Metatarsal neutral axis and head torsion in hominoids

Human head torsion is significantly different from that of apes for all metatarsals except the fifth. In particular, the first metatarsal head of apes is rotated laterally but has a plantar orientation in humans, while the metatarsal 2-4 heads are generally medially rotated in apes and laterally rotated in humans. These results have been used to identify if ape characteristics were present in the Australopithecus afarensis foot. A complete fourth metatarsal of A. afarensis from Hadar showed head torsion falling within the distribution of humans and outside the distribution of apes. Even though this method is extremely informative, it is applicable only to complete fossil bones. Here we propose a method for inferring metatarsal head torsion from fragmentary metatarsals. We measured the neutral axis angle at three locations of the diaphysis, at 35, 50, and 65% of bone length in a sample of Homo sapiens, Pan troglodytes, Gorilla gorilla, and Pongo pygmaeus. Results show that, for all species, correspondence between the neutral axis angle and the metatarsal head rotation increases from proximal to distal. The values and patterns of the neutral axis angle at the 35% level (distal) of metatarsal 2-5 are strongly correlated to those of metatarsal head torsions. This method of estimating metatarsal head torsion from diaphyseal neutral axis orientation, besides validating the results obtained by previous studies, shows that it is possible to use fragmentary early hominin metatarsals to better understand the pattern of acquisition of human-like, pronated distal foot.