Economy and Endurance in Human Evolution.
The evolutionary pressures shaping humans' unique bipedal locomotion have been a focus of research since Darwin, but the origins of humans' economical walking gait and endurance running capabilities remain unclear. Here, I review the anatomical and physiological determinants of locomotor economy (e.g., limb length and posture) and endurance (e.g., muscle volume and fiber type) and investigate their development in the hominin fossil record. The earliest hominins were bipedal but retained ape-like features in the hind limb that would have limited their walking economy compared to living humans. Moreover, the evolution of bipedalism and the loss of the forelimbs in weight support and propulsion would have reduced locomotor endurance in the earliest hominins and likely restricted ranging. Australopithecus evinced longer hind limbs, extended limb posture, and a stiff midfoot, suggesting improved, human-like economy, but were likely still limited in their endurance compared to modern humans. The appearance of skeletal traits related to endurance (e.g., larger limb joints, spring-like plantar arch) in Homo was somewhat mosaic, with the full endurance suite apparent only ∼1 million years ago. The development of endurance capabilities in Homo appears to parallel the evolutionary increase in brain size, cognitive sophistication, and metabolic rate.