|In order to conserve more energy, the most efficient method of travel was walking on two legs / Monthira via Shutterstock|
When comparing ourselves to even our closest relatives on the evolutionary chain, we find that humans are an incredible outlier in that we travel around upright on two legs. Being bipedal has been a trait of hominids for several million years in our evolutionary chain.
But why did our species branch out to eventually walk on two legs with our backs straight? What sort of advantages would this evolutionary change have had on our ancestors?
During the beginning of the 1900s, scientists believed that while walking on two legs was a good trait, it was our big brains that made hominids unique compared to other apes. At the time, only Neanderthals and Homo erectus had been the hominids found, and they both had large brains. However, later in the century, they found more skulls and remains of ancestral hominids that in fact had small brains, but their skeletons showed evidence of being bipedal, as evidenced by the position of the area where the spinal cord leaves the skull and a broad pelvis paired with the angle of the thigh bones. One of these was the remains of the famous Lucy skeleton.
Recent decades have had anthropologists finding evidence that even older branches of ape ancestors, such as the Sahelanthropus tchadensis and the Orrorin tugenensis were also upright walkers. The skeletons were seven and six million years old, respectively, but the evidence is not completely conclusive. Still, with early hominids, such as the four-million-year-old Ardipithecus ramidus showing extensive evidence of bipedal motion, it shows that this trait has long been in the human bloodline. Though capable of upright walking, most of the very early hominids still retained ape-like features, such as long, curved digits and short legs, as explained by Smithsonian.com. It was during the time of the Homo erectus less than two million years ago that hominids became terrestrial.
There have been many interesting theories as to why exactly our ancestors started to walk around on two legs. Charles Darwin was one of the first to give his theories, stating that in order to free their hands for using tools, such as stones and creating weapons like spears, they would have needed to travel using only two limbs. Unfortunately, this is unlikely the case given that the earliest tools were dated to about 2.5 million years ago, whereas bipedalism had already developed more than 4 million years before then.
|One of the rejected theory is that our ancestors began to stand tall in order to see above the tall grass found in savannah habitats / Bobrova Natalia via Shutterstock|
By 2010, anthropologist Owen Lovejoy revived and revised Darwin’s hypothesis and stated that it may have had more to do with human beings developing monogamy. Many of these early hominid species lived in forest habitats, and as the climate changed over time, it became more difficult to procure food over the changing seasons, especially for females raising offspring. Thus, early humans adjusted by creating a setup where males became the sole providers to their females and children, and in turn, females became exclusive to these males. Bipedalism then evolved as males needed their hands and arms to carry more food. This theory may actually hold the most potential, as chimpanzees were observed to walk on two legs when they carried rare food.
Other theories suggest that bipedalism became our ancestor’s method of locomotion given that it was more efficient. With the climate changes occurring back in the era, hominids found their natural habitats were shrinking, and they needed to descend from their forest perches to travel across vast stretches of grassland to look for other forest patches elsewhere. In order to conserve more energy, the most efficient method of travel was walking on two legs. Researchers even found that chimps used up 75% more energy walking than humans did, which suggests there may be truth in the theory.
Aside from theories that were well received, there are those that have been rejected by most scholars. One example is that our ancestors began to stand tall in order to see above the tall grass found in savannah habitats. Another is that they needed to expose less of their body to the sun’s heat while they were in an open environment. Unfortunately, neither of these was likely given that hominids were almost always living in at least partially wooded areas. According to PBS.org, the very stature of ancestral humans would have been an issue, as back then hominids were only about four feet tall at most. Standing up tall in such an environment would have also made them much more visible targets for faster and stronger predators.
Some claim that the changes were adaptational and necessary for acquiring different kinds of food. This is evident in theories, such as the aquatic theory that explains that humans developed bipedalism to better wade in the water, or that it helped reach fruits in trees more easily. These two, however, have been quickly debunked.