Why Did Humans Evolve to Walk on Two Legs?
Among primates, humans are the only ones who habitually walk on two legs. Why did humans come to acquire upright bipedalism, an inherently unstable form of locomotion? My specialty is a field of study called "biomechanics," which involves analyzing the mechanisms of human body movements, such as bipedal walking and object manipulation by hand, from a mechanical engineering perspective and applying this knowledge to engineering and medicine. As one of my research themes, I am advancing the study of human evolution, specifically to clarify why and how human bipedalism emerged.
When you think of research on human evolution, you might imagine fossil excavations. Fossils are indeed the most direct evidence of human evolution and are crucial for understanding it. However, not many early human fossils have actually been discovered, and it is practically impossible to clarify why and how bipedalism evolved based on fossil information alone. For this reason, attempts have long been made to approach the evolution of upright bipedalism by considering bipedal monkeys as models for early humans and comparing their bipedal locomotion with that of humans. Within this context, our group has been conducting research focusing on the bipedal walking of Japanese macaques.
Japanese macaques normally walk on four legs, but when trained in bipedalism, like those in monkey performances, they walk on two legs extremely well. They walk so skillfully that at first glance they appear to be the same as humans, but our research has revealed, through analysis of their body movements and the forces acting on their feet from the floor, that the walking of the two species is far more different than it appears. From the perspective of locomotor efficiency in bipedal walking, human bipedalism is superior.
Why can't Japanese macaques walk like humans? Conversely, what evolutionary changes would need to occur in the body of a Japanese macaque, which is naturally quadrupedal, to enable efficient locomotion similar to that of humans? If we can figure this out, it could be a major key to clarifying the evolutionary path of human-like bipedalism.
Therefore, our group is trying to uncover this "key" by creating a mathematical model of the musculoskeletal system of the Japanese macaque and analyzing its bipedal locomotion from a mechanical engineering perspective. Specifically, we are conducting virtual evolutionary experiments on bipedalism. We make virtual modifications to parts of the Japanese macaque's body structure (for example, the foot) within a computer to make them more human-like, and then use physical simulations to predict how their bipedal walking changes. From this, we decipher the evolution of human bipedalism. Unfortunately, we cannot yet say that we have figured out the evolution of human upright bipedalism! However, based on our globally unique approach, we dream of the day in the near future when that path will be revealed.
It may seem like a surprising combination at first glance, but in this way, mechanical engineering is also contributing to the study of human evolution.