft chins can be presented in a child when neither parent presents a cleft chin. Cleft chins are common among people originating from Europe, the Middle East and South Asia.[21]
There is a possible genetic cause for cleft chins, a genetic marker called rs11684042, which is located inchromosome 2.[22]
InPersian literature, the chin dimple is considered a factor of beauty and is metaphorically referred to as "the chin pit" or "the chin well": a well in which the poor lover is fallen and trapped.[23]
A double chin is a loss of definition of the jawbone or soft tissue under the chin. There are two possible causes for a double chin, which have to be differentiated.
In overweight people, commonly the layer ofsubcutaneous fat around theneck sags down and creates awrinkle, creating the appearance of a second chin. This fat pad is occasionallysurgically removed and the corresponding muscles under the jaw shortened (hyoid lift).[24]
Another cause can be a bony deficiency, commonly seen in people of normal weight. When the jaw bones (mandible and by extension themaxilla) do not project forward enough, the chin in turn will not project forward enough to give the impression of a defined jawline and chin. Despite low amounts of fat in the area, it can appear as if the chin is melting into the neck. The extent of this deficiency can vary drastically and usually has to be treated surgically.[citation needed] In some patients, the aesthetic deficit can be overcome withgenioplasty alone; in others, the lack of forward growth might warrantorthognathic surgery to move one or two jaws forward. If the patient suffers fromsleep apnea, earlymaxillomandibular advancement is usually the only causal treatment and necessary to preserve normal life expectancy.
^Stringer CB, Hublin JJ, Vandermeersch B (1984). "The origin of anatomically modern humans in western Europe". In Smith FH, Spencer F (eds.).The Origins of Modern Humans: A World Survey of the Fossil Evidence. New York: Alan R. Liss, Inc. pp. 51–135.
^Robinson L (1913). "The story of the chin".Knowledge.36:410–420.
^Daegling DJ (1993). "Functional morphology of the human chin".Evolutionary Anthropology: Issues, News, and Reviews.1 (5):170–177.doi:10.1002/evan.1360010506.S2CID85195021.
^DuBrul EL, Sicher H (1954).The Adaptive Chin. Springfield, IL: C.C. Thomas.
^Günther H. "Anomalien und Anomaliekomplexe in der Gegend des ersten Schlundbogens".Zeitschrift für menschliche Vererbungs- und Konstitutionslehre.23:43–52.;Lebow MR, Sawin PB (1941). "Inheritance of human facial features: a pedigree study involving length of face, prominent ears and chin cleft".Journal of Heredity.32:127–32.doi:10.1093/oxfordjournals.jhered.a105016.
