Craniofacial Evolution and Alveolar Bone Loss: A Lesson From Hominins

In hominins, the reduction of prognathism during craniofacial evolution was a significant derived trait differentiating Homo from earlier hominins and other apes and might have contributed significantly to calvarial expansion and encephalization. Gnathic remains of Australopithecus africanus , A. robustus , Homo habilis, and H. erectus from the Plio-Pleistocene boundary 5.3-2.6 MYA were studied for evidence of alveolar bone (AB) loss indicative of periodontal disease(s). AB loss may provide critical insights into craniofacial evolution and the divergence of Homo from the Australopithecines. AB loss in Plio-Pleistocene gnathic remains provides the fossilized hard evidence of the antiquity of periodontal diseases, the first recognized diseases in hominins' evolution.

Seventy-one gnathic remains of Australopithecines and Homo species from Ditsong Museum of Natural History, Pretoria, and the School of Anatomy of the University of the Witwatersrand, Johannesburg, were examined using scanning electron microscopy (SEM) and macrophotography. Specimens were scanned at the microfocus X-ray tomography laboratory (MIXRAD) at Necsa, Pretoria, optimizing on highest spatial resolution and image contrast. Linear distances from the enamel-dentine junction (EDJ) to the remaining AB crest were also measured.

Morphometric analyses showed that there is progressive AB loss as hominins speciated from Australopithecines to Homo. Homo remains showed statistically significant variation when analysing the linear distance between the EDJ and the remaining AB when compared with both Australopithecine taxa. AB loss was confirmed by microfocus X-ray tomography and, in Homo species only, showed a vertical pattern of bone loss with crateriform lesions and furcation defects. SEM and microfocus X-ray tomography, macrophotography, and linear measurements from the EDJ to the remaining alveolar bone showed that Homo had greater alveolar bone loss with intrabony defects and craters when compared with Australopithecines' taxa. There were no significant differences between the 2 Australopithecine species examined.

The presented data show that Homo species developed significant AB loss. The data propose that random mutations of genes controlling odontometric values selected for a reduction of the size of the crowns during hominins' evolution. Smaller crowns ultimately resulted in weaker masticatory forces yet allowing masticatory function and thus survival in the presence of AB loss. Together with the speciation of smaller crowns, with reduction of masticatory muscle mass and thus masticatory forces, there was a reduction of prognathism leading to calvarial expansion with subsequent encephalization, speciating the Homo clade and later, the emergence of Homo sapiens .