Human jaw shrinkage is the phenomenon of continued size reduction of the human

mandible In jawed vertebrates, the mandible (from the Latin ''mandibula'', 'for chewing'), lower jaw, or jawbone is a bone that makes up the lowerand typically more mobilecomponent of the mouth (the upper jaw being known as the maxilla). The jawbone i ...

and

maxilla In vertebrates, the maxilla (: maxillae ) is the upper fixed (not fixed in Neopterygii) bone of the jaw formed from the fusion of two maxillary bones. In humans, the upper jaw includes the hard palate in the front of the mouth. The two maxil ...

over the past 12,000 to 15,000 years. Modern human lifestyles and diets are vastly different now from what they were for most of human evolutionary history. Human jaws, as well as oral cavities, have been shrinking ever since the spread of agriculture during the

Neolithic Revolution The Neolithic Revolution, also known as the First Agricultural Revolution, was the wide-scale transition of many human cultures during the Neolithic period in Afro-Eurasia from a lifestyle of hunter-gatherer, hunting and gathering to one of a ...

( years ago). This has been confirmed by bone remains dated to this time period.Pinhasi, R., Eshed, V., & von Cramon-Taubadel, N. (2015). Incongruity between affinity patterns based on mandibular and lower dental dimensions following the transition to agriculture in the Near East, Anatolia, and Europe. PLOS ONE, 10(2), e0117301.Lieberman, D. E., Krovitz, G. E., Yates, F. W., Devlin, M., & Claire, M. S. (2004). Effects of food processing on masticatory strain and craniofacial growth in a retrognathic face. Journal of human evolution, 46(6), 655-677. Researchers are able to infer the basic lifestyle practices of past cultures, enabling them to link jaw size with lifestyle behaviors. Bones from burial sites of past

hunter-gatherer A hunter-gatherer or forager is a human living in a community, or according to an ancestrally derived Lifestyle, lifestyle, in which most or all food is obtained by foraging, that is, by gathering food from local naturally occurring sources, esp ...

societies are associated with larger jaws and mouths, while bones retrieved from former farming cultures have decreased jaw size. Bones from farming societies indicate the presence of dental

malocclusion In orthodontics, a malocclusion is a misalignment or incorrect relation between the teeth of the upper and lower dental arches when they approach each other as the jaws close. The English-language term dates from 1864; Edward Angle (1855–1 ...

s, commonly known as non-straight teeth. Within recent centuries, as food has become more processed and soft in form, a rapid increase in non-straight teeth, smaller jaws, and mouths has been observed, as well as decreased space for wisdom teeth, and associated health conditions. Such conditions include

sleep apnea Sleep apnea (sleep apnoea or sleep apnœa in British English) is a sleep-related breathing disorder in which repetitive Apnea, pauses in breathing, periods of shallow breathing, or collapse of the upper airway during sleep results in poor vent ...

, constricted airways, and decreased respiratory fitness.Ehrlich, P. R., & Blumstein, D. T. (2018). The great mismatch. BioScience, 68(11), 844-846.Kahn, S., Ehrlich, P., Feldman, M., Sapolsky, R., & Wong, S. (2020). The jaw epidemic: Recognition, origins, cures, and prevention. ''BioScience'', ''70''(9), 759-771. Medical professionals have been making similar observations and documenting them for hundreds of years. Changes in diet, lifestyle, and breathing patterns have led to maladaptive

phenotypic In genetics, the phenotype () is the set of observable characteristics or traits of an organism. The term covers the organism's morphology (physical form and structure), its developmental processes, its biochemical and physiological propert ...

expression in terms of morphological craniofacial development that starts in childhood but persists throughout the lifespan.

Evolutionary history

The general trend of jaw and

oral cavity A mouth also referred to as the oral is the body orifice through which many animals ingest food and vocalize. The body cavity immediately behind the mouth opening, known as the oral cavity (or in Latin), is also the first part of the alime ...

shrinkage, as well as dental malocclusion presence, has been observed in burial remains across

Eurasia Eurasia ( , ) is a continental area on Earth, comprising all of Europe and Asia. According to some geographers, Physical geography, physiographically, Eurasia is a single supercontinent. The concept of Europe and Asia as distinct continents d ...

. Analyses of remains from areas thought to be ''in situ'' (origin) to agriculture, such as those in the

Levant The Levant ( ) is the subregion that borders the Eastern Mediterranean, Eastern Mediterranean sea to the west, and forms the core of West Asia and the political term, Middle East, ''Middle East''. In its narrowest sense, which is in use toda ...

region dated to approximately 12,000 years ago, are thought to be where humans first changed from hunting and gathering to a more agricultural lifestyle, with some populations relying on agriculture more than others.Pinhasi, R., Eshed, V., & Shaw, P. (2008). Evolutionary changes in the masticatory complex following the transition to farming in the southern Levant. American Journal of Physical Anthropology: The Official Publication of the American Association of Physical Anthropologists, 135(2), 136-148. Burial sites ranging from 15,000 years ago to approximately 4,000 years ago, spread throughout Europe and modern-day Turkey, have been determined to be the remains of farmers, hunter-gatherers, transitional farmers, and semi-sedentary hunter-gatherers; comparisons and analyses of dental dimensions and jaw

morphology Morphology, from the Greek and meaning "study of shape", may refer to: Disciplines *Morphology (archaeology), study of the shapes or forms of artifacts *Morphology (astronomy), study of the shape of astronomical objects such as nebulae, galaxies, ...

have been made between these four lifestyle practices. Clear morphological differences were found based on lifestyle practice, as the jaws and teeth associated with more farming were shown to be smaller on average and often accompanied with malocclusion. Hunter-gatherer populations overwhelmingly had larger jaws, almost always providing adequate space for teeth, including wisdom teeth, and tongue crowding was rare. Indigenous hunter-gatherer populations living today, such as

Aboriginal Australians Aboriginal Australians are the various indigenous peoples of the Mainland Australia, Australian mainland and many of its islands, excluding the ethnically distinct people of the Torres Strait Islands. Humans first migrated to Australia (co ...

and the Hadza people of Tanzania, have better oral health and less malocclusion than the average human living in a developed society today.Crittenden, A. N., Sorrentino, J., Moonie, S. A., Peterson, M., Mabulla, A., & Ungar, P. S. (2017). Oral health in transition: The Hadza foragers of Tanzania. ''PLOS ONE'', ''12''(3), e0172197. Within Hadza populations, a difference in oral health between males and females has been shown; Hadza women predominantly eat agricultural foods, because they spend most of their time in villages, while the men mostly live in the bush, which consists of hunting, tracking, and gathering. This lifestyle difference causes Hadza men's diets to be dominated by wild foods and leads to them having less periodontal disease, straighter teeth, and fewer cavities than their female counterparts. Even when comparing medieval skulls (approximately 500 to 1,500 years ago) with modern skulls, there is stark contrast in terms of jaw size and malocclusion rates. Due to the exponential increase in advancement since the Agricultural Revolution 12,000 years ago, humans' immediate environments, diets, and culture have changed dramatically. This short length of time, relative to evolutionary timescale, means human genetics are still essentially the same as before these modern changes in lifestyle practices. Specific human developmental pathways were naturally selected for, over vast periods of time; however, these pathways no longer fully match our current environments, leading to the rise in new pathologies and disease; this is also known as

evolutionary mismatch Evolutionary mismatch (also "mismatch theory" or "evolutionary trap") is the evolutionary biology concept that a previously advantageous phenotypic trait, trait may become maladaptation, maladaptive due to change in the biophysical environment, e ...

Etiology

The main contributing factor to the recent increase in malocclusion is widely considered to be due to a sharp reduction in chewing stress, especially during critical periods of craniofacial growth. Experiments done on non-human subjects have shown that induced nasal blockages and/or dietary changes earlier in life lead to maladaptive morphological change in their jaws, intended to simulate what we are observing globally in human children. Significant craniofacial changes due to diet have even been experimentally shown in pigs during development; researchers fed groups either a hard-consistency diet or a soft-consistency diet, for eight months in total.Ciochon, R. L., Nisbett, R. A., & Corruccini, R. S. (1997). Dietary consistency and craniofacial development related to masticatory function in minipigs. Journal of craniofacial genetics and developmental biology, 17(2), 96-102. Drastic differences in jaw and facial musculature, facial structure, and tooth-crowding were observed; researchers directly related the findings to what we are observing more in human populations.

Breathing mode

Orthodontics has also allowed us to identify another contributing factor to shrinking mandibles and overall craniofacial morphological change. An overwhelming proportion of orthodontics patients, who are attempting to correct malocclusion of their teeth, share the characteristic of breathing primarily through the

. Oftentimes, this habitual mouth breathing is caused by obstructed nasal airways during childhood.Harvold, E. P., Tomer, B. S., Vargervik, K., & Chierici, G. (1981). Primate experiments on oral respiration. American journal of orthodontics, 79(4), 359-372.Harari, D., Redlich, M., Miri, S., Hamud, T., & Gross, M. (2010). The effect of mouth breathing versus nasal breathing on dentofacial and craniofacial development in orthodontic patients. The Laryngoscope, 120(10), 2089-2093. Modern humans have spent more time indoors and, as a result, are exposed to higher concentrations of allergens, which accumulate to higher concentrations indoors. Children are experiencing allergies at higher rates, causing congested nasal airways, propagating them to breathe through the mouth more often. Chronic

mouth breathing Mouth breathing, medically known as chronic oral ventilation, is long-term breathing through the mouth. It often is caused by an obstruction to breathing through the nose, the innate breathing organ in the human body. However, by the early 20 ...

in children has been shown to cause posterior-jaw positioning and more crooked teeth and impacts overall jaw development negatively;Surtel, A., Klepacz, R., & Wysokińska-Miszczuk, J. (2015). The influence of breathing mode on the oral cavity. Polski merkuriusz lekarski: Organ Polskiego Towarzystwa Lekarskiego, 39(234), 405-407. These morphological changes further constrict airways, also leaving less room for the tongue to rest, leading to higher rates of

obstructive sleep apnea Obstructive sleep apnea (OSA) is the most common sleep-related breathing disorder and is characterized by recurrent episodes of complete or partial airway obstruction, obstruction of the respiratory tract#Upper respiratory tract, upper airway lea ...

. Decreased mandible size has been directly identified as a risk factor for obstructive sleep apnea. Obstructive sleep apnea in non-obese children has been shown to be a direct result of abnormal oral–facial development, with abnormal development being directly tied to decreased muscle tone of oral and facial muscles.Guilleminault, C. (2013). Pediatric obstructive sleep apnea and the critical role of oral-facial growth: evidences. Frontiers in neurology, 3, 184. This

hypotonia Hypotonia is a state of low muscle tone (the amount of tension or resistance to stretch in a muscle), often involving reduced muscle strength. Hypotonia is not a specific medical disorder, but it is a potential manifestation of many different dis ...

of craniofacial muscles can be caused by lack of chewing stress, jaw posture and rest position, chronic nasal airway obstruction, and even respiratory inefficiency. Nasal breathing has been shown to be advantageous to mouth breathing due to a number of factors, such as how the nasal cavity humidifies incoming air, easing the burden on the lungs, while also filtering out a majority of incoming debris and dust. Nasal breathing also promotes a slower breathing rate.Russo, M. A., Santarelli, D. M., & O’Rourke, D. (2017). The physiological effects of slow breathing in the healthy human. ''Breathe'', ''13''(4), 298-309. Reduced breathing rates have been shown to promote improved health and longevity. Children who are confirmed clinically to be mouth breathers often show considerably higher rates of concentration difficulties, craniofacial bone abnormalities, malocclusion, cross-bite, chronic

gingivitis Gingivitis is a non-destructive disease that causes inflammation of the gums; ulitis is an alternative term. The most common form of gingivitis, and the most common form of periodontal disease overall, is in response to bacterial biofilms (also ...

, candida infections, and

halitosis Bad breath, also known as halitosis, is a symptom in which a noticeably unpleasant breath odour is present. It can result in anxiety among those affected. It is also associated with depression and symptoms of obsessive compulsive disorder. Th ...

. Due to increasingly sedentary lifestyles, overall population fitness levels are thought to contribute as well. Due to a lack of respiratory efficiency, people are overbreathing through the mouth, even when performing non-strenuous tasks.Tomkinson, G. R., Lang, J. J., & Tremblay, M. S. (2019). Temporal trends in the cardiorespiratory fitness of children and adolescents representing 19 high-income and upper middle-income countries between 1981 and 2014. British Journal of Sports Medicine, 53(8), 478-486. Breathing chronically through the mouth causes a change in rest posture for the jaw; over time, this can significantly alter jaw development in children, as well as adults to an extent.

References

{{Reflist Human head and neck Human evolution

Evolutionary history

Etiology

Breathing mode

See also

References