Fossil Evidence for the Evolution of Hominin Locomotion
Authors: Ward Carol V. 
Published: June 2021

The fossil record is the only source of information we have on the order and timing of the acquisition of the distinctive characteristics that make us human. Our ability to accurately reconstruct our evolutionary history depends entirely on the accuracy with which we can interpret these fossils. While comparative analysis can generate important hypotheses about how and why fossils vary in their bony anatomy, experimental biology is providing new tests of the models on which these hypotheses are based, and new interpretations of what observed variation reveals about the biology and behavior of our fossil relatives.

One of the most distinctive characteristics of hominins is our unique mode of terrestrial bipedal posture and locomotion. Even the earliest hominins display characters associated with upright bipedality, although none shares all characters typical of modern humans. The retention of primitive, ‘ape‐like’ features has been argued to reflect stabilizing selection for arboreal competency, perhaps leading to compromised bipedal gait. However, studies of australopith footprints suggest that the kinematics of bipedality in australopiths was equivalent to that of humans. Articular surface geometry, which can be influenced by loading vectors during ontogeny, also suggests a fully human‐like gait, and new fossils suggest that australopiths had human‐like lower limb proportions. Australopiths are also argued to have more powerful upper limbs with more curved phalanges, but experimental evidence calls into question the strength of these inferences. Variation among australopith samples has been suggested to reflect adaptive differences among species, although consideration of intraspecific variation in extant hominoids and experimental links between activity and bone form may provide tests of such interpretations.

The transition from australopith early Homo postcranial form is often tied to the abandonment of the trees, although this does not explain why observed changes would have occurred. Experiments testing the adaptive benefits of more modern Homo morphology on physiology, locomotor efficiency and upper limb function are providing hypotheses to explain why these changes may have been favored by selection. The fossil record suggests some of these shifts may not have characterized the origins of Homo or even of Homo erectus, but rather appeared within the evolution of early H. erectus. A decrease in skeletal robusticity in anatomically modern Homo sapiens 40–30,000 years ago has traditionally been explained as a response to behavioral shifts involving the dramatic increase in technology. The increasing evidence for systemic influences on skeletal robusticity suggests, instead, that a genetically determined physiological shift in human biology near this time may underlie these differences, and that specific activities may have had less influence than often interpreted.

A growing body of experimental work is providing a new lens through which we can view the fossil record and leading to insights that are refining the picture of how we evolved. The fossil record continues to raise new questions to guide experimental investigations. Experimental work, in turn, provides validated models to test the hypotheses we use to interpret the fossils. It is through a combination of the recovery of new fossils, detailed comparative analysis of hard and soft tissues, and modern experimental biology that a picture of how we evolved will continue coming into focus.

This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.