Gracility of the modern Homo sapiens skeleton is the result of decreased biomechanical loading

Timothy M. Ryan; Colin N. Shaw

doi:10.1073/pnas.1418646112

New Research In

Edited by Clark Spencer Larsen, The Ohio State University, Columbus, OH, and accepted by the Editorial Board November 20, 2014 (received for review September 29, 2014)

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Significance

Compared with other primates and earlier human species, contemporary humans possess relatively lightly built skeletons. Previous studies suggest that skeletal gracility results from a lack of physical activity because of increased reliance on culture, is a biomechanical consequence of bipedal locomotion, or reflects systemic physiological differences between modern humans and other primates. We found that bone strength in the hip joint of human foragers is comparable to similarly sized nonhuman primates, and is significantly more robust than sedentary agriculturalists. These results conclusively demonstrate the utility of trabecular bone structure for differentiating activity and mobility patterns among prehistoric hominins and reveal that low levels of physical activity contribute to reduced bone strength, and consequently increased fracture risk, in contemporary human populations.

Abstract

The postcranial skeleton of modern Homo sapiens is relatively gracile compared with other hominoids and earlier hominins. This gracility predisposes contemporary humans to osteoporosis and increased fracture risk. Explanations for this gracility include reduced levels of physical activity, the dissipation of load through enlarged joint surfaces, and selection for systemic physiological characteristics that differentiate modern humans from other primates. This study considered the skeletal remains of four behaviorally diverse recent human populations and a large sample of extant primates to assess variation in trabecular bone structure in the human hip joint. Proximal femur trabecular bone structure was quantified from microCT data for 229 individuals from 31 extant primate taxa and 59 individuals from four distinct archaeological human populations representing sedentary agriculturalists and mobile foragers. Analyses of mass-corrected trabecular bone variables reveal that the forager populations had significantly higher bone volume fraction, thicker trabeculae, and consequently lower relative bone surface area compared with the two agriculturalist groups. There were no significant differences between the agriculturalist and forager populations for trabecular spacing, number, or degree of anisotropy. These results reveal a correspondence between human behavior and bone structure in the proximal femur, indicating that more highly mobile human populations have trabecular bone structure similar to what would be expected for wild nonhuman primates of the same body mass. These results strongly emphasize the importance of physical activity and exercise for bone health and the attenuation of age-related bone loss.

Footnotes

↵¹To whom correspondence should be addressed. Email: tmr21{at}psu.edu.

Author contributions: T.M.R. and C.N.S. designed research, performed research, analyzed data, and wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission. C.S.L. is a guest editor invited by the Editorial Board.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1418646112/-/DCSupplemental.