Miocene hominoid craniofacial morphology and the emergence of great apes

The initial cladogenic event between Hominoidea (apes, including humans) and Cercopithecoidea (Old World monkeys) consisted primarily of changes in the craniofacial region. These changes, seen in taxa commonly known as victoriapithecids and proconsulids, arose in a mosaic fashion. The divergence in the postcranium was more subtle; there are strong suggestions that apes initially adopted a tail-less pronograde arboreal quadrupedalism, while cercopithecoids became better adapted to a more terrestrial lifestyle. Recent phylogenetic analysis suggests that gibbons (Hylobates) have reversed derived craniofacial characters autapomorphically, contradicting the interpretation that the origin of apes sensu stricto coincides with the emergence of suspensory adaptations. The suspensory postcranium evolved later and appeared first in Eurasia; recent palaeobiogeographic reconstructions suggest that suspensory apes subsequently re-colonized Africa, as suggested nearly thirty years ago on neontological grounds. To test whether these two models of hominoid evolution are compatible, catarrhine craniofacial and postcranial traits, including those from Eurasian fossils, were subjected to parsimony analysis. The results demonstrate a mosaic pattern of derived characters, with gibbons reversing some traits of the face, which suggests their derivation from a 'great ape' face. Combined with the palaeobiogeography, a much longer, step-wise transition from primitive catarrhines to extant great apes than previously envisioned is supported. The pattern of craniofacial change is difficult to interpret in functional/adaptational terms, but the origin of brachiation may have arisen through character displacement due to competition with the emerging modern Old World monkey radiation in Eurasia.