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

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. Anat Rec, 298:1572Rec, 298: -1588Rec, 298: , 2015. V C 2015 Wiley Periodicals, Inc.

Growth of the pectoral girdle in a sample of juveniles from the kellis 2 cemetery, Dakhleh Oasis, Egypt

“…During the evolution of the human shoulder suspension apparatus, a reorientation of the forces of the sternocleidomastoid and trapezius muscles altered the growth, shape, and size of the pectoral girdle (Osborn and Homberger, ). These muscle forces are also reoriented during infancy allowing the infant to sit upright, control head movement, and eventually engage in independent bipedal locomotion with swinging arms during early childhood (i.e., toddler years) (Ledebt, ; Osborn and Homberger, ; Sutherland et al, ). Growth patterns of the pectoral girdle reported here may reflect the changing roles of the shoulder and clavicle during the transition from crawling to upright walking.…”

“…Growth patterns of the pectoral girdle reported here may reflect the changing roles of the shoulder and clavicle during the transition from crawling to upright walking. For instance, an elongated clavicle helps to hold the upper limb away from the axial skeleton (Voisin, ), which would permit the arms to hang down and freely swing during upright bipedal locomotion (Osborn and Homberger, ). In this study, the clavicle experiences its greatest post‐natal growth acceleration during early childhood.…”

“…The main functions of the pectoral girdle are to increase upper limb mobility by reducing articular constraints and stabilize the head and trunk during bipedal locomotion (Palastanga et al, ; Scheuer and Black, ). The scapula largely serves as a bony attachment site for several distinct muscles (e.g., deltoid, trapezius, rhomboid major and minor, levator scapulae, teres major and minor, serratus anterior, supraspinatus, infraspinatus, latissimus dorsi, triceps brachii, omohyoid) in the cranial (head), cervical (neck), brachial (upper arm), and upper dorsal (back) regions of the body; and, as such, plays a central role in head and upper body posture and upper limb function (Jobe et al, ; Osborn and Homberger, ). The clavicle is interposed between the scapula and thorax, and articulates with the former at the acromioclavicular joint and with the latter at the sternoclavicular joint (Palastanga et al, ).…”

“…It has been suggested that the shoulder suspension apparatus (i.e., head, neck, and shoulders) evolved in tandem with the adoption of upright posture, increased manual dexterity, and bipedalism (Osborn and Homberger, ; Pontzer et al, ). Muscle forces can influence bone growth and eventual skeletal size (Hall, ; LeVeau and Bernhardt, ; Roux, ); and, it has been demonstrated that the size of skeletal features associated with the adult shoulder suspension apparatus is causally related to mechanical forces (Osborn and Homberger, ).…”

“…It has been suggested that the shoulder suspension apparatus (i.e., head, neck, and shoulders) evolved in tandem with the adoption of upright posture, increased manual dexterity, and bipedalism (Osborn and Homberger, ; Pontzer et al, ). Muscle forces can influence bone growth and eventual skeletal size (Hall, ; LeVeau and Bernhardt, ; Roux, ); and, it has been demonstrated that the size of skeletal features associated with the adult shoulder suspension apparatus is causally related to mechanical forces (Osborn and Homberger, ). Given that the pectoral girdle largely serves as a muscle attachment site for several muscles of the head, neck, and upper arm, it is hypothesized that growth patterns in the scapula and clavicle will reflect, in part, changes in muscle forces acting on the pectoral girdle as an infant learns to sit up, cruise, and eventually walk; and, as manual dexterity increases once the upper limbs are no longer tied to locomotor constraints (e.g., crawling).…”