User:BartHarleyJarvis/Human jaw shrinkage

Human jaw shrinkage is the phenomenon of continued size reduction of the human mandible and maxilla over evolutionary time spans. Modern human lifestyles and diets are vastly different now than they were for most of human evolutionary history. Human jaws, as well as oral cavities, have been shrinking ever since the Neolithic agricultural revolution (~12,000 years ago), as confirmed by bone remains dated to this time period. Researchers know the basic lifestyle practices of many of these past cultures, being able to link burial sites of past hunter-gatherer societies with larger jaws and mouths, while bones retrieved from former farming cultures have higher rates of decreased jaw size, along with the presence of dental malocclusions, commonly known as non-straight teeth. As food has become more processed and soft in form, within recent centuries, a rapid increase in non-straight teeth, smaller jaws and mouths, a lack of space for wisdom teeth, and associated health conditions have been observed, such as sleep apnea, constricted airways, and decreased respiratory fitness. 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 expression in terms of morphological craniofacial development that starts in childhood but persists throughout the lifespan.

Contents

 * 1Evolutionary history
 * 2Etiology
 * 3See also
 * 4References

Evolutionary history[edit]
The general trend of jaw and oral cavity shrinkage, as well as dental malocclusion presence, have been observed in burial remains across Eurasia, including analysis of remains from areas thought to be in situ (origin) to agriculture, such as those in the Levant region, dated to about 12,000 years ago, which are thought to be where humans were first transitioning from hunting and gathering to a more agricultural lifestyle, with some populations relying on agriculture more than others. Burial sites ranging from 15,000 years ago to about 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 analysis of dental dimensions and jaw morphology have been made between these four types of lifestyle practice. 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. Even when comparing medieval skulls (~500-1500 years ago) with modern skulls, there is stark contrast in terms of jaw size and malocclusion rates.

Due to the exponential increase in advancement ever since the Agricultural Revolution 12,000 years ago, human’s 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.

Etiology[edit]
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 of 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're observing globally in human children. Significant craniofacial changes due to diet have even been experimentally shown in pigs during development, in which researchers fed groups either a hard consistency diet or soft consistency, for eight months in total. Drastic differences in jaw and facial musculature, facial structure, as well as tooth-crowding were observed; researchers directly related the findings to what we are observing more in human populations.

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 oral cavity (mouth), often due to obstructed nasal airways during childhood. Allergens tend to be present in higher concentrations indoors, and in-turn, humans have spent more and more of their time indoors over the last few centuries. Children are experiencing allergies at higher rates, causing congested nasal airways, propagating them to breathe through the mouth more often. Chronic mouth-breathing in children has been shown to cause posterior-jaw positioning, more crooked teeth and impacts overall jaw development negatively; this also causes constricted airways leading to higher rates of sleep apnea. 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. This hypotonia 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. 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, candida infections, and halitosis. Due to increasingly sedentary lifestyles, overall population fitness levels are thought to contribute as well, due to a lack of respiratory efficiency, people are over-breathing through the mouth, even when performing non-strenuous tasks. 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.

See also[edit]

 * Neoteny in humans

References[edit]

 * 1) ^ Jump up to:a b c d e f 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.
 * 2) ^ Jump up to:a b c 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.
 * 3) ^ Jump up to:a b c d e Ehrlich, P. R., & Blumstein, D. T. (2018). The great mismatch. BioScience, 68(11), 844-846.
 * 4) ^ Jump up to:a b c d e f g h i Kahn, S., Ehrlich, P., Feldman, M., Sapolsky, R., & Wong, S. (2020). The jaw epidemic: Recognition, origins, cures, and prevention. BioScience, 70(9), 759-771.
 * 5) ^ Jump up to:a b 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.
 * 6) ^ Jump up to:a b 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.
 * 7) ^ Gibson, K. R., & Calcagno, J. M. (1993). Brief communication: Possible third molar impactions in the hominid fossil record. American journal of physical anthropology, 91(4), 517-521.
 * 8) ^ Luther, F. (1993). A cephalometric comparison of medieval skulls with a modern population. The European Journal of Orthodontics, 15(4), 315-325.
 * 9) ^ Lombardi, A. V. (1982). The adaptive value of dental crowding: a consideration of the biologic basis of malocclusion. American journal of orthodontics, 81(1), 38-42.
 * 10) ^ Jump up to:a b 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.
 * 11) ^ Jump up to:a b Harvold, E. P., Tomer, B. S., Vargervik, K., & Chierici, G. (1981). Primate experiments on oral respiration. American journal of orthodontics, 79(4), 359-372.
 * 12) ^ Jump up to:a b 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.
 * 13) ^ Jump up to:a b c 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.
 * 14) ^ Chi, L., Comyn, F. L., Mitra, N., Reilly, M. P., Wan, F., Maislin, G., Schwab, R. J. (2011). Identification of craniofacial risk factors for obstructive sleep apnoea using three-dimensional MRI. European Respiratory Journal, 38(2), 348-358.
 * 15) ^ Guilleminault, C. (2013). Pediatric obstructive sleep apnea and the critical role of oral-facial growth: evidences. Frontiers in neurology, 3, 184.
 * 16) ^ Jump up to:a b 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.