If upright walking is so energetically favorable, why do apes still “knuckle-walk”?

I wanted to really quickly give kudos to Afarensis, who dutifully ripped apart a dumb question that a creationist asked in regards to recent research on bipedalism. If you come from the camp that thinks there isn’t such a thing a stupid question… then I’m sorry to rain on your parade. There is such a thing as a stupid question and the following is a prime example of idiocy in its purest form. Here’s question,

“If upright walking is so energetically favorable, why do apes still “knuckle-walk”?”

Questions like this have been part of the creationist canon for some time. They use it, albeit ignorantly, because they think it is a valid attack point and weakens evolution. Unfortunately for them, as Afarensis points out, there’s a lot of flaws in thinking that way.

I want to take the time to answer this question and also add/clarify that the recent research I reviewed, on bipedalism energetics, simply shows that bipedalism is more energetically favorable for us. And that’s because our bodies are adapted to this form of locomotion. I’ll provide a lot of comparative anatomy to argue this point.  For example, the vertebrae in our backs are organized in a way to distribute the weight down to our pelvis which has a very different structure compared to other great apes. Our legs are longer and the bones are more robust than our arms because they bear more weight than the arms. The knee joint and the foot also exhibit differences that beneficial to bipedal walking.

Bipedalism is not a favorable form of locomotion for chimpanzees because their bodies are adapted primarily for terrestrial quadrupedal knuckle walking. Compared to humans, their arms are longer than their legs, their backs aren’t as specialized in weight distribution, and their hands, rather than their feet, exhibit robusticity because they bear lots of weight and force.

Now it is not all about the differences in bones and the skeletons. The illustration to your right, from a American Museum of Natural History webpage, compares some the soft tissue involved in the biomechanic differences between chimps and humans.  In the top left of the illustration, you see drawings of the three semicircular canals located in the inner ear, the primary organs for maintaining balance. In humans, two of the three canals are specialized to  stabilize the head. Also, in humans, there are fewer muscle connections between the head and the shoulders when compared to the chimpanzee. That’s cause quadruped chimps have to fight gravity in order to hold their heads up while walking on fours. Our head just sits on our necks.

In contrast to the head and neck, humans have more connections between the gluteus maximus muscle in the butt and the hip than chimpanzees do, which stabilizes the femur into the pelvis and helps keeps the trunk and leg moving together. Both the Achilles tendon of the heel and the tendon of the arch of the foot are much smaller in chimpanzees than they are in humans; in a running person they act like springs, absorbing and releasing energy.

So to ask, “why do apes still knuckle walk?” is straight up stupid. They don’t walk upright because it hurts, it is draining and inefficient.

You maybe thinking, “Hey, I’ve been to the zoo! I saw the YouTube video clip of the chimp and dog you shared with us. I’ve seen chimps run around on twos!” Chimps sometimes walk in short bipedal bursts. I’m not saying they don’t. But as the paper calculated, when they do walk bipedally, they experience more tension on their bones and joints than we do. That added tension and force costs more energy for chimps to walk bipedally because they aren’t adpated for it.

I imagine a similar but opposite conclusion can be made if the energetics of human knuckle-walking quadrupedalism is calculated and compared to that of a chimp. Try walking on all fours and not look like a idiot at the same time. You’ll soon experience tension on your bones and joints. After maybe about a dozen yards or so, fatigue will overcome your ability to keep up this form of locomotion. Why? Because our bodies are not adapted for and haven’t been selected for quadrupedal locomotion.

Any one, creationist or not, who thinks that bipedalism is a better form of locomotion should revise their train of thought. Apes still knuckle-walk because that’s part of the ecological niche they occupy. We humans walk on our two legs because that’s part of the ecological niche we occupy. Natural selection, or some other evolutionary force, continues to select for each of us to move about that way. Their bodies are damn efficient at knuckle-walking. Our bodies are damn efficient at bipedal walking. Simple as that.