Evidence for an elongated Achilles tendon in Australopithecus

Area of Science:

• Paleoanthropology
• Primate Anatomy
• Biomechanics

Background:

• Modern humans possess the longest Achilles tendons (AT) among primates, potentially enhancing locomotor efficiency.
• AT elongation is hypothesized to be key in the evolution of the genus Homo.
• Estimating AT length in extinct hominins is challenging due to tendon non-fossilization.

Purpose of the Study:

• To investigate the relationship between calcaneal superior facet area and AT length in extant primates.
• To develop a reliable method for estimating AT length in fossil hominins.
• To infer the AT morphology and locomotor capabilities of Australopithecus.

Main Methods:

• Photographic analysis of calcanei from 145 extant anthropoid primates across 12 genera.
• Quantification of the relative superior calcaneal facet size.
• Correlation of facet size with published AT length data.

Main Results:

• A strong predictive relationship was found between relative superior facet area and AT length in primates (R² = 0.83, p < .001).
• Significant differences in facet area were observed between great apes and humans (p < .001).
• Application to Australopithecus calcanei suggests a longer, more human-like AT than previously inferred.

Conclusions:

• The superior calcaneal facet area serves as a reliable proxy for AT length in primates.
• Australopiths likely possessed a longer Achilles tendon, impacting their locomotor abilities.
• These findings offer new insights into the evolution of human locomotion and the genus Homo.