A Comparative Study of Leg Musculature in Humans and Non‐human Primates and its Evolutionary Implications

Abstract

In humans, the lateral compartment of the leg consists of the fibularis longus (FL) and fibularis brevis (FB) muscles, which originate from the lateral shaft of the fibula. These two muscles serve to plantar flex and evert the foot. While FL inserts on the first metatarsal, FB inserts on the fifth metatarsal. Occasionally, a third muscle is present, the fibularis digiti quinti (FDQ), which is described in previous studies as an accessory tendon arising from FB and inserting on variable parts of the fifth digit, which extends the fifth toe. The sparse literature for great apes indicates that FDQ presents similarly to humans as an accessory tendon. In monkeys, however, FDQ presents as a distinct muscle, with its own belly and a long, thin tendon that runs between FL and FB and inserts on the fifth distal phalanx. Previous studies of humans have found that 32–33% have a variant of the FDQ muscle, which typically manifests itself as an accessory tendon traveling from FB to the fifth digit.In this study, we test the hypothesis that FDQ is present in a third of human legs and compare the variations of FDQ in humans to that of chimpanzees and macaque monkeys. Our sample includes cadavers of humans (n=25), chimpanzees (n=2), and macaque monkeys (n=90). All cadavers (n=117) were surveyed for the presence or absence of the FDQ muscle, noting the origin and insertion if present.Since the FDQ is observed among different primates, it is viewed as an evolutionary primitive trait that is variably retained among the higher order of primates. While previous human research documents FDQ muscles as arising from the FB tendon, this study provides the first documentation of the FDQ in a single human with its own separate muscle belly and tendon, arising from the fibula and inserting on the fifth digit. Comparatively, this human FDQ is more similar to monkeys than previously documented human FDQ muscles. The variation observed in FDQ among humans and apes may reflect a reduced reliance on function – extension of the fifth toe. Although humans and apes appear to share a morphologically similar FDQ, their locomotion patterns are quite different. While the human foot is less dexterous, a necessary condition for walking bipedally, the ape foot is adept for an arboreal environment, which requires great dexterity for grasping while climbing. This suggests that while the foot of humans and apes has very different functional adaptations, they still share a reduced need to extend the fifth digit, leading to a less primitive condition as compared to monkeys.This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.