The AL 333-160 fourth metatarsal from Hadar compared to that of humans, great apes, baboons and proboscis monkeys: Non-conclusive evidence for pedal arches or obligate bipedality in Hadar hominins

摘要

Based on comparisons to non-statistically representative samples of humans and two great ape species (i.e. common chimpanzees Pan troglodytes and lowland gorillas Gorilla gorilla). Ward et al. (2011) concluded that a complete hominin fourth metatarsal (4th MT) from Hadar, AL 333-160, belonged to a committed terrestrial biped with fixed transverse and longitudinal pedal arches, which was no longer under selection favoring substantial arboreal behaviors. According to Ward et al., the Hadar 4th MT had (1) a torsion value indicating a transverse arch, (2) sagittal plane angles between the diaphyseal long axis and the planes of the articular surfaces indicating a longitudinal arch, and (3) a narrow mediolateral to dorsoplantar base ratio, an ectocuneiform facet, and tarsal articular surface contours all indicating a rigid foot without an ape-like mid-tarsal break. Comparisons of the Hadar 4th MT characters to those of statistically representative samples of humans, all five great ape species, baboons and proboscis monkeys show that none of the correlations Ward et al. make to localized foot function were supported by this analysis. The Hadar 4th MT characters are common to catarrhines that have a midtarsal break and lack fixed transverse or longitudinal arches. Further comparison of the AL 333-160 4th MT length, and base, midshaft and head circumferences to those of catarrhines with field collected body weights show that this bone is uniquely short with a large base. Its length suggests the AL 333-160 individual was a poor leaper with limited arboreal behaviors and lacked a longitudinal arch, i.e. its 4th MT long axis was usually held perpendicular to gravity. Its large base implies cuboid-4th MT joint mobility. A relatively short 4th MT head circumference indicates AL 333-160 had small proximal phalanges with a restricted range of mobility. Overall, AL 333-160 is most similar to the 4th MT of eastern gorillas, a slow moving quadruped that sacrifices arboreal behaviors for terrestrial ones. This study highlights evolutionary misconceptions underlying the practice of using localized anatomy and/or a single bony element to reconstruct overall locomotor behaviors and of summarizing great ape structure and behavior based on non-statistically representative samples of only a few living great ape species.