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.
The evidence for post-canine megadontia comes from measuring post-canine tooth surface area of hominid specimens that have been found and comparing these numbers between hominid species. Australopithecus, dated to have lived 2 to 3 million years ago, is the earliest hominid genus to demonstrate post-canine enlargement, with average post-canine 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 post-canine size is observed, starting in the Homo genus and culminating with Homo sapiens which has an average post-canine 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.
The shift towards 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 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.
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