What caused the rise of bipedalism? The need for food? Tools? Advantage of seeing over tall grass? There are many posited reasons why bipedalism evolved in humans, but there’s no way to really know how this occurred. However, we can infer what caused bipedalism from what was occurring around the time that humans and chimps split.
There is no way for us to know exactly how and why bipedalism evolved in humans, but we can make inferences based on what we know was occurring around the time that humans and chimpanzees split. At that time, there was a major climate shift occurring. To better understand how the rise of bipedalism occurred, we have to have a better understanding of the elusive Last Common Ancestor (LCA) between humans and apes.
Since the elusive LCA lived in an African rainforest as Darwin inferred, all traces of the LCA probably rotted away on the forest floor. Since gorillas and chimps walk on their knuckles, the LCA of chimps and gorillas must have knuckle-walked as well (Lieberman, 2013: 36). So the LCA of the two must have been similar to them. Using this same logic, we can say the same about the human/chimpanzee LCA.
Looking at chimps and gorillas when they walk on two legs, you can see that they lurch forward, while humans don’t do so. When you look at how chimps walk, for instance, they have their legs far apart and their body sways from side-to-side like a drunk human. Conversely, when a human walks, they imperceptibly sway their torsos meaning we can expend more energy moving forward than stabilizing the upper body. The large bone that makes up the pelvis, the illium, is tall and faces backward in apes but sideways in humans. This was crucial for the rise of bipedalism in humans as it allowed the small gluteal muscles on the side of the hip to stabilize the upper body over each leg while walking when only one leg is on the ground.
Of course another important adaption for bipedal walking is an S-shaped spine.Apes, like other quadrupeds, have spines that slightly curve forward, so when standing upright, their bodies tilt naturally forward. Due to this, when standing upright, an apes’ torso is positioned unstably in front of its hips. In contrast to an apes’ spine, human spines have two pairs of curves. The lower curve in the lumbar is made possible by having more vertebrae than apes (5 for humans, 3 or 4 for apes). Several of these vertebrae have a wedge shape in which the top and bottom surfaces are not parallel. Just like architects use wedge-shaped stones to make arches, the wedge-shaped vertebrae curve the lower spine inward above the pelvis positioning the torso stably above the hips. Human chest and neck vertebrae make another softer curve at the top of the spine which orients the head downward.
The final skeletal difference that allowed for bipedalism is the human feet. As we walk, we land first on the heel then as the rest of the foot makes contact with the ground we stiffen the foot’s arch which enables us to push the body upward and forward mostly with the big toe. The shape of our feet is due to the shape of the foot’s bones as well as numerous bones and ligaments that secure the bones in place.
Since it’s impossible to know exactly what selected for bipedalism, we have to make inferences. But most evidence supports the idea that regularly standing upright and walking made it easier for early humans to find food more effectively due to the scarcity of food due to the climate shift that was occurring at the time that humans and chimps diverged (Lieberman, 2013: 40).
We don’t know what the LCA looked like nor how it lived or moved, but by making inferences to what we know, we can say that it was big. The LCA probably was most likely a fruit eater as well. So some walking on two legs would help it find food better.
Another reason why bipedalism got selected for was because walking on two legs conserved energy while traveling. The LCA most likely walked on its knuckles which expends more energy. Human walking conserves 75 percent more energy than walking on all fours. (Lieberman, 2013: 42) Basically, we could walk further with using less energy. I don’t even need to say what that means.
As one might expect, other selective pressures are hypothesized to have favored bipedalism in the first hominins. Additional suggested advantages of being upright include improved abilities to make and use tools, to see over tall grass, to wade across streams, and even to swim. None of these hypotheses bear up under scrutiny. The oldest stone tools appear millions of years after bipedalism evolved.(Lieberman, 2013: 43)
The best evidence we have suggests that bipedalism evolved during the climate change so early hominins could acquire ‘fallback foods’ like fibrous plant roots and the like. At the time of the climate change as chimps and humans split, food became harder to find and so early Man needed to subsist on fibrous plant roots. Smaller canines were critical here, and this is seen in the dental records we have from this time.
Stone tools appear millions of years after bipedalism evolved. Bipedalism evolved, in my opinion, as a postural adaptation as well as to better find these ‘fallback foods’ when early Man couldn’t eat his regular diet.
Bipedalism didn’t occur to get us on our feet to free our hands. Rather, it occurred so we could forage more efficiently and expend less energy as bipedal walking expends 75 percent more energy than knuckle walking (as the LCA is thought to have walked).
Consider lumbar regions. In any population of chimps, you’d find that about half of them have three lumbar vertebrae, the other half have four, and a very tiny number have five, thanks to heritable genetic variations. If having five lumbar vertebrae gave some apes a few million years ago a slight advantage when standing and walking, they would have been more likely to pass that variation off to their offspring. The same selective processes must have applied to other features that must have improved the LCA’s ability to be bipedal, such as how wedged its lumbar vetebrae were, the orientation of its hips , and the stiffness of its feet (Lieberman, 2013: 45).
A drawback of becoming bipedal is coping with pregnancy. As any pregnant woman tells you, the further she is in her term, the harder it becomes to stand upright. This requires the mother to contract her back muscles more or lean backward, which shifts her center of mass back over her hips. Though this pose saves energy, it places extra stress on the lumbar vertebrae of the lower back as the vertebrae attempt to slide away from one another. Thus, back pain is common in mothers. However, we can see how natural selection helped human mothers by increasing the number of wedged vertebrae, over which women arch their lower spines: three in females versus two in males (Lieberman, 2013: 46). The extra curving from the vertebrae reduces the sheer force on the spine.
Another disadvantage of becoming bipedal is speed. According to Lieberman, early humans’ inability to gallop limited our early ancestors to being about half as fast as a typical ape when sprinting (Lieberman, 2013: 46). Two limbs are also much less stable than four which makes it harder to make sharp turns when running. Another thing that bipedalism hindered was climbing up trees. It’s much easier and faster to climb up trees as a quadruped than biped. Becoming bipedal also lead to the ailments we have today such as ankle, back, and knee problems.
We can never really know exactly how and why bipedalism evolved. However, we have enough information on how ancient human ancestors lived and walked to make inferences on how and why bipedalism evolved. With a basic understanding of kinesiology, one can begin to know how and why bipedalism evolved in humans. Clearly, it was way more advantageous for our early ancestors to become bipeds than stay quadrapeds.