Evolution of Hominin Forelimbs in the Context of Bipedalism

Abstract

The evolution of bipedalism in the hominin lineage coincided with a major shift in the locomotion function of the forelimbs, from producing external forces in contact with the substrate in the arboreal and quadrupedal last common ancestor with chimpanzees, to producing no external forces but swinging as angular momentum counterweights to the legs in striding bipedalism. The shift in forelimb function has been an important topic of study in human evolution, with fossil forelimbs used to interpret the behavior of extinct species and the degree to which they relied on terrestrial bipedalism as a locomotion strategy. This thesis uses biomechanical models and experiments of human walking and running in three studies to investigate how forelimb variation observed in hominin fossils affect the mechanics and costs of bipedal locomotion in order to refine interpretations of the evolution of bipedalism in the hominin lineage. The first study addressed the question, Why do humans walk with straight arms but run with bent arms? In order to answer the question an experiment was conducted with a modern human sample walking and running with both straight and bent forelimbs. The results of the study indicated that a mechanical tradeoff exists when bending the forelimb at the elbow; bent forelimbs reduce shoulder muscle torque at the cost of increased elbow muscle torque. Net metabolic rate results showed that the mechanical tradeoff favors straight forelimbs during walking, as bent forelimbs increased metabolic rate by 11%. However, the cost of running was equivalent with straight and bent forelimbs, leaving the question of why humans run with flexed elbows unanswered. The second study addressed the effect of distal forelimb length on the muscle torques at the elbow during walking and running. An experiment was conducted with modern humans walking and running holding hand weights that moved the center of mass of the distal forelimb away from the elbow, experimentally lengthening the segment. Longer distal forelimbs increased the required elbow muscle torque for both gaits, but the effect size was approximately three times greater for running compared to walking. In the hominin fossil record a shift towards relatively shorter distal forelimbs occurred in Homo erectus, coincident with the evolution of endurance running. The results of the second study shed light on the evolution of hominin forelimbs, linking forelimb biomechanics during running to selection for shorter distal forelimbs. The third study addressed functional scaling of forelimb swing dynamics across a range of body sizes and compared functional scaling to geometric scaling of interlimb proportions. Data from an experiment of modern human walking, combined with a theoretical scaling model of shoulder muscle function, indicated that under the constraint of similar shoulder muscle function bigger hominins require relatively shorter forelimbs compared to small variants. Extinct hominin limb proportions are well predicted by the functional scaling model, which outperforms a geometric scaling model that does not incorporate mechanical function. The results of the third study suggest that the shift to relatively short forelimbs in the genus Homo, previously interpreted as a signal of a transition from an ancestral mix of arboreal and terrestrial bipedal locomotion to obligate terrestrial bipedalism, is more simply explained as a shift to bigger body size in Homo. The results of this thesis shed new light on the evolution of human-like walking and running and the origins of the genus Homo. Previous interpretations of hominin locomotion behavior that posit a compromised and costly bipedal gait in hominins before Homo lack biomechanical underpinnings and rely solely on morphological evidence. The results presented here provide the first mechanistic approach to understanding the evolution of hominin forelimbs and lead to the conclusion that human-like walking function evolved in Australopithecus, followed by the coincident evolution of larger body size and endurance running in the genus Homo.