Foot stiffness underlies its mechanical function, and is central to the evolution of human bipedal locomotion.1–5
The stiff and propulsive human foot has two distinct arches, the longitudinal and transverse.3–5 By
contrast, the feet of non-human primates are flat and softer.6–8 Current understanding of foot stiffness is
based on studies that focus solely on the longitudinal arch,9–14 and little is known about the mechanical
function of the transverse arch. However, common experience suggests that transverse curvature dominates
the stiffness; a drooping dollar bill stiffens significantly upon curling it along the transverse direction, not
the longitudinal. We derive a normalized curvature parameter that encapsulates the geometric principle 15
underlying the transverse curvature-induced stiffness. We show that the transverse arch accounts for almost
all the difference in stiffness between human and monkey feet (vervet monkeys and pig-tailed macaques)
by comparing transverse curvature-based predictions against published data on foot stiffness. 6,7 Using this
functional interpretation of the transverse arch, we trace the evolution of hominin feet16–20 and show that
a human-like stiff foot likely predates Homo by ∼ 1.5 million years, and appears in the ∼ 3.4 million year
old fossil from Burtele. 19 A distinctly human-like transverse arch is also present in early members of Homo,
including Homo naledi,
20 Homo habilis,
16 and Homo erectus.
17 However, the ∼ 3.2 million year old Australopithecus
afarensis 18 is estimated to have possessed a transitional foot, softer than humans and stiffer
than other extant primates. A foot with human-like stiffness probably evolved around the same time as other
lower limb adaptations for regular bipedality, 3,18,21,22 and well before the emergence of Homo, the longitudinal
arch, and other adaptations for endurance running.2
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