Meeting Abstract

83.4  Friday, Jan. 7  The Human Head-Neck-Shoulder Complex: A Configuration to Free the Hands OSBORN, M.L.*; HOMBERGER, D.G.; Louisiana State Univ., Baton Rouge mosbor1@tigers.lsu.edu

In humans, the large mastoid process and robust clavicle are part of a head-neck-shoulder complex that evolved in connection with the acquisition of an upright posture and freed hands. Our biomechanical analysis shows that when the fore limbs are used to manipulate or carry objects, the upper trapezius muscles contract to counterbalance the added weight on the shoulders. Simultaneously, the contracting sternocleidomastoid muscles prevent the head from being retroflexed by the contracting trapezius muscles. Our hypothesis was tested through a natural experiment provided by the asymmetry of biomechanically relevant paired features in males of known handedness, based on the premise that bones subjected to greater forces tend to be larger. Our analytical approach posits that the features of each member of a population are integrated, forming an individually unique concatenated complex. Our morphometric data from 54 right-handed individuals revealed four features within the complex (i.e., longer left mastoid process, wider right mastoid process, broader left scapula, and greater right humerus diameter), which were asymmetrical in at least 70% of the individuals. Combinations of at least two of these asymmetrical features from different skeletal elements were found to be predictive of right-handedness of an individual. The diameter of the humerus, though the most predictive of the analyzed features (see also Stirland 1983), was not 100% predictive by itself. Differences between individuals are likely to have been caused by different long-term forces exerted on the features due to idiosyncrasies, and athletic or professional activities. The forces that mold the analyzed features in extant humans were probably also part of the selective regime favoring their evolution within the Homo lineage.