Evolution of hominid bipedalism and prehensile capabilities

Abstract In this paper, I present an updated version of the hylobatian model for the proximate ancestors of the Hominidae. The hylobatians are hypothesized to have been relatively small creatures that were especially adapted for vertical climbing on tree trunks and vines and for bipedalism on horizontal boughs. They were no more disposed toward suspensory behaviours than are modern chimpanzees and bonobos. According to this evolutionary scenario, bipedalism preceded the emergence of the Hominidae. The earliest hominids would be recognized as diurnally terrestrial bipeds that stood with full extension of the knee joints and walked with greater extension of the lower limbs than is common in non-human primates that are induced to walk bipedally on the ground. The wealth of hominid fossils from the Hadar Formation, Ethiopia, and the Laetolil Formation, Tanzania, are generally compatible with the hylobatian model. They show that by ca. 4 Ma B.P. habitually terrestrial, bipedal hominids had evolved from arboreal ancestors. The Hadar hominids had curved fingers and toes, strong great toes and thumbs, and other features that suggest that they were rather recently derived from arboreal hominids and that they probably continued to enter trees, perhaps for night rest and some foraging. The hominid hand bones from Hadar evince no features that are distinctly related to knuckle-walking. They relate neatly to counterparts in the hand of O.H. 7, a specimen that was found with stone tools. However, there is no evidence that the Hadar hominids of 3 Ma ago engaged in tool behaviour.