A human’s most notable trait is our ability to walk upright. Bipedalism is a trait that has evolved over millions of years, and with its evolution came many other anatomical changes as well. The changes have made walking and running significantly more efficient than bipedal and quadrupedal walking in Chimpanzees (Sockol 2007). Some of the most significant anatomical changes range from the rotation of the pelvis, the bicondylar angle in the femur, and changes in the skull, which brought humans closer to where they are today.
As early human ancestors began walking on two legs, their bodies changed, and one of the most significant changes was in the pelvis. According to the Smithsonian, approximately 1.95 million years ago, the pelvis began to change, which showed that “…early human species had given up climbing for walking.” Interestingly, the human “…pelvis changed more than any other bone in [the human] body,” and that had other consequences (DeSilva 182). Those changes led to a much more difficult birthing process as the new anatomical changes created a new birthing “…path by which primates typically are birthed [and that] does not work for [humans]” (DeSilva 180). The rotation of the pelvis may have made walking easier; however, the change is also an example of how evolution can favor some benefits over others: walking efficiency vs. birthing efficiency.
Along with the change in the pelvis came the change in the legs. Around 1.89 million years ago, the elongated femurs of Homo erectus allowed them to take longer strides making their walking more efficient (Smithsonian). In addition to its elongation, the femur began to angle inward from the pelvis to the knee, known as the “bicondylar angle.” The angling of our femur inwards is beneficial for keeping the body balanced. However, much like the pelvis, there are tradeoffs as well, and the bicondylar angle can lead to increased “patellar subluxation” (dislocation of the kneecap); however, the body also developed a lateral patellar lip that has evolved to correct that instability (DeSilva 176).
Another anatomical feature that changed when humans began walking upright was the position of their foramen magnum. The foramen magnum is the hole in the bottom of the skull that helps connect the head to the rest of the body. As ancient bipedal ancestors evolved, the position of their foramen magnum began to rotate from the back of the skull to the bottom, and that change is seen in humans today. Researchers sampled a group of modern primates and found that among “…the primates sampled, humans exhibit the most anteriorly positioned foramen magnum” (Russo and Kirk 2013).The research supports a key marker for identifying bipedal fossils. When paleontologists unearth fossils, they look for the position of the foramen magnum to determine the probability that the fossil belonged to a bipedal animal.
Bipedalism has had many subsidiary consequences other than the creation of upright walking, and those changes balance positive ones that make walking more efficient and negative ones, such as the rotation of the pelvis that makes childbirth more challenging or a vulnerable knee. However, without bipedalism, humans would lose their most distinguishing trait, allowing anthropologists to examine how evolution can mitigate the vulnerabilities it creates.
References:
DeSilva, Jeremy. First Steps: How Upright Walking Made Us Human. Harper, an Imprint of HarperCollinsPublishers, 2022.
Russo, Gabrielle A., and E. Christopher Kirk. “Foramen magnum position in bipedal mammals.” Journal of Human Evolution, vol. 65, no. 5, Nov. 2013, pp. 656–670, https://doi.org/10.1016/j.jhevol.2013.07.007.
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.