The Natural History of the Third Molar within the Genus Homo
Modern Homo sapiens often experience dental pathologies due to impaction of their third molars, and many people must have these four teeth surgically removed. The human face is flat and displays little to no prognathism, so if this morphology is the cause of third molar pathology, at what point during the course of the evolution of the adult human face and jaws did third molar impaction become a frequent problem? Exactly what changes in jaw morphology are associated with third molar impaction?
RQ: At what point in evolution did the increasing degree of orthognathism begin to cause third molar impaction?
H1: Chronic M3 impactions in adults are influenced by degrees of prognathism, mandibular angles, and retromolar spaces.
P1: There will be no observable instances of M3 impactions in adults of the Homo species with significant degrees of prognathism or large retromolar spaces, such as in H. habilis, H. erectus/ergaster, H. heidelbergensis, or H. Neanderthalensis.
P2: All cases of M3 impactions in adults will be found only in early modern and contemporary H. sapiens, because they have the flattest faces and the smallest retromolar spaces.
H2: Chronic M3 impactions in adults are not influenced by degrees of prognathism, mandibular angles, and retromolar spaces.
P1: M3 impactions will be observed in a Homo species that preceded early modern H. sapiens, such as H. habilis, H. erectus/ergaster, H. heidelbergensis, or H. neanderthalensis.
P2: M3 impactions will be found in ALL adult specimens of the genus Homo, which would show that such pathologies have existed since Homo has existed.
My literature review will cover studies of the mandible and maxillae for Homo habilis, H. erectus/ergaster, H. heidelbergensis, and H. neanderthalensis, and early modern and contemporary H. sapiens, noting in particular changes in retromolar space, mandibular body and length, and M3 and palate retraction. It will also cover human tooth formation in relation to impaction (which can occur even when there is adequate space in the mouth), as well as current studies of asymptomatic third molars, and modern Homo sapiens populations that do not develop third molars at all.
Preliminary reading list
Markus Bastir, Antonio Rosas, and H. David Sheets. The Morphological Integration of the Hominoid Skull: A Partial Least Squares and PC Analysis with Implications for European and Middle Pleistocene Mandibular Variation. From Modern Morphometrics in Physical Anthropology, 2005. Ed. Dennis Slice.
Dodson TB. Management of asymptomatic wisdom teeth.
Phillips C, Norman J, Jaskolka M, et al. Changes over time in position and periodontal probing status of retained third molars.
Marcelo Fernandes et al. Actuarial life-table analysis of lower impacted wisdom teeth in general dental practice.
Amandeep S. Johal and Joanna M. Battagel. Dental crowding: a comparison of three methods of assessment.
Coenraad F. A. Moorrees and Robert B. Reed. Biometrics of crowding and spacing of the teeth in the mandible.
My methods will consist of examining the third molar regions of the mandibles and maxillae of five (if possible) individual specimens each of Homo habilis, H. erectus/ergaster, H. heidelbergensis, H. neanderthalensis, and early modern and contemporary H. sapiens. For each specimen, I will use calipers and measuring tape to measure the:
- retromolar space (distance between the posterior edge of the third molar—or the cavity where its root would have been—and leading edge of the ascending ramus)
- mandibular angle
- mandibular body length
- position of third molar’s degree of retraction of the upper jaw (as described by Whitehead, p. 219 of lab manual)
- index of overlap of palate’s retraction and the masseter’s anterior extension
Measurements and descriptions will be organized into tables or diagrams showing the comparative position of the third molar over time, and will pinpoint which, if any, particular group of hominins first experienced widespread and chronic third molar impaction.
1. Did chronic third molar pain (from impactions, infections, cysts) affect social behavior? If so, in what ways? Was it rare or common to die of complications from such infections? Might individuals who experienced chronic and severe pain have been in enough distress to effect their appeal to the opposite sex and thus reduce their reproductive fitness? What is the earliest archaeological evidence for the treatment of third molar pathology?
2. The human face became flatter over time as our mastication apparatus (teeth and jaw muscles) got smaller and smaller due to changes in diet—from large amounts of tough, fibrous material to cooked meats and tubers and processed grains. Also, canine teeth in males became smaller as females selected for less aggression and more pair bonding to aid in cooperative breeding, thus eliminating the need for a large muzzle to house the CP3 complex and its corresponding diastema.
3. Are there there any other pathologies related to orthognathism in humans?