Differences between Human and Great Ape Distal Humeral Articular Axes
Authors: Zelazny Kaya G. , Sylvester Adam D. , Ruff Christopher B. 
Published: June 2021

Apes differ in their use of the forelimb, with the humerus loaded during knuckle‐walking, quadrumanous clambering, and manipulation, each of these activities differing in the biomechanical forces produced. Morphological variation that changes the orientation of applied forces alters the biomechanical efficiency of specific behaviors; the range of variation in a species may therefore be selected upon to optimize efficiency during behaviors typical of that species. In this study, we measured the angle between the humeral diaphyseal axis and the axis of the distal articular surface (N= 646 humeri) to determine whether the shaft‐articular angle in species that make use of the forelimb during locomotion is different than that in humans. We also measured this angle for a number of fossil hominin humeri to determine whether these differences can be used to interpret fossil hominin behavior.Axes were measured on the basis of a principal component analysis of landmarks placed across the humerus; for the shaft, we placed 214 semi‐landmarks above the olecranon fossa and below the greater and lesser tubercles, while the articular axis was based on 21 trochlear and capitular fixed landmarks. Shaft‐articular angles varied between 69.53 and 103.02 degrees in extant apes including humans, with lower angles indicating humeri with a relatively proximal capitulum and distal trochlea. Means varied significantly between hominid taxa. The shaft‐articular angle was highest in gorillas and lowest in humans. In gorillas, the articular axis was roughly perpendicular to the shaft (mean = 85.4°), while in humans, the angle between the shaft and distal articular surface was significantly more acute (mean = 78.8°). A shaft‐articular angle close to 90° is likely to provide more stability when the forelimb is used for support during locomotion; while gorillas had the highest angles, Pan and Pongo also had higher shaft‐articular angles than our human sample. There was, however, significant variation within humans on the basis of population; while the means of our modern industrial populations were low, other populations had ranges more similar to Pan and Pongo. Although there was no difference on the basis of sex within humans as a whole, sex differences were identified within specific populations. Comparison of shaft‐articular angle in fossil hominin humeri may nevertheless be informative. We compared three fossil humeri with relatively complete shafts (KNM ER 739, STW 431 and Gombore IB) to our extant sample and discovered that all three had shaft‐articular angles slightly above 90 degrees. These results fall outside of the range of modern humans, but within the ape range. While this difference may represent a phylogenetic signal, preliminary results from more fragmentary fossils indicate that this pattern is not present in all fossil hominins. This suggests that these results may also reflect a functional difference, and that KNM ER 739, STW 431 and Gombore IB may have used their upper limb for support during locomotion.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.