Form

While postcanine megadontia denotes the enlargement of the premolars and molars found in early hominid ancestors, it did not affect the structural organization of the cusps that make up those teeth, and thus, were used similarly to the premolars and molars that modern humans possess today. The premolars and molars of modern hominids and those affected by postcanine megadontia both have two and between four and five cusps respectively.^[25][26] The enlargement of the teeth affected by postcanine megadontia, without a difference in the arrangement or number of cusps that compose them, imparted an increased ability for grinding and crushing matter compared to modern day humans. While the form of the teeth themselves is not affected by postcanine megadontia, the ratio of molar teeth volume to total teeth volume is dramatically larger in specimens with postcanine megadontia compared to their modern human counterparts, an adaptation that shows signs of evolutionary convergence when compared to the form and function of the teeth found in many modern herbivores.^[27]

Functionality

Postcanine megadontia is associated with specific food material properties. This allows for great insight into the diets of early hominins that exhibited the trait. Post-canine megadontia is most commonly linked to diets rich in foods that are “small, chemically sealed, and resistant to bolus formation.”^[28] Having these larger teeth in comparison to the oral cavity size breaks down food particles more, which will increase the effectiveness of the natural processes of food processing that occur within the mouth.^[28] For example, the large transverse dimensions of corpses of Homo floresiensis from the Liang Bua cave on Flores Island in Indonesia suggest that these early hominids had postcanine megadontia and a diet with great masticatory stress.^[29] However, as Homo evolved, the amount of masticatory stress involved in eating decreased as “behavioral adaptations for extraoral food processing” were further developed. Thus, tool making Liang Bua corpses would have a comparatively smaller transverse dimensions of the skull, meaning their “mascilatory functional morphology” deviates greatly from the Pleistocene Homo.^[29]'

Biomechanics

Megadontia works by increasing the ratio of minimum to maximum second moments of area or the efficiency of a shape to resist bending or malformation.^[30] This resistance is tied to whichever specific axis about which the bending is being applied.^[31] Also called mandible robusticity, this characteristic allows for much stronger mastication of food. However, calculations made using biomechanical models does not necessarily perfectly predict the efficiency of different mandibular compositions in resistance to bending forces. Shearing is also an important factor in mastication effectiveness, and resistance to shear is proportional to the cross sectional area of the teeth. In postcanine Megadontia specimens, the cross sectional area is on average much larger than modern hominids which implies greater shear stress resistance.^[31]

Genetics

Some important genes involved in the development and regulation of tooth formation include BMP4, FGF8, and homeobox genes such as MSX1, PAX9, PITX2, SHOX2, Barx1, and Shh to name a few.^[32][33] Research suggests that homeobox genes are mainly responsible for much of the variance in tooth morphogenesis observed in fossilized hominids.^[32] It is theorized that tooth enlargement is due to several different gene mechanisms, none of them fully understood, and that selection acts on molars and pre-molars as a singular unit rather than on individual teeth.^[32]

There are a few theories concerning the selection mechanisms. One theory is that selection acts on the variation in the molars and pre-molars presented by the homeobox genes in hominid species.^[32] Another theory suggests that postcanine megadontia resulted from the spatial reassignment by homeobox genes that increased postcanine tooth size while simultaneously decreasing the size of the canines.^[32]

For the transition from megadontia to normal-sized postcanine teeth and its inverse relationship to brain size, one hypothesis proposes that an inactivation of the MYH16 gene, which resulted in an increase in brain size, decreased temporal muscle mass.^[30] The decrease in muscle allowed for the brain to grow, which might have allowed early hominids to develop tools.^[30] A second hypothesis suggests that the SRGAP2 gene is responsible instead.^[30] The inhibition of this gene allows for an increase in brain development.^[30]

Compared with modern human dental morphology

Compared to present day humans, early hominids such as Paranthropus aethiopicus and Australopithecus garhi^[34] had significantly larger dental morphology in their molars and premolars and smaller incisors. The hominids possessing postcanine megadontia had thick molar enamel, premolars with molarized roots, and lower molars that had additional capsules.^[35] Rather than inheriting their early hominid ancestors’ large sized molars, human molars evolved significantly, reducing instead to a size more similar to their front teeth. Contrary to megadont hominins’ dominant second molars, modern humans’ first molar is the largest, and their mandibles can rarely fit a third molar.^[36]

Compared to primates and other species

The morphology of teeth among species offers insight into their diet and phylogeny. The constant chewing in primate’s diets created a selection in primate molar shape, notably possessing cusp tips to ingest seeds.^[18] In order to achieve maximum chewing efficiency as the food toughness increases, folivorous primates tend to have larger postcanines than frugivorous primates.^[37] In primates, positive allometry exists between the size of postcanine teeth in primates and cranial length. This relationship has also been suggested in other groups of mammals, but the differences in postcanine size in primates are less variant compared to other mammals.^[38][39] Other species with herbivorous diets have adaptations in their postcanines in order to eat plant material,^[40] but the term postcanine megadontia typically refers to the dental adaptation in the hominid group.^[2]