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Abstract
Bipedalism is a defining feature of the hominin lineage, but the nature and efficiency
of early hominin walking remains the focus of much debate. Here, we investigate walking
cost in early hominins using experimental data from humans and chimpanzees. We use
gait and energetics data from humans, and from chimpanzees walking bipedally and quadrupedally,
to test a new model linking locomotor anatomy and posture to walking cost. We then
use this model to reconstruct locomotor cost for early, ape-like hominins and for
the A.L. 288 Australopithecus afarensis specimen. Results of the model indicate that
hind limb length, posture (effective mechanical advantage), and muscle fascicle length
contribute nearly equally to differences in walking cost between humans and chimpanzees.
Further, relatively small changes in these variables would decrease the cost of bipedalism
in an early chimpanzee-like biped below that of quadrupedal apes. Estimates of walking
cost in A.L. 288, over a range of hypothetical postures from crouched to fully extended,
are below those of quadrupedal apes, but above those of modern humans. These results
indicate that walking cost in early hominins was likely similar to or below that of
their quadrupedal ape-like forebears, and that by the mid-Pliocene, hominin walking
was less costly than that of other apes. This supports the hypothesis that locomotor
energy economy was an important evolutionary pressure on hominin bipedalism.