Chimpanzee locomotor energetics and the origin of human bipedalism

Abstract

Bipedal walking is evident in the earliest hominins [Zollikofer CPE, Ponce de Leon MS, Lieberman DE, Guy F, Pilbeam D, et al. (2005) Nature 434:755-759], but why our unique two-legged gait evolved remains unknown. Here, we analyze walking energetics and biomechanics for adult chimpanzees and humans to investigate the long-standing hypothesis that bipedalism reduced the energy cost of walking compared with our ape-like ancestors [Rodman PS, McHenry HM (1980) Am J Phys Anthropol 52:103-106]. Consistent with previous work on juvenile chimpanzees [Taylor CR, Rowntree VJ (1973) Science 179:186-187], we find that bipedal and quadrupedal walking costs are not significantly different in our sample of adult chimpanzees. However, a more detailed analysis reveals significant differences in bipedal and quadrupedal cost in most individuals, which are masked when subjects are examined as a group. Furthermore, human walking is approximately 75% less costly than both quadrupedal and bipedal walking in chimpanzees. Variation in cost between bipedal and quadrupedal walking, as well as between chimpanzees and humans, is well explained by biomechanical differences in anatomy and gait, with the decreased cost of human walking attributable to our more extended hip and a longer hindlimb. Analyses of these features in early fossil hominins, coupled with analyses of bipedal walking in chimpanzees, indicate that bipedalism in early, ape-like hominins could indeed have been less costly than quadrupedal knucklewalking.

Fig. 1.

Net cost of transport (ml of O₂ kg⁻¹ m⁻¹) for chimpanzee quadrupedal walking (blue), chimpanzee bipedal walking (red), and human walking (yellow). Dashed lines indicate trendlines for running and walking in birds and mammals. The running trendline is for 65 terrestrial species (15). Walking data (open symbols) were collected from the literature [see supporting information (SI) Table 2].

Fig. 2.

Comparison of walking mechanics in chimpanzees and humans. (a) GRF vectors and joint torque for humans and chimpanzees. Figures show joint positions at 50% stance (forelimb and hindlimb shown separately for quadrupedal chimpanzees). Positive torque values indicate flexion, whereas negative values indicate extension. Note the large hip flexion moments in chimpanzees relative to humans. Horizontal dashed lines indicate zero joint torque. (b) V_musc per newton of bodyweight (cm³/N) at each joint and in the whole limb for chimpanzee hindlimbs (dark blue) and forelimbs (light blue) during quadrupedal walking, chimpanzee hindlimbs during bipedalism (red), and human hindlimbs (yellow). (c) Mean t_c (in seconds) during walking in chimpanzees and humans. Note that Froude numbers are similar for all groups (Fr = 0.2; Table 1), but absolute speeds are slightly higher for humans (Table 1).

Fig. 3.

Comparison of differences in V_musc/t_c (white bars) and COL (gray bars) between gaits and species. Error bars indicate ±1 SEM percent difference.

Fig. 4.

Comparison of thigh angle, knee flexion, and t_c for C4 versus other chimpanzees (n = 4). Thigh angles is measured as the angle between the thigh segment and the horizontal (e.g., thigh angle is 90° when the thigh is perpendicular to the ground). Knee angle is measured as the angle between the thigh and leg where full knee extension is 180°. t_c is the time elapsed from touchdown to toe-off. Asterisks indicate significant differences between quadrupedal (blue) and bipedal (red) and strides (P < 0.05, Student's paired t test).