Post-canine megadontia

Post-canine megadontia is an enlargement of the molars and premolars, which is found in early hominid ancestors such as Paranthropus aethiopicus.

Archaeological Evidence

ملف:Paranthropus bonsei compared to Homo sapiens.png

Paranthropus boisei (left) vs. Homo sapiens (right). The average post-canine tooth area for P. boisei was 756mm² versus 334mm² in H. sapiens.

Australopithecus, dated to have lived 2 to 3 million years ago, is the earliest hominid genus to demonstrate post-canine enlargement. The greatest canine enlargement was seen in Paranthropus boisei and a steady decline in post-canine size was observed in the Homo genus.

Studies of premolar size in hominid species that predate Australopithecus afarensis show long, unicuspid teeth at the P₃ location, while species dated after A. afarensis have been shown to have wider, bicuspid teeth at same location, which is hypothesized to show the beginnings of canine to premolar evolution in hominids.

Homo floresiensis, a hominid species from the late Pleistocene found in cave deposits in Liang Bua, Indonesia, shows a smaller molar size that is closer to the hominid lineage. However, the remaining teeth show similarities to the bigger tooth sizes of the earlier genuses Australopithecus and Homo.

Timeline

A timeline depicting observed evidence of post-canine megadontia from 3 million years ago to the hominid species Homo floresiensis

The origins of post-canine megadontia dates back to about 4-5 million years ago with the discovery of Ardipithecus ramidus in the Middle Awash region of Ethiopia. Distinctive features in A. ramidus such as dentition with reduced canines, the skull, hindlimb and forelimb reveals it to be the transitional species between chimpanzees and hominid species.

It was the arise of Australopithecus africanus found in several regions of South Africa (Taung, Sterkfontein, Makapansgat) 2-3 million years ago that first demonstrated the enlargement of the pre-molars and molars. In terms of morphology, A. africanus shares many similar characteristics with A. Afrensis as well as other geneses in Paranthropus.

The first species of Paranthropus aethiopicus was discovered in Lake Turkana, Kenya and its successor, Paranthropus robust, was seen in the northern parts of South Africa; Swartkrans, Kromdraa and Drimolen (also referred to as the Cradle of Humankind). Paranthropus boisei, the last species included in the genus Paranthropus, was first found in Olduvai Gorge, Tanzania and around Ethiopia and Kenya. P. bosei was known for massive facial and dental bones and structure, primarily larger mandibles, molars, and premolars, which was an adaptation allowing them to consume hard plant foods with the ability of high force chewing.

Evolutionary Implications

Postcanine megadontia is commonly associated with the repeated consumption of tough plant-like material, which can be referred to as "low-quality food stuffs". The substances were integral to the diet of extinct hominids, and their molars were subject to the constant occlusal attrition from the stress of vigorous mastication. The development and evolution of this trait was characterized by a thick coating of enamel surrounding the molars and premolars, mitigating the detrimental effects of the tough diet. As such, this postcanine dentition is capable of “crushing and grinding” the tough shoots and leaves common to the diet of an early hominid. Australopithecus Paranthropus, for example, was perhaps the most noteworthy hominid to display this trait, an adaption perhaps due to its varied and encompassing diet . Note, postcanine megadontia is hypothesized to have no correlation to durophagy, but is rather a crucial development in hominids that allowed for preservation of occlusal quality.

Increased postcanine size can be correlated with the evolution of other physiological traits . Inverse trends of brain enlargement and molar size point to diet and food processing as a linking factor; encephalization is a crucial consideration in the development of tool usage and extraoral food processing that was observed in Homo species, but not in Australopithecines. Post canine enlargement has also been significantly positively correlated with basal metabolic rate, independently of body size. Larger primates tend to need larger teeth to process more food to meet the energy requirements of a larger body, however the evolution of postcanine megadontia is more likely due to the quality of the diet. The tough, “low quality” foodstuff consumed by robust Australopithecines coupled with their lack of food processing technology, lead to an enlargement of the occlusal surface of the molars

A study that analyzed the development of molars in hominids and Miocene apes found that a larger “ratio of the areas of [molar 1] and [molar 3]” could correlate to an increase in fruits found in the diet of these species. An increased ratio of the areas of molars was found to have a negative correlation with the amount of “leaves, flowers, and shoots” in the diet, suggesting that species like Ardipithecus, which had a greater ratio of areas of molars, had gradually transitioned to including more fruit in their diet as the size of their molars increased.

Form and Function

Functionality

Post-canine 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.” 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. For example, the large transverse dimensions of corpses of Homo floeresiensis 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. However, as Homo evolved, the amount 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.'

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. This resistance is tied to whichever specific axis about which the bending is being applied. 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.

Genetics

One hypothesis proposes that an inactivation of the MYH16 gene, which resulted in an increase in brain size, decreased temporal muscle mass. The decrease in muscle allowed for the brain to grow, which might have allowed early hominids to develop tools. A second hypothesis suggests that the SRGAP2 gene is responsible instead. The inhibition of this gene allows for an increase in brain development.

Comparative Biology

Post-canine Megadontia Compared with Modern Human Dental Morphology

Compared to present day humans, early hominids such as Paranthropus aethiopicus and Australopithecus garhi had significantly larger dental morphology in their molars and premolars and smaller incisors. The hominids possessing post-canine megadontia had thick molar enamel, premolars with molarized roots, and lower molars that had additional capsules. 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.

Post-canine Megadontia 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. In order to achieve maximum chewing efficiency as the food toughness increases, folivorous primates tend to have larger post-canines than frugivorous primates. In primates, positive allometry exists between the size of post canine 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. Other species with herbivorous diets have adaptations in their post canines in order to eat plant material, but the term postcanine megadontia typically refers to the dental adaptation in the hominid group.