Neanderthal's heel: How the Neanderthal's external calcaneal shape uniquely differs from that of modern humans

Neanderthals exhibit unique morphological adaptations in their calcaneus compared to Homo sapiens, shedding light on distinct evolutionary pressures and functional requirements shaping their respective foot anatomy. This study assesses which calcaneal traits represent plesiomorphic, autapomorphic, and/or functional adaptations. Geometric morphometrics was used to analyze the external shape of 7 Neanderthal and 64 H. sapiens calcanei spanning early Upper Pleistocene hunter-gatherers to postindustrial populations. Shape, size, and allometric differences were assessed among groups, enabling detailed examination of the whole calcaneus and key subregions such as talar facets, the cuboid facet, and the tuberosity. Compared to calcanei of H. sapiens, calcanei of Neanderthals were shorter, wider, and taller-distinct adaptations likely favoring more load-bearing over the rearfoot and high mobility. Similar but less pronounced traits appear in highly mobile, unshod modern humans, while sedentary, postindustrial populations show more gracile calcanei, likely influenced by footwear and reduced activity. Neanderthals also displayed ancestral features such as a shorter sinus tarsi. Notably, Neanderthal subtalar joint shape and calcaneocuboid orientation suggest a habitually pronated foot posture. Neanderthals also exhibit a reduced lateral plantar process compared to H. sapiens, implicating differences in plantar ligament attachment and foot arch support. Differences in the cuboid facet and tuberosity of Neanderthals and H. sapiens imply alternative strategies for foot stabilization and force transmission through proximal tarsal joints, likely reflecting a different longitudinal arch expression, one adapted to Neanderthals' body mass and high-activity lifestyle.