Bipedalism
Our bipedal body structure is unique amongst living apes. In fact, our ancestors started on the path to becoming human when they began walking on two legs.
DEVELOPING INTO A BIPED
About seven million years ago, our early ancestors climbed trees and walked on four legs when on the ground.
By five million years ago, our ancestors had developed the ability to walk on two legs but their gait was quite different from our own and their skeletons retained some features that helped them climb trees.
By 1.8 million years ago, our ancestors had developed long legs and an efficient striding gait that made it easier to travel longer distances. Our ancestors had also developed the ability to run. This new stride became possible when changes to the shoulders, chest and waist allowed the body to stay balanced during prolonged running.
Many changes were required to achieve this, including:
re-shaping of the pelvis and lower limbs to bring the knees and feet directly under the body’s centre of gravity. This allowed the body to balance on one leg while the other leg was raised to take a step.
modifications to the feet and toes so they developed into stable platforms with an energy efficient spring to push off the ground with each step.
alterations to the skull and backbone which created an upright body in which the head was balanced directly above the backbone.
AUSTRALIAN MUSEUM
WHAT THE EVIDENCE SUGGESTS BY DONALD JOHANSON
Few attributes of being human have attracted more intense thought than the simple fact that, unlike all other living mammals, we walk upright on two legs. Human bipedalism represents a major evolutionary innovation that at first glance seems improbable. Bipedal primates, unlike the quadrupedal kind, are slow, clumsy, and notoriously unstable and prone to falls and injury. Yet despite the negatives, bipedalism has permitted us to multiply to a world population of over six billion, allowing us to assume a position of primacy over all other life on the planet. Such a powerful adaptation demands explanation.
Biologists and anthropologists have wrestled with this question for centuries and have postulated a diverse series of explanations for why we became bipedal. Nearly all of the speculation has fallen far short, however, of a coherent, powerful understanding, with little explanatory or evolutionary relevance, of this unique adaptation.
Most explanations target a single cause such as standing up to look over tall grass on the savannah of Africa. This is, in fact, logically one of the least rewarding of all explanations. Our earliest bipedal ancestors were small in stature. For example, Lucy was only three-and-a-half feet tall as an adult and weighed roughly 60 pounds. Imagine this puny little hominid venturing out of the forest, where she and her ancestors had lived for millions of years, onto the new, challenging, and highly dangerous world of the grasslands. A major shift in habitat is not a good time to experiment with a totally new mode of locomotion; leaving the speed and agility of quadrupedalism behind for upright walking would have been a terrible choice.
Standing up in the grasslands to get an enhanced view would have immediately announced to carnivores galore that a new entrée had appeared on the menu! Hyenas, saber-toothed cats, lions, large, wild dogs, and other fierce creatures populated the savannah. Each specialized carnivore would have been capable of devouring a tiny, bipedal hominid. Being slow, clumsy runners, the early hominids would have quickly fallen prey. Likewise, lacking the ability to fall back onto all fours and outrun the carnivores would have signaled the end of human evolution.
Standing up to reach fruit in low-hanging branches is also a lame explanation. Quadrupeds like chimpanzees can stand up on two legs and then fall to all fours to escape danger with great speed. Why become bipedal if it is possible to stand and retain the benefits of quadrupedalism?
Upright posture assisted our ancestors in keeping their bodies cool, and some have offered this as an explanation for bipedalism. Sure, it's hot under the African sun, but why not simply rest in the shade of a nice acacia tree during the hottest part of the day like baboons do, restricting foraging to the cooler times in the morning and afternoon?
Next, the aquatic "theory" is perhaps one of the all-time worst explanations. No primates collect food in the water, while most avoid being in water at all costs. If this were a good explanation, then we should have lost our limbs like the whales, which evolved from quadrupeds. Furthermore, African lakes and rivers are dangerous places occupied by hippopotami, which kill more humans each year in Africa than any other animal, as well as snakes, crocodiles, giant otters, and other dangerous creatures.
If we stood up to make tools, then why is there a disconnect between the earliest evidence for bipedalism and the first stone tools? Evidence for bipedalism extends as far back as 4.2 million years ago, perhaps even six million years ago, but stone tools do not appear in the archeological record until 2.6 million years ago—so we can rule out tool-making as an explanation.
The simple reason that these explanations fail is that most scholars have asked the wrong question: Why did our ancestors become upright? The correct question is: What were the advantages for early hominids that resulted in such a behavioral change from quadrupedalism to bipedalism?
THE REAL ADVANTAGE
Most animals expend the greatest amount of energy in reproduction, feeding, and ensuring safety. Owen Lovejoy's explanation points out that if males contributed to the survival of their mates and their offspring, then their own genes would survive into the next generation. Bipedalism would have allowed males to provide their mates and offspring with high-quality food as well as to help protect them from any looming dangers. This explanation makes biological and behavioral sense. Natural selection cannot create a behavior like bipedalism, but it can act to select the behavior once it has arisen. Surely some early hominids were better at upright walking than others, a behavior that permitted them to forage widely and bring food back to a home base.
I believe that our ancestors, pre-Lucy, became upright in the protection of the forests, a familiar environment in which they faced fewer dangers. Then, armed with a new type of locomotion, bipedalism, they were pre-adapted to move onto the savannahs and expand their territory. Once we moved into more open environments, we brought a whole package of advantages with us. Our hands were free to make and use tools, we could walk long distances to collect and carry food (and in doing so benefit from enhanced thermoregulation to prevent overheating), we could look over tall grass if we needed to, and so on.
Bipedalism was a behavioral innovation that led the way to making everything possible for our evolution, even if it is still not perfected. Humans continue to suffer from fallen arches, hernias, severe lower back pain, and other bad side effects of bipedalism. But it was still the single step in our ancient past that led to the tool-making, brain enlargement, and intelligence that have led to our preeminence on the planet today.
BY DONALD JOHANSON
DISADVANTAGES
Negative effects on our bodies. For example, the curvature of the lower spine, or lordosis, makes it easier to balance on two legs. However, pain is frequently experienced when the back curves too much due to weakened muscles or illness.
Does not promote speed and agility.
Suffer from fallen arches, hernias, severe lower back pain.
Difficulty in transporting food and increased risk of arthritis and back injuries due to the upright posture.
Head-up posture is associated with elevated blood pressure and reduced cognitive abilities.
Bipedalism has placed a burden on humans' circulatory systems, which are required to pump blood from the heart to the head and the feet. One result of this overworking of the circulatory system is bulging, or varicose, veins.
Why is bipedalism unusual?
The primate version of bipedalism is unusual because the back is close to upright (completely upright in humans), and the tail may be absent entirely.
Many primates can stand upright on their hind legs without any support. Chimpanzees, bonobos, gorillas, gibbons and baboons exhibit forms of bipedalism.