Arboreality and Bipedality in the Hadar Hominids

TLDR

Studies of Hadar hominids’ locomotor skeleton reveal traits of both arboreal climbing and terrestrial bipedalism, suggesting they spent time in trees for feeding, sleeping, or predator avoidance while also moving bipedally on the ground, a pattern supported by ecological context and modern primate models. The study presents new data on phalangeal length, tarsal morphology, and body proportions to strengthen the argument for arboreality in Hadar hominids. The authors analyze phalangeal length and curvature, tarsal and metatarsophalangeal joint morphology, and body proportions. Limb and pedal proportions, along with hip, knee, and foot anatomy, indicate that Hadar bipedality differed from modern humans, and that selection for terrestrial bipedality intensified as forests shrank, with Homo erectus possibly freeing hominids from tree dependence.

Abstract

Numerous studies of the locomotor skeleton of the Hadar hominids have revealed traits indicative of both arboreal climbing/suspension and terrestrial bipedalism. These earliest known hominids must have devoted part of their activities to feeding, sleeping and/or predator avoidance in trees, while also spending time on the ground where they moved bipedally. In this paper we offer new data on phalangeal length and curvature, moφhology of the tarsus and metatarsophalangeal joints, and body proportions that further strengthen the argument for arboreality in the Hadar hominids. We also provide additional evidence on limb and pedal proportions and on the functional anatomy of the hip, knee and foot, indicating that the bipedality practiced at Hadar differed from that of modern humans. Consideration of the ecology at Hadar, in conjunction with modern primate models, supports the notion of arboreality in these earliest australopithecines. We speculate that selection for terrestrial bipedality may have intensified through the Plio-Pleistocene as forests and woodland patches shrunk and the need arose to move increasingly longer distances on the ground. Only with Homo erectus might body size, culture and other factors have combined to ‘release’ hominids from their dependence on trees.