Insights into the evolution of human bipedalism from experimental studies of humans and other primates

Abstract

SUMMARYAn understanding of the evolution of human bipedalism can provide valuable insights into the biomechanical and physiological characteristics of locomotion in modern humans. The walking gaits of humans, other bipeds and most quadrupedal mammals can best be described by using an inverted-pendulum model, in which there is minimal change in flexion of the limb joints during stance phase. As a result, it seems logical that the evolution of bipedalism in humans involved a simple transition from a relatively stiff-legged quadrupedalism in a terrestrial ancestor to relatively stiff-legged bipedalism in early humans. However, experimental studies of locomotion in humans and nonhuman primates have shown that the evolution of bipedalism involved a much more complex series of transitions, originating with a relatively compliant form of quadrupedalism. These studies show that relatively compliant walking gaits allow primates to achieve fast walking speeds using long strides, low stride frequencies, relatively low peak vertical forces, and relatively high impact shock attenuation ratios. A relatively compliant, ape-like bipedal walking style is consistent with the anatomy of early hominids and may have been an effective gait for a small biped with relatively small and less stabilized joints, which had not yet completely forsaken arboreal locomotion. Laboratory-based studies of primates also suggest that human bipedalism arose not from a terrestrial ancestor but rather from a climbing, arboreal forerunner. Experimental data, in conjunction with anatomical data on early human ancestors, show clearly that a relatively stiff modern human gait and associated physiological and anatomical adaptations are not primitive retentions from a primate ancestor, but are instead recently acquired characters of our genus.

Laetoli footprints reveal bipedal gait biomechanics different from those of modern humans and chimpanzees

“…But even small differences in limb posture could have had important evolutionary implications. If the Laetoli G1 footprints were made by A. afarensis, and members of that taxon typically used a bipedal gait that involved relatively more flexed limbs, then A. afarensis probably would have experienced somewhat higher bipedal locomotor costs compared with modern humans [36][37][38] despite having similarly long lower limbs [39]. A higher cost of bipedal locomotion relative to modern humans could have had important adaptive consequences for A. afarensis, affecting mobility, home range size and many other aspects of interactions with the environment.…”

Does a crouched leg posture enhance running stability and robustness?

“…However, bipedalism can be found within a few families of mammals, reptiles and within all birds. Among mammals, various groups of primates (Schmitt, 2003), the macropods (Windsor and Dagg, 2010) and a few groups of heteromyd rodents (Djawdan, 1993) locomote bipedally. Within reptiles, some families of lizards are also capable of bipedal locomotion (Aerts et al, 2003), especially when running at high speeds (Irschick and Jayne, 1999).…”

Üner Tan Syndrome: Review and Emergence of Human Quadrupedalism in Self-Organization, Attractors and Evolutionary Perspectives

“…Examples of diagonal sequence footfall patterns in a tetrapod and a non-human primate, also showing the ground reaction forces (arrows) on the ipsilateral and contralateral forelimbs and hind limbs. Notice greater ground reaction forces on the left forelimb and the contralateral right hind limb during walking on all four extremities in a tetrapod, and greater vertical force on the left hind foot (F hind ) than the contralateral (right) fore foot (F fore ) during diagonal sequence quadrupedal locomotion in a non-human primate (modified from Schmitt, 2003).…”

“…By contrast most primates support their body weight more on their hind limbs than their forelimbs during diagonal sequence quadrupedal locomotion, whether they are standing or moving, on the ground or in trees (Kimura et al, 1979;Demes et al, 1994;Schmitt, 2003;Schmitt & Hanna, 2004). The increased hind limb weight support in non-human primates is generally interpreted as an adaptation that reduces stress on the forelimb joints, facilitating the forelimb mobility, especially for arboreal locomotion (Reynolds, 1985;Larson, 1998).…”