One of the defining traits that set humans apart from other species is their ability to be habitual bipeds, but it is evident that humans were not always bipedal. There have been many theories of what led ancestral humans to one day decide to get up off of all fours and walk on two feet. Yet, no one knows what really led to habitual bipedalism. From looking at human anatomy compared to other primates we see a lot of anatomical features that contribute to upright walking, one such skeletal feature is the direction of the femora. In humans, the femora are narrow and point inward creating what is called the valgus angle, but one of their closest relative’s chimps, cannot lock their knees. As a result, humans have a striding gait and chimps waddle when they walk. There are plenty of other skeletal features that play a role in bipedalism, but like the game of “who came first the chicken or the egg,” many have pondered which morphological feature evolved first.
For years scholars battled with each other over whether bipedalism or enlarged brains came first. With the help of Raymond Dart’s founding of the Taung Child and Donald Johanson’s founding of Lucy, the argument was eventually resolved. These two discoveries showed evidence of two primates who were bipedal and had small ape sized brains. Which makes sense because human infants are born with relatively small and underdeveloped brains. They learn to sit up, crawl, stand, and then walk and as they grow their brains are enlarged. The one factor that maintains the same throughout this entire process of becoming bipedal was the ability to balance. Although human ancestors were already able to balance on all fours, in the push to bipedalism the pelvis evolved causing a chain reaction of evolutionary morphology in the skeletal system as other anatomical features fell in line.
When most people hear the term pelvis they think of the bone in which babies are pushed through, but the pelvis is more than that. According to paleontologist Caroline Vansickle, the pelvis is composed of two hip bones and a triangular bone called the sacrum that forms a ring in which babies are pushed through. Humans have a bowl-shaped pelvis rotated laterally while the chimp’s pelvis is long and narrow and tilts forward. Human’s enlarged pelvis accommodates for the stress of bearing the weight of the upper body, thus creating an erect posture. In their article “The evolution of the human pelvis: changing adaptations to bipedalism, obstetrician and thermoregulation,” biologist Laura Tobias Gruss and Evolutionary anthropologist Daniel Schmitt compare the pelvis structure of other apes to Homo sapiens (humans/homo) and how the difference in structures contribute to bipedalism. “When apes walk bipedally, they compensate by leaning their trunk towards the supported side or stretching out their arms, but such side-to-side weight shifts are energetically costly. In humans, the gluteal muscles on the support side are able to balance the trunk more efficiently by pulling up the unsupported side of the pelvis. To reiterate, the shape of the pelvis controls the function of other limbs and muscles in creating balance when walking.
Many have argued that the pelvis evolved because of childbirth and that babies are born with smaller brains to make childbirth easier. These same people argue that human babies cannot walk at birth because their brains are underdeveloped. Which that fact cannot be denied that the pelvis would need to be modified for childbirth, but the latter part of the argument takes two steps back into the direction of which came first the enlargement of brains or bipedalism. As mentioned early evidential support found that bipedalism developed first. To clarify, the pelvis would have to be modified to consider childbirth, but it is also beneficial in creating a system of balance. It also has to be taken into account that, the argument only supports one gender, men have basically the same pelvis just with a smaller angle and they cannot give birth. So why would natural selection act on their pelvis (in reference to childbirth) if it didn’t serve a function for them? With that stated it is plausible to say that with all the facts supporting the pelvis ability to create balance, the pelvis evolved first and it continued to be modified to support other functions.
