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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 Agricultural revolution, as this has been confirmed by bone remains, dated to this time period (Pinhasi et al., 2015; Lieberman et al., 2004). 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, also known as non-straight teeth (Pinhasi et al., 2015). As food has become more processed, soft, and more liquid in form, within recent centuries, a rapid increase in non-straight teeth, smaller jaws and mouths, lack of space for wisdom teeth, and associated health conditions have been observed, such as sleep apnea, as well as more constricted airways (Ehrlich & Blumstein, 2018; Khan et al., 2020; Lieberman et al., 2004). Medical professionals have been making similar observations and documenting them for hundreds of years (Khan et al., 2020). Changes in diet, lifestyle, and breathing patterns early in life have led to maladaptive phenotypic expression in terms of morphological craniofacial development throughout the lifespan (Khan et al., 2020).

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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 (Pinhasi et al., 2008; 2015). 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 (Pinhasi et al., 2008; 2015). Hunter-gatherer populations overwhelmingly had larger jaws, almost always providing adequate space for teeth, including wisdom teeth, and tongue crowding was rare (Kahn et al., 2020; Gibson & Calcagno, 1993). Even when comparing medieval skulls (~500-1500 years ago) with modern skulls, there is stark contrast in terms of jaw size and malocclusion rates (Luther, 1993).

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 (Khan et al., 2020). 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 (Ehrlich & Blumstein, 2018).

The main contributing factor to the recent increase in malocclusion is widely thought to be due to a sharp reduction in chewing stress, especially during critical periods of craniofacial growth (Lombardi, 1982; Pinhasi et al., 2015). 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 (Ciochon et al., 1997). 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 (Ciochon et al., 1997).

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 (Harvold et al., 1981; Harari et al., 2010). 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 (Ehrlich & Blumstein, 2018). Children are experiencing allergies at higher rates, causing congested nasal airways, propagating them to breathe through the mouth more often (Ehrlich & Blumstein, 2018). Chronic mouth-breathing in children has been shown to cause posterior-jaw positioning, more crooked teeth and impacts overall jaw development negatively (Harvold et al.,1981; Surtel et al., 2015); this also causes constricted airways leading to higher rates of sleep apnea (Harari et al., 2010). Decreased mandible size has been directly identified as a risk factor for obstructive sleep apnea (Chi et al., 2011). 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, 2013). 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.

(Talk about risks of mouth breathing/pros of nasal breathing from Surtel article (and tomkinson too even)

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 et al., 2019). 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 (Khan et al., 2020).