The Human Shoulder Suspension Apparatus: A Causal Explanation for Bilateral Asymmetry and a Fresh Look at the Evolution of Human Bipedality

7Citations
Citations of this article
35Readers
Mendeley users who have this article in their library.

This article is free to access.

Abstract

The combination of large mastoid processes and clavicles is unique to humans, but the biomechanical and evolutionary significance of their special configuration is poorly understood. As part of the newly conceptualized shoulder suspension apparatus, the mastoid processes and clavicles are shaped by forces exerted by the musculo-fascial components of the cleidomastoid and clavotrapezius muscles as they suspend the shoulders from the head. Because both skeletal elements develop during infancy in tandem with the attainment of an upright posture, increased manual dexterity, and the capacity for walking, we hypothesized that the same forces would have shaped them as the shoulder suspension apparatus evolved in ancestral humans in tandem with an upright posture, increased manual dexterity, and bipedality with swinging arms. Because the shoulder suspension apparatus is subjected to asymmetrical forces from handedness, we predicted that its skeletal features would grow asymmetrically. We used this prediction to test our hypothesis in a natural experiment to correlate the size of the skeletal features with the forces exerted on them. We (1) measured biomechanically relevant bony features within the shoulder suspension apparatus in 101 male human specimens (62 of known handedness); and (2) modeled and analyzed the forces within the shoulder suspension apparatus from X-ray CT data. We identified eight right-handed characters and demonstrated the causal relationship between these right-handed characters and the magnitude and direction of forces acting on them. Our data suggest that the presence of the shoulder suspension apparatus in humans was a necessary precondition for human bipedality.

Cite

CITATION STYLE

APA

Osborn, M. L., & Homberger, D. G. (2015). The Human Shoulder Suspension Apparatus: A Causal Explanation for Bilateral Asymmetry and a Fresh Look at the Evolution of Human Bipedality. Anatomical Record, 298(9), 1572–1588. https://doi.org/10.1002/ar.23178

Register to see more suggestions

Mendeley helps you to discover research relevant for your work.

Already have an account?

Save time finding and organizing research with Mendeley

Sign up for free