Evolutionary Variation
The pectoral muscles — the pectoralis major and pectoralis minor — evolved from a primitive muscle sheet that connected the coracoid to the humerus. In late reptilians and early mammals, this muscle structure was displaced dorsally; while most of its components evolved into the pectoralis major, some fibers eventually attached to the scapula and evolved into the supraspinatus, the infraspinatus, and parts of the subscapularis.
Read more about this topic: Infraspinatus Muscle
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