Post-canine megadontia is a relative enlargement of the molars and premolars compared to the size of the incisors and canines. This phenomenon is seen in some early hominid ancestors such as Paranthropus aethiopicus.
Archaeological evidence The evidence for postcanine megadontia comes from measuring postcanine tooth surface area of hominid specimens and comparing these measurements to other hominid species. Australopithecus, dated to have lived 2 to 3 million years ago, is the earliest hominid genus to demonstrate postcanine enlargement, with average postcanine tooth area ranging from approximately 460mm2 and going all the way up to the largest tooth area, 756mm2, which is seen in Paranthropus boisei . After Australopithecus, a trend of steady decline in postcanine size is observed, starting in the genus Homo and culminating with Homo sapiens which has an average postcanine tooth area of only 334mm2. Studies of premolar size in hominid species that predate Australopithecus afarensis show long, uni-cuspid teeth at the P3 location, while species dated after A. afarensis have been shown to have wider, bicuspid teeth at the 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 of H. floresiensis show similarities to the bigger tooth sizes of the earlier genera Australopithecus and Homo.
Timeline and map
The shift towards postcanine 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 suggest it to be near the split between the chimpanzee and hominin lineages. It was the origin 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. afarensis as well as other genera in Paranthropus. The first specimen of Paranthropus aethiopicus was discovered in Lake Turkana, Kenya and its successor, Paranthropus robustus, was found in the northern parts of South Africa (Swartkrans, Kromdraai and Drimolen). Paranthropus boisei, the last species included in the genus Paranthropus, was first found in Olduvai Gorge, Tanzania and around Ethiopia and Kenya. P. boisei 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.
The first species in the genus Homo, Homo habilis, has been found in Tanzania and Kenya at sites dating between 2.1 and 1.5 million years ago. Species within the genus Homo showed no difference within molar size up until Homo floresiensis, where smaller molars were beginning to be expressed. Species after H. floresiensis such as H. heidelbergensis, H. neanderthalensis, and H. sapiens began to show instead wider frontal teeth and a decrease in size of the molars compared to earlier species. H. sapiens shows significantly smaller molars, mandible, and a prominence of the chin.
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 adaptation 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 mass 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, but the evolution of postcanine megadontia is more likely due to the quality of the diet. The tough, "low-quality food stuffs" 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. Many modern primates who lack such dietary features have been shown to occasionally rely on "fall-back foods" of these sorts, implying robust Australopithecines developed postcanine megadontia as they had to more heavily rely on such foods.
Form and function
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