Choice of Size Parameter Alters Interpretation of Fossil Hominin Distal Humeral Morphology

Abstract

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.