Morphological Variation in the Distal Humerus of Extant Hominids and Fossil Hominins

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URL: http://jhir.library.jhu.edu/handle/1774.2/61454

Abstract

Relative to most other postcranial regions, the distal humerus is commonly preserved in the fossil hominin record. It is also integral to upper limb function, and therefore potentially useful in reconstructing locomotion. However, variability within species and overlapping morphological ranges has made identifying functionally or phylogenetically discriminating features difficult. This project uses 3D morphometrics with sliding semilandmarks in an attempt to overcome these difficulties. It also explores the impacts of distal humeral allometry and developmental plasticity in order to clarify interpretations.

A total of 250 modern (Holocene) humans, 125 extant great apes, and 10 fossil hominin specimens were included in the study. Results show that articular, periarticular, and shaft morphology differs between modern humans and Pan, Gorilla, and Pongo. The relationship of fossil hominins to extant species varies by region. Articular morphology is human-like, characterized by low to moderate articular relief. However, both periarticular and shaft morphology are more ape-like, driven by anteroposteriorly compressed periarticular surfaces, proximally positioned epicondyles, and robust diaphyses.

Allometry of these characteristics was tested by regressing principal component scores against body mass, distal humeral centroid size, and biepicondylar breadth. This also tested the influence of choice of size measure. Body mass and centroid size were highly correlated but uncovered different relationships between small fossil hominins (A.L. 288-1, SKX 10924, SK 24600) and extant taxa. With respect to centroid sizes, small fossils appear unique. However, they are similar to small extant nonhuman hominoids with respect to body mass. This is caused by differences in the relationship of mass to centroid size among extant taxa.

To determine the responsiveness of the distal humerus to mechanical loading, bilateral asymmetry of cross-sectional properties and centroid sizes of regions across the distal humerus were calculated and compared within modern humans. Asymmetry was significant for all properties, but declined distally from 40% through the periarticular region. Asymmetries were correlated with ZP asymmetry at midshaft, but correlations declined with distance.

In total, these results suggest that fossil hominin distal humeri have more similarities to extant apes than previously reported, in regions particularly responsive to loading, which may support claims of retained arboreal behaviors.

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