Locomotion and Posture in Ancestral Hominoids Prior to the Split of Hylobatids

Abstract

Torso-orthograde (TO)-positional behavior and an associated suite of postcranial morphological specializations , both of which are often linked to the evolution of large body size , are considered hallmarks of hominoid evolution . These features are thought to have played an important role in the Old World monkey-ape divergence and the development of upright bipedal locomotion in our own lineage. Compared to early theorists, who considered hylobatids an appropriate model for investigating the initial stages of hominoid evolution, more recent reconstructions of the last common ancestor of hominoids have advocated a larger-bodied, more generalized ape very different from hylobatids as the most likely morphotype. While field observations from the large-bodied non-human hominids (i.e., Asian and African apes) confirm that they do in fact utilize an expansive TO-positional repertoire, no detailed data from hylobatids has been available until now to fully evaluate this hypothesis in light of all hominoid taxa. Using recently published positional behavior data from a population of adult white-handed gibbons ( Hylobates lar ) from Khao Yai National Park , Thailand , and integrating these data with a diverse anthropoid primate comparative sample, we reevaluate differences in positional behavior of extant cercopithecoids and hominoids. The comparative dataset agrees with recent suggestions that all living apes share a diverse TO-positional repertoire, relative to that of cercopithecoid monkeys. The question, whether or not TO-anatomical specializations and behavior among the Hominoidea represent homologous or homoplastic traits or both, could not yet be answered conclusively, despite the fact that homologous evolution is the more parsimonious explanation, due to a lack of support from the fossil record. Nevertheless, an upright (TO) and enhanced exploitation of the arboreal tree canopy by our ape ancestors were likely key evolutionary novelties that helped to shape the origin of hominins. Only an expansive understanding of the diversity of both extant and extinct apes will help to comprehensively unravel our enigmatic origins.