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The Function and Evolution of the Mastoid Process and Clavicle in Humans: A Natural Experiment
Author(s) -
Osborn Michelle L,
Homberger Dominique G
Publication year - 2010
Publication title -
the faseb journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.709
H-Index - 277
eISSN - 1530-6860
pISSN - 0892-6638
DOI - 10.1096/fasebj.24.1_supplement.lb9
Subject(s) - mastoid process , clavicle , anatomy , extant taxon , collar , shoulders , process (computing) , medicine , orthodontics , biology , surgery , computer science , evolutionary biology , mechanical engineering , engineering , operating system
We hypothesize that, in humans, the evolution of a uniquely enlarged mastoid process and clavicle was connected to the acquisition of an upright posture and of freed hands. Our biomechanical model shows that when the arms are moved or loaded, the contracting upper trapezius muscles counterbalance the added weight on the shoulders. The synchronously contracting sternocleidomastoid muscles prevent the head from being retroflexed. To test our hypotheses, we analyzed the asymmetry of biomechanically relevant character pairs in 54 right‐handed male skeletons, based on the premise that bones subjected to greater forces are larger. The humerus diameter, mastoid process width, and rise of the superior nuchal line were more often larger on the right side, while the clavicle length and scapular breadth were so on the left side. These biomechanically relevant characteristics were found in varying aggregates of 2–5 for each individual. This variability may be due to differing occupations. The forces that mold the mastoid process and clavicle in extant humans probably also selected for their robust size within the Homo lineage. Funding: LSU Robert C. West Fund to MLO; LSU Foundation to DGH