Comparisons of Forelimb Muscular Anatomy in Humans, Macaques, and Chimpanzees: An Evolutionary Perspective

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The forearm extensor muscles are physiologically organized into three functional groups; one group is made up of muscles that extend and abduct or adduct the hand at the wrist. Two of these muscles are Extensor Carpi Radialis Longus (ECRL) and Extensor Carpi Radialis Brevis (ECRB). Both originate from the distal end of the humerus; ECRL inserts onto the base of metacarpal (MC) 2 and ECRB inserts onto the base of MC3. Previous research has demonstrated that 24% of human forearms include an extra extensor muscle, which has important clinical implications. Here, we test the hypothesis that this extra extensor muscle is present in a quarter of human forearms, but not in other primates due to locomotor constraints. Our sample includes prosected human (n=25), macaque (n=90), and chimpanzee (n=2) cadavers. All cadavers (n=117) were surveyed for the presence or absence of accessory extensor muscles, noting the origin and insertion of extra muscles. Results indicate the presence of an additional extensor, most likely Extensor Carpi Radialis Accessorius (ECRA) in 5% of the human sample, and complete absence in both the macaque and chimpanzee samples. Given the similar anatomy among these three species, it is noteworthy that accessory extensor muscles do not appear in non‐human primate forearm muscles, especially in the chimpanzee, which is closely related to humans. However, based on patterns in human evolution, these results are somewhat expected. Throughout primate, including human, evolution, locomotor patterns have changed and allowed for adaptations in the wrist and forelimb. Macaques are terrestrial quadrupeds and thus have stable wrists. Brachiating animals, like chimpanzees, also have stable wrists to support their body weight while suspended. Both of these locomotor patterns are reflected in the musculoskeletal anatomy of these animals. Humans have evolved more flexible wrists, since their forearms are unnecessary for bipedal locomotion. Thus, as a byproduct of primate evolution, species such as humans develop extra extensor muscles for use in tool making and other needs, whereas other primates do not in order to maintain wrist stability for more important demands like locomotion. This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal .

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