https://anatomypubs.onlinelibrary.wiley.com/doi/10.1002/ar.24387
05 March 2022

Abstract
Modern humans have the longest Achilles tendon
(AT) of all the living primates. It has been
proposed that this anatomy increases locomotor
efficiency and that its elongation may have
played a crucial role in the origin and early
evolution of the genus Homo. Unfortunately,
determining the length of the AT in extinct
hominins has been difficult as tendons do not
fossilize. Several methods have been proposed
for estimating the length of the AT from
calcaneal morphology, but the results have
been inconclusive. This study tested the
relationship between the area of the superior
calcaneal facet and AT length in extant
primates. The superior facet is instructive
because it anchors the retrocalcaneal bursa,
a soft tissue structure which helps to reduce
friction between the AT and the calcaneus.
Calcanei from 145 extant anthropoid primates
from 12 genera were photographed in posterior
view and the relative superior facet size
quantified. AT lengths were obtained from
published sources. The relative area of the
superior facet is predictive of AT length in
primates (R2 = 0.83; p < .001) and differs
significantly between the great apes and
humans (p < 0.001). When applied to fossil
Australopithecus calcanei, our results
suggest that australopiths possessed a
longer, more human-like, AT than previously
thought. These findings have important
implications for the locomotor capabilities
of Australopithecus, including their capacity
for endurance running and climbing.

"Our results suggesting that the AT in
Australopithecus had reached modern
human-like length is especially interesting
given the growing body of literature examining
the evolution of the capacity for endurance
running in humans (e.g., Lieberman et al.,
2009). Endurance running is unique to humans
among all extant primates (Bramble & Lieberman,
2004). Researchers have hypothesized that
specific anatomical changes, including the
elongations of the AT, enhanced the endurance
running abilities of early members of the
genus Homo.

"However, Bramble and Lieberman (2004) never
argue that all of the anatomical adaptations
for distance running would necessarily occur
in concert in Homo erectus, only that the suite
of features adaptive to endurance running would
be present by that time in our evolutionary
history. In this light, our finding of increased
AT length in Australopithecus is perhaps not all
that surprising. It is reasonable to expect that
some of the adaptations thought to be related to
endurance running were already present in earlier
hominins."

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