you characterized human walking as controlled falling--does animal walking take more effort because they have 4 legs, they have to push off each one? what about the relative stresses of human vs animal running? is there an example of a well-muscled animal that doesnt have such trouble with fragility, or a social animal that can care for wounded members of its group? we've sort of gotten away from anthropology huh lmao Sorry! I guess... what pressures selected for the above in humans?
Actually, a lot of this is still anthropology! Bipedalism- one of the defining characteristics of hominins- and its origins are extremely important to physical anthropologists and primatologists.
Human walking is divided into two parts: stance phase and swing phase. As an experiment, stand up, take a slow walking step, and really think about what you’re doing. Then sit down again because things tend to get weird when you actually think about what you’re doing when you walk- it’s kind of like remembering to breathe or being aware that you can feel your tongue in your mouth. By and large as a species*, our brains are so used to the motions of walking that thinking about it can throw us off our stride. This is why learning to walk again after an injury can be so challenging- actively thinking about locomotion is not something the human brain likes to do. To walk, we pick up a leg, swing it forward, land on it, roll off the toe of the other leg, pick it up, swing forward, land on it… it goes on forever. Here’s an illustration of what that looks like!
This doesn’t actually require a lot of energy, from a caloric standpoint. It’s just falling with style. Running is quite similar; it’s just more energy put into it. It’s still the same motion; the leg just gets lifted a little higher. There’s a phase where both legs are off the ground (which you can’t see in this gif, unfortunately)- same as when a horse moves from walking to galloping.
Let’s compare that to a horse- the first gif is a horse walking, the second is a horse galloping.
Much of the stress in four-legged running comes from body weight; a horse is going to be a lot heavier than a human, so that’s a lot of force put on the knees. Running is always going to be more stressful than walking, but for humans, our relatively small body size is going to make it comparatively less stressful and more efficient.
Now, this isn’t to say that other animals aren’t efficient for what they are/can do; it’s just to say that we’re more efficient. There’s only two animals that can really keep up with us: domesticated dogs and domesticated horses. We’re going to keep looking at horses because horses and the energetics of their movement are really well-studied; horses have had a long working relationship with humans. Unlike most other animals, it’s unlikely we domesticated them just to eat- equines are real dynamos and are able to do a tremendous amount of work. Now, this is where things kind of get into physics, but bear with me for a moment. Consider for just a second: animals (us included) as machines. There’s an input: oxygen and calories. There’s an output that we call “work,” which is using a force to move an object a distance when both the force and the motion of the object are in the same direction. That’s what we mean when we’re talking about “work,” the ability to move an object (at bare minimum, the animal’s body) a distance. Horses have been selectively bred over millennia for stamina and speed, and as a result the domesticated horse’s maximum work output is about 3.5 times higher than what it should be**. So back to that question of two feet versus four feet: It’s not just about quadrupedalism versus bipedalism, but also about aerobic potential, lung capacity, metabolism… there’s a lot to it.
Efficiency versus the capacity for speed is one of those evolutionary tradeoffs. There’s quite a lot of them in locomotion, including the fragility of various species. Horses and other leggy ungulates are typically more fragile than most other quadrupeds. If we look at predators- let’s use coyotes as an example- they might not be as fast initially as an ungulate, but they’ve also got some padding around their ankles. They’re less likely to shatter a bone. Bigger carnivores, like lions? Even more padding, with hunting strategies to match. Lions like to chase their prey into ambushes rather than just chase it down. To escape these ambushes, prey species need speed; like all things evolutionary, those fragile legs are a tradeoff.
As far as social animals caring for wounded members of their group, you’ll sometimes see this in other primates- they’ll lick each others’ wounds, pick off debris, that sort of thing. There’s some evidence that they’ll chew plants with medicinal properties, but interpreting these actions is really difficult because there’s so much we don’t know about great apes’ cognition. There’s an excellent book, The Evolution of Sickness and Healing, that has loads of information. While it’s a little older- it was published in 1997- it’s a great jumping off point. The whole thing’s available for free online here. Chapter 2 in particular has a lot of good information on what chimpanzees have been observed doing, including one of my favorite Jane Goodall anecdotes about a 23-year-old male chimpanzee (so, an adult) who got hurt in a fight with another chimp and started screaming; his elderly mother came running from about half a kilometer away and started grooming him, which got him to calm down. Even for chimps, mom’s attention can make it all better.
As always, footnotes and references/further reading under the jump!