Reevaluating the functional implications of Australopithecus afarensis navicular morphology

Area of Science:

• Paleoanthropology
• Human Evolution
• Functional Morphology

Background:

• The evolution of the human longitudinal arch is debated, with navicular tuberosity size previously used to infer its presence.
• A small navicular tuberosity is thought to indicate a longitudinal arch, while a large one suggests a flat foot, a hypothesis challenged by other fossil evidence.

Purpose of the Study:

• To reevaluate the morphometric affinities of Australopithecus afarensis naviculars in comparison to other fossil hominins and primates.
• To investigate the relationship between navicular tuberosity size and locomotor adaptations in hominins and primates.

Main Methods:

• Multivariate cluster analyses were performed on navicular morphometric data from 170 fossil hominins and anthropoid primates.
• Navicular tuberosity size was quantified using the ratio of tuberosity volume to the talar facet surface area.

Main Results:

• All fossil hominin naviculars, including those of A. afarensis, clustered closest to modern humans.
• Navicular tuberosity size did not differ between arboreal and terrestrial anthropoid taxa, with Ateles exhibiting the largest tuberosity.
• A large navicular tuberosity may be associated with developed leg musculature for ankle plantarflexion.

Conclusions:

• The morphology of A. afarensis naviculars supports the presence of a longitudinal arch, consistent with the Laetoli footprints.
• The size of the navicular tuberosity is not a reliable indicator of a flat foot and may reflect muscle development instead.