Objective: This study investigates bilateral asymmetry in the humerus of modern human populations with differing activity patterns to assess the relative plasticity of different bone regions in response to environmental influences, particularly the biomechanical demands of handedness. Methods: External breadths, cross-sectional properties, and centroid sizes were used to quantify directional and absolute asymmetry of humeral diaphyseal, distal periarticular, and articular regions in six populations with differing subsistence strategies (total n = 244). Geometric section properties were measured using computed tomography at six locations along the distal humerus, while centroid sizes of the distal articular and periarticular regions, as well as eight segments of the diaphysis, were extracted from external landmark data. Bilateral asymmetries were compared between populations and sexes. Each property was also tested for correlation with bilateral asymmetry at 40% of bone length, which has been shown to correlate with handedness. Results: Asymmetry is highest in the diaphysis, but significant through all distal bone regions. Asymmetry increases in the region of the deltoid tuberosity, and progressively declines distally through the shaft and distal periarticular region. Articular asymmetry is higher than periarticular asymmetry, approaching levels seen just proximal to the olecranon fossa, and is weakly but significantly correlated with diaphyseal asymmetry. Hunter-gatherers from Indian Knoll have significantly higher levels of asymmetry than other groups and are more sexually dimorphic, particularly in crosssectional properties of the diaphysis. Conclusions: Humeral dimensions throughout the diaphysis, including regions currently used in taxonomic assignments of fossil hominins, likely respond to in vivo use, including population and sex-specific behaviors.

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.

Allometric trends in human distal humeral morphology may account for a substantial portion of morphological variation observed among fossil hominins, affecting both taxonomic assessments and behavioral reconstructions. However, previous allometric studies have focused on the relationship of humeral morphology to size parameters of the distal humerus itself (i.e., centroid size), which is potentially confounded by interspecific differences in scaling of humeral dimensions to body mass, a factor that creates direct biomechanical demands on the forelimb in all non‐human primates.In this study, we compared the relationship between distal humeral centroid size and body mass in extant hominids (N=246 modern humans, 47 Pan, 44 Gorilla, 22 Pongo) and three fossil hominins (A.L. 288‐1, StW 431, KNM‐ER 1503/1504) where body mass could be estimated from femoral head size. We then scaled principal components extracted from geometric morphometric (GM) analyses of three regions of the distal humerus (articular, periarticular, and distal diaphyseal) by both body mass and centroid size in order to evaluate differences resulting from choice of size parameter. Finally, we analyzed the relationship of nine fossil hominins to humans of equivalent body mass (A.L. 288‐1, StW 431, KNM‐ER 1503/1504) and centroid size (KNM‐KP 271, A.L. 288‐1, StW 431, KNM‐ER 1504, KNM‐ER 739, SKX 10924, SK 24600, TM 1517, Gombore IB‐7594).While centroid size was highly correlated with body mass in all taxa (r=0.88–0.97), humans were highly positively allometric and had significantly smaller centroid sizes relative to body mass than other hominids, as did A.L. 288‐1 and KNM‐ER 1504. StW 431, however, showed a relationship between distal humeral size and body mass more similar to that seen in non‐human great apes. There was significant allometry in the traits differentiating modern humans and other hominids that resulted in large‐bodied humans showing more great ape‐like distal humeral characteristics than smaller humans in the articular and distal diaphyseal but not the periarticular region. This increased differences in size‐scaled morphology between modern humans and fossil hominins in several cases, most prominently A.L. 288‐1. The differences between humans and other hominids in the relationship of centroid size to body mass create the appearance of a distinct fossil hominin morphological group composed of A.L. 288‐1 and other small fossil humeri (SKX 10924, SK 24600) when principal components extracted from the GM analyses are regressed on centroid size. When body mass is used as the independent variable for regression, however, this distinct fossil grouping disappears, and it becomes clear that this morphology is also seen in small‐bodied chimpanzees and orangutans.This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.