The Evolution of Third Molar Agenesis and Impaction
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CitationCarter, Katherine. 2016. The Evolution of Third Molar Agenesis and Impaction. Doctoral dissertation, Harvard University, Graduate School of Arts & Sciences.
AbstractUp to 70% of modern humans experience problems with their third molars, whether it is failure of proper eruption (impaction) or not erupting at all (agenesis). Thus, it is a commonly used example for explaining why humans are still evolving and the relevance of evolutionary medicine. Agenesis dates back to at least early Homo erectus, and is seen with greater frequency during the evolution of later Homo. However, there are currently few data that support any of the three hypotheses (agenesis as selection against impaction, developmental delay, and the probable mutation effect) proposed to explain the evolution of agenesis and impaction. Furthermore, while most researchers associate changes in the physical properties of food with an increased frequency of third molar pathologies, it is unclear whether these changes took place during the advent of agriculture, the beginning of industrialization or both. Considerable variation exists among modern human populations in the rate of agenesis and impaction, but there are inconsistencies in reports of how modern variation partitions among sex, population, and morphological traits. Thus, this study first aims to explain modern variance in agenesis and impaction, then to use morphological data to assess how the third molar pathologies of four different populations from Serbia, Egypt, Japan and SE USA changed with the adoption of agriculture and the beginnings of industry. This will lead to an understanding of the evolution of third molar agenesis and impaction that will help understand the evolutionary origins of an important condition affecting modern humans.
First, we aimed to consolidate all available data on worldwide third molar agenesis frequencies, with a particular emphasis on exploring the factors leading authors to find contradictory results for the demographic and morphological predictors of this anomaly. A total of 12,376 studies were originally identified, then narrowed down to 1,312 for title/abstract screening. Based on our inclusion and exclusion criteria, we selected 92 studies, containing 100 effect sizes and 63,314 subjects, for systematic review and meta-regression. The worldwide rate of agenesis was found to be 22.63% (95% CI: 20.64%-24.76%), though the estimates ranged from 5.32% to 56.0%. Our subgroup analyses revealed that females are 14% more likely to have agenesis of one or more third molars than males and that maxillary agenesis was 36% more likely than mandibular agenesis in both sexes. Further, we found that having agenesis of one or two molars was most common, while agenesis of three molars was least common. Finally, we found large differences among agenesis frequency depending on geographic region. This information is expected to be of use not only to clinicians and patients but also to policy makers, given the implications for third molar extraction protocols.
Next, we performed a meta-analysis to synthesize the abundant existing literature on third molar impaction frequencies worldwide. A total of 7,936 records were originally identified, then narrowed down to 864 for title/abstract screening. Based on our inclusion and exclusion criteria, we selected 49 studies, containing 83,484 subjects, for systematic review and meta-regression. The worldwide rate of impaction was found to be 24.40% (95%CI: 18.97%-30.80%), though the estimates ranged from 3.08% to 68.60%. Our subgroup analyses revealed that mandibular impaction was 57.58% more likely than maxillary impaction, while we did not detect any difference in impaction frequency between males and females. In addition, we found that mesioangular impaction was most common, followed by vertical impaction, with distoangular and horizontal impaction less frequent. Further, we found that having impaction of one or two third molars was most common, while impaction of three or four third molars was least common. Finally, we found small differences among impaction frequency depending on geographic region. Given the rich debate surrounding third molar treatment policy, understanding demographic and morphological differences in impaction rates is an important first step in assessing appropriate treatment protocols.
Finally, we applied the results of these analyses to explore the evolutionary origins of third molar agenesis and impaction. Humans are unique in having high frequencies of pathological third molars, including impacted molars that fail to erupt and agenesis, or missing, molars. Many researchers attribute an increased prevalence of third molar agenesis and impaction to highly processed modern diets. Three competing hypotheses exist to explain the evolution of third molar agenesis: 1) agenesis as the result of selection against impaction, 2) agenesis as developmental delay, and 3) the probable mutation effect. These hypotheses, however, remain untested. Here we test these hypotheses using data from four populations sampled before, during and after the transitions to agriculture and two populations before, during, and after the transition to industry. We found the selection against impaction hypothesis contributed to the rise in third molar agenesis at the end of the industrial transition, but impaction remained at such low frequencies during the agricultural transition that this mechanism is unlikely to have been a causative force. Furthermore, we found a negative relationship between dental arch space and third molar agenesis in the industrial transition, but a positive relationship between these two variables in the agricultural transition. Overall, these data suggest that two different mechanisms influenced the prevalence of human third molar agenesis: one operating since the time of Homo erectus and one with much more recent origins.
Citable link to this pagehttp://nrs.harvard.edu/urn-3:HUL.InstRepos:33493544
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