Arboreality and Bipedality in the Hadar Hominids

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.

The evolution of the human pelvis: changing adaptations to bipedalism, obstetrics and thermoregulation

“…It is well known that there is considerable and often rancorous debate about the nature of locomotion in early hominins: was it like modern humans, like that of chimpanzees, or something in between? It is not the goal of this paper to revisit this debate; the details can be found in the original works [7,26,27,42], in more recent summaries [10,11,21,[43][44][45][46][47] and a myriad of articles in between. With the recent description of Ar.…”

“…africanus, which post-date Ar. ramidus by 1-2 Myr, has been described in detail previously (see [6,7,9,11,26,27,[53][54][55][56]), and the two species are for the purpose of this paper considered similar (though see reference [56] for a sense of the variation) and will be discussed together. As in Ar.…”

Integrating the genotype and phenotype in hominid paleontology

“…Virtually all paleoanthropologists agree that there was a fundamental shift toward bipedality in this taxon (55)(56)(57)(58)(59)(60)(61) One of the pivotal traits in this argument is the length of the A. afarensis phalanges. As evidenced by modern arboreal hominoids, long digits can be advantageous for climbing (62), and A. afarensis phalanges are intermediate in length between long modern ape phalanges and derived, abbreviated modern human phalanges.…”

Functional morphology of the ankle and the likelihood of climbing in early hominins

“…Others have regarded retained primitive morphologies of the early hominin postcranial skeleton as evidence for arboreality (e.g., refs. [12][13][14][15]. Still others have argued that if early hominins were engaged in any significant amount of arboreality, then they were climbing in a manner kinematically distinct from any known anthropoid primate (16)(17)(18)(19).…”