Plausible Theories of Bipedalism as of March 2024
Anthropologists have made valiant efforts to propose theories explaining why human bipedalism evolved. Researchers have proposed theories ranging from the more outlandish such as the fighting hypothesis and the aquatic ape hypothesis to more believable ones such as the energy efficiency theory and the arboreal model, and it is important to separate the noise from the signals. So, while it was fun to cover the entertaining hypotheses, it is mandatory to explore the most supported theories.
One of those hypotheses is the arboreal hypothesis that claims human bipedalism adapted from arboreal ancestors. The hypothesis revolves around the central idea that arboreal human ancestors would have been outstretched in the canopies of trees, reaching for fruits, testing the integrity of branches, and grasping them as they move. A group of researchers attempted to garner support for the theory by observing wild chimpanzees in Uganda. After observing them for approximately 247 hours, the researchers concluded that chimpanzee “[b]ipedalism was observed only on arboreal substrates, and was almost all postural, and not locomotor,” which demonstrates that “…the origin of bipedal posture in hominids to be related to foraging advantages in fruit trees” (Stanford 2006). Another research team explored the same theory by conducting a year long study on the locomotion of Orangutans. After obtaining “2811 observations of orangutan locomotor bouts” they found that “[h]uman bipedalism [may be]…an exploitation of a locomotor behavior retained from the common great ape ancestor” (Thorpe et al. 2007). The evidence in the locomotion of Orangutans affirms the findings in chimpanzee locomotion further supporting the arboreal model. While there is strong support for the arboreal model, there is another strong hypothesis as well: the energy efficiency hypothesis.
The energy efficiency hypothesis states that bipedalism evolved because it was far more efficient than quadrupedalism. The increased efficiency allowed early humans to travel longer distances and forage for resources more effectively than their quadrupedal cousins. A research team tested the hypothesis by analyzing the energy exerted by humans and chimpanzees as they walked and found that, on average, “…human walking is 75% less costly than both quadrupedal and bipedal walking in chimpanzees” (Sockol et al. 2007). The energy exhaustion differences are well explained by humans’ “…more extended hip[s] and longer hindlimb” (Sockol et al. 2007). The results successfully replicate earlier studies such as Rodman and McHenry’s 1980 study that examined the energy costs between chimpanzees and humans as they locomote. The need to conserve energy as early humans needed to travel long distances may have been a driving force in the adaptation, and the data seems to agree with the hypothesis rendering the energy efficiency hypothesis as one of the more robust explanatory theories for human bipedalism.
The origins of bipedalism remain unclear; however, some theories are more robust than others. The validity of each hypothesis lies in an anthropologist’s ability to test it. The energy efficiency hypothesis found its merit through procedural studies that developed significant findings. However, perhaps the most robust one at the moment is the arboreal model, which claims that bipedalism has adapted from a need to stretch through treetops. None of the theories are uniformly accepted by all, but the energy efficiency theory and, especially the arboreal model are among the strongest.
References:
Rodman, Peter S., and Henry M. McHenry. “Bioenergetics and the origin of hominid bipedalism.” American Journal of Physical Anthropology, vol. 52, no. 1, Jan. 1980, pp. 103–106, https://doi.org/10.1002/ajpa.1330520113.
Sockol, Michael D., et al. “Chimpanzee locomotor energetics and the origin of human bipedalism.” Proceedings of the National Academy of Sciences, vol. 104, no. 30, 24 July 2007, pp. 12265–12269, https://doi.org/10.1073/pnas.0703267104.
Stanford, Craig B. “Arboreal bipedalism in wild chimpanzees: Implications for the evolution of hominid posture and locomotion.” American Journal of Physical Anthropology, vol. 129, no. 2, 15 Nov. 2005, pp. 225–231, https://doi.org/10.1002/ajpa.20284.
Thorpe, S. K., et al. “Origin of human bipedalism as an adaptation for locomotion on flexible branches.” Science, vol. 316, no. 5829, June 2007, pp. 1328–1331, https://doi.org/10.1126/science.1140799.