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.

Cited Works (10)

Year	Entry
2001	Hsieh Y, Robling AG, Ambrosius WT, Burr DB, Turner CH. Mechanical loading of diaphyseal bone in vivo: the strain threshold for an osteogenic response varies with location. J Bone Miner Res. December 2001;16(10):2291-2297.
2000	Haapasalo H, Kontulainen S, Sievänen H, Kannus P, Järvinen M, Vuori I. Exercise-induced bone gain is due to enlargement in bone size without a change in volumetric bone density: a peripheral quantitative computed tomography study of the upper arms of male tennis players. Bone. September 2000;27(3):351-357.
1977	Jones HH, Priest JD, Hayes WC, Tichenor CC, Nagel DA. Humeral hypertrophy in response to exercise. J Bone Joint Surg. March 1977;59A(2):204-208.
1988	Bertram JEA, Biewener AA. Bone curvature: sacrificing strength for load predictability? J Theo Biol. March 7, 1988;131(1):75-92.
2002	Bass SL, Saxon L, Daly RM, Turner CH, Robling AG, Seeman E, Stuckey S. The effect of mechanical loading on the size and shape of bone in pre-, peri-, and postpubertal girls: a study in tennis players. J Bone Miner Res. December 2002;17(12):2274-2280.
2014	Warden SJ, Roosa SMM, Kersh ME, Hurd AL, Fleisig GS, Pandy MG, Fuchs RK. Physical activity when young provides lifelong benefits to cortical bone size and strength in men. Proc Natl Acad Sci USA. April 8, 2014;111(14):5337-5342.
2000	Ruff CB. Body size, body shape, and long bone strength in modern humans. J Hum Evol. February 2000;38(2):269-290.
2002	Robling AG, Hinant FM, Burr DB, Turner CH. Improved bone structure and strength after long‐term mechanical loading is greatest if loading is separated into short bouts. J Bone Miner Res. August 2002;17(8):1545-1554.
1983	Biewener AA. Allometry of quadrupedal locomotion: the scaling of duty factor, bone curvature and limb orientation to body size. J Exp Biol. July 1983;105(1):147-171.
2006	Ruff C, Holt B, Trinkaus E. Who's afraid of the big bad Wolff? "Wolff's law" and bone functional adaptation. Am J Phys Anthropol. April 2006;129(4):484-498.