Previous research has suggested that bipedalism is a less physiologically costly form of locomotion (compared to quadrupedalism), suggesting a selective advantage for walking upright. And there must be some advantage to explain why hominins aren’t quadrupeds, because bipeds can’t move faster (to outrun a predator, for example). A study by Michael Sockol et al., published in the PNAS last week, discusses the biomechanics of chimpanzee vs. human locomotion, attempting to answer the question:
Do differences in anatomy and gait between bipedal and quadrupedal walking, as well as between chimpanzees and humans, explain observed differences in cost [of transportation]?
They trained 5 chimps to walk quadrupedally and bipedally on a treadmill, measured cost of transportation (in ml of O2 / kg m), and compared the results with similar tests on human volunteers. Not surprisingly, cost of transport was higher for chimps than humans, regardless of type of locomotion.
What I found most interesting was that for one of the chimps, bipedal locomotion was actually less costly than quadrupedalism. Translation, there’s variation in the chimpanzee sample, and some chimps are more effective bipeds than others.
Sockol et al.’s (2007) Figure 4. Comparison of thigh angle, knee flexion, and contact time for Chimpanzee 4 versus other chimpanzees. Thigh angle is measured as the angle between the thigh segment and the horizontal. Knee angle is measured as the angle between the thigh and leg where, and contact time is the time elapsed from touchdown to toe-off. Asterisks indicate significant differences between quadrupedal (blue) and bipedal (red) strides.
Compared to other chimps in the study, chimpanzee 4 showed no significant difference for thigh angle or knee angle between bipedal and quadrupedal strides, but did show significantly longer contact time while bipedal. This provides insight into how bipedalism may have developed in the common ancestor of chimps and hominins: variation in bipedal ability gave a selective advantage to those for whom bipedalism was less costly.
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Sockol, M., Raichlen, D., & Pontzer, H. (2007). Chimpanzee locomotor energetics and the origin of human bipedalism Proceedings of the National Academy of Sciences, 104 (30), 12265-12269 DOI: 10.1073/pnas.0703267104
Tags: Anthropology, bipedal-locomotion, bipedalism, chimpanzee, chimps, Evolution, primatology
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