The ability to walk on two legs is called “bipedalism”, and though it is one of the characteristics that defines us a humans, the skill is actually pretty unique in the animal kingdom. So why is that? Why do humans walk on two legs? And why isn’t bipedalism more common among other animals? Let’s take a look.
Many species that are now extinct were bipedal, including some dinosaurs (think Tyrannosaurus Rex). But by and large, quadrupedalism, or the ability to walk on four feet, occurs much more commonly in nature. That said, quite a handful of species have taken short forays into bipedalism at some point or another in their evolutionary history.
While most members of the animal kingdom are quadrupeds, humans aren’t completely alone in walking on two feet. It is not particularly unusual for mammals, like tree kangaroos and jumping rodents, to be bipedal, and all birds are bipedal when on the ground. A lot of amphibians will resort to bipedalism when running, and primates usually have bipedal tendencies and are capable of sustaining permanent bipedalism when injured. Some years ago, numerous sightings of a bipedal bear ignited a social media frenzy. It was later discovered that the bear had successfully adapted to walking on two legs because of an injury.
Why Is Bipedalism Unique?
Permanent bipeds are comparatively rare, and most of them will hop on two feet rather than take separate steps. The only other types of species aside from humans who walk by raising one foot at a time are gibbons and larger sized birds.
Evolutionarily speaking, it might not seem very sensible to be a biped because it comes with a lot of disadvantages. Bipeds are taller, and therefore, more easily exposed to predators. Meanwhile, injuring just one leg makes movement close to impossible. Furthermore, bipeds generally can’t compete with the ground speeds of the fastest quadrupeds, particularly over short distances.
Anatomically speaking, holding up the head puts a lot of strain on the spine, especially the lower back, and as a result, spinal injuries and slipped discs are common in bipeds. Walking on two legs also puts more strain on the circulatory system than walking on four legs, and conditions like varicose and bulging veins are not uncommon.
Despite the disadvantages, bipedal hominid (the early version of humans) footprints dating back to 3.5 million years have been discovered, while fossil evidence suggests humans could have been bipedal for up to 6 or 7 million years. So the question becomes, why? Why is it that humans are bipedal?
Why Do Humans Walk on Two Legs?
There are a handful of theories as to why humans have evolved to become permanently bipedal, but one of the more popular ones is that humans started walking on two legs to adapt to the phenomenon of global cooling that was happening worldwide at the time. As Africa’s once lush forest were replaced by barren grasslands, humans began moving to the savanna where they had less places to hide from predators, and standing on two legs allowed them to better see what was coming.
Another somewhat related theory suggests that as the savannas grew, humans needed to travel farther and farther to find clusters of trees, and walking on two legs is better suited for endurance.
Meanwhile, the postural feeding theory essentially suggests that because human’s close relatives, chimpanzees are bipedal when the eat, it is likely that this just formed as a habit of convenience. Essentially, it would simply be easier to reach up and grab food from trees.
Regardless of why hominids became bipedal, it has all worked out for the best.
How Did Bipedalism Help Humans?
To answer that question in one word, tools. As the newly free human hand evolved to become increasingly dexterous and sophisticated, rudimentary tools, like spears and knives, started coming into the picture. These tools helped humans hunt and protect themselves. As the use of tools gradually became more common, these secret weapons began to bestow upon early human beings distinct evolutionary advantages.
Even more importantly, the process of thinking up these tools pushed human beings toward an evolutionary emphasis on the brain. Now fast forward to today, where humans stand comfortably at the top of the food chain surrounded by a world of hyper complex tools made with the help of their brains and hands.
However, the human body still does have to take a punishing toll to accommodate these abilities. It is not exactly ergonomically intuitive to balance the now much larger human head and brains, generally weighing about 30 pounds, on top of a long tower-like structure. However, the pelvis has widened to help distribute the weight and an extra curve in the spine makes it easier to stand up and straight. Furthermore, shorter arms mean less weight to carry around.
When it is all said and done, the odd pain in the back or neck is more than worth the sacrifice considering the unexpected consequences of what liberating the arms and hands has allowed humans to become.
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