Metric and morphological analysis of the foot in the Middle Pleistocene sample of Sima de los Huesos (Sierra de Atapuerca, Burgos, Spain)

The preservation of Homo fossil foot remains prior to modern humans and Neandertals is very scarce throughout the fossil record. Understanding foot morphology in human evolution is essential to know taxonomic processes and to acquiring information about corporal size. Here, we present a comprehensive study of the large foot remains sample recovered from the Middle Pleistocene site of Sima de los Huesos (SH). The minimum number of individuals (MNI) has been established at 16 by the foot remains. As is the case with other elements of the foot, metric and morphological differences in the feet between Middle and Late Pleistocene hominins tend to be subtle. However, an exclusive combination of traits can be recognized in the feet of the SH hominins. The SH hominins and Neandertals display tali with short necks and broad lateral malleolar facets, broad calcanei with long bodies and projected sustentaculum tali, stout naviculars, as well as robust metatarsals and phalanges. Nevertheless, the feet from SH are characterized by a very broad lateral malleolar facet in the talus and a very broad sustentaculum tali in the calcaneus, even more so than in Neandertals with respect to both traits. More importantly, the head of the talus from SH is narrower than the Neandertal's broad talus head, and the short intermediate cuneiforms found at SH distinguish them from the other comparative samples. The body mass estimation based on the trochlear breadth of the talus provides a mean of 69.7 ± 10.0 kg for the SH hominins, similar to that determined by the femoral head and significantly lower than that obtained from the bi-iliac breadth.