Search

Among living mammals, only the African apes and some anteaters adopt knuckle-walking as their primary locomotor behavior. That Pan and Gorilla both knuckle-walk has been cited as evidence of their common ancestry and a primitive condition for a combined Homo, Pan, and Gorilla clade. Recent research on forelimb ontogeny and anatomy, in addition to recently described hominin fossils, indicate that knuckle-walking was independently acquired after divergence of the Pan and Gorilla lineages. Although the large-bodied, largely suspensory orangutan shares some aspects of the African ape bauplan, it does not regularly knuckle-walk when terrestrial. While many anatomical correlates of knuckle-walking have been identified, a functional explanation of this unusual locomotor pattern has yet to be proposed. Here, we argue that it was adopted by African apes as a means of ameliorating the consequences of repetitive impact loadings on the soft and hard tissues of the forelimb by employing isometric and/or eccentric contraction of antebrachial musculature during terrestrial locomotion. Evidence of this adaptation can be found in the differential size and fiber geometry of the forearm musculature, and differences in torso shape between the knuckle-walking and non-knuckle-walking apes (including humans). We also argue that some osteological features of the carpus and metacarpus that have been identified as adaptations to knuckle-walking are consequences of cartilage remodeling during ontogeny rather than traits limiting motion in the hand and wrist. An understanding of the functional basis of knuckle-walking provides an explanation of the locomotor parallelisms in modern Pan and Gorilla. Anat Rec, 301:496-514, 2018. © 2018 Wiley Periodicals, Inc., (© 2018 Wiley Periodicals, Inc.)

Study Design: We propose that chronic spondylolytic defects at L5 are influenced by insufficient differential mediolateral distances between inferior articular facets of L4 and the superior facets of S1, which results in these structures impinging on adjacent sides of the par interarticularis during hyperlordosis. Individuals with adequate increase in interfacet distances from L4 through S1 are less likely to develop or maintain defects., Objectives: We test the above hypothesis by comparing the transverse interfacet dimensions throughout the lumbar columns of normal and spondylolytic individuals., Summary of Background Data: Spondylolysis is a common condition, yet its etiology is poorly understood. It is generally considered to be the result of a vaguely defined fatigue fracture through the pars interarticularis. The cause(s) of spondylolysis, however, have not been clearly identified., Methods: Lumbar vertebrae from the Hamann-Todd osteological collection at the Cleveland Museum of Natural History were examined. Thirty individuals with bilateral spondylolysis at L5 were compared with 30 age- and sex-matched controls. Differences in transverse distances between articular facets and in transverse breadths of vertebral bodies were compared using two-tailed t tests., Results: Results show that normal individuals have a significantly greater increase in interfacet dimensions progressing down the spine from L4 to S1 than do those with spondylolysis. These differences are not the result of normal individuals having increasingly large vertebrae, as results are significant even when standardized for vertebral body breadth. Vertebral body size itself does not differ systematically between groups., Conclusions: Spondylolysis is the direct result of contact pressures on both sides of the pars interarticularis resulting from inadequate separation between the inferior articular processes of L4 and the superior articular facets of S1. Individuals lacking sufficient increase in transverse interfacet dimensions in their lumbar columns are at greater risk of developing and maintaining spondylolytic defects.

Calcanei from African apes, modern humans, and Australopithecus afarensis are compared to investigate the anatomical and mechanical changes that occurred in this bone as a result of the transition to terrestrial bipedality. Features analyzed include the cross-sectional area and volume of the calcaneal tuber, the geometry and orientation of the articular surfaces, and the surface topography of the calcaneal corpus. Calcaneal morphology is unequivocal in its partitioning of quadrupedal pongids and bipedal hominids.

Talocrural joints of the African apes, modern humans, and A.L.288-1 are compared in order to investigate ankle function in the Hadar hominids. Comparisons between the hominids and African pongids clearly illustrate the anatomical and mechanical changes that occurred in this joint as a consequence of the evolutionary transition to habitual bipedality. Features which are considered include the obliquity of the distal tibial articular surface, the shape of the talar trochlea, and the location and functional implications of the talocrural axis. In every functionally significant feature examined the A.L.288-1 talocrural joint is fully bipedal. Moreover, the Hadar ankle complex also shows the functional constraints which are necessarily imposed by the adaptation to habitual bipedalism.