Eli5: why do the vast majority of mammals walk on 4 legs, while humans only walk on 2?
In: 2
Because 4 legged locomotion is just plain easier and more efficient. Pretty much any 4 legged animal can outrun a human, and standing upright doesn’t require as much active effort. The amount of adaptation to our spine and pelvis to allow 2 leggedness is just silly, and it’s causing us back issues and difficulties with birth, compared to other mammals. Notice how other two-legged creatures, like birds, aren’t actually upright. The spine and pelvis just are not meant to do that, ours are a barely-functional evolutionary hack.
But it seems that our ancestors benefited from 2 leggedness in the form of improved tool usage. As in, if you’re already carrying something you’re gimping your four legged locomotion anyway (like a chimp or gorilla), so you might as well get good at the whole bipedal thing instead. At least that’s the most convincing hypothesis for our evolution that I’ve come across. That, or when transitioning from trees to the ground, it was easier to move from knuckle walking to upright walking (and get that sweet free-hands bonus), than it would have been to go full efficient quadrupedal.
4 legs are siginificantly faster. Each leg provides power and support, so more legs allow for more power.
But our evolution made humans experts at endurance. Pursuit predation is the act of following ones prey through hell and high water, by following signs and tracks rather than keeping pace with said prey. Our sweat glands and resitance to stress magnify that endurance even more.
Horses can run faster, but humans can run farther. We used our intelegence & our endurance to make up for our lack of speed.
Four legged locomotion requires less brain development and allows for higher sustained speeds in the short term. Humans just cannot outrun most animals, because two legs is slower than four, and we require a complicated cerebellum which requires nutrients and required additional evolution to make happen.
However, two legged locomotion is more efficient than four, allowing dramatically greater stamina. It also allows tool or weapon use while moving. An in-shape human can outlast most prey animals in the long term, briskly walking after them as they run from us, until they simply sit down from exhaustion.
Way back in the day, our ancestors evolved away from the ground, to live in the trees. You’ll notice that your hand is almost perfectly suited to grasping long, thin objects from the top, with four fingers curling downwards to create a hook that clings on to branches and an opposable thumb making a hook in the other direction to provide a “lock” we can open or close at will so we don’t slip off of a high branch. This method of locomotion via grasping from branch to branch to branch in a treed area is called “brachiation”, and many of our relatives among the great apes and monkeys still utilize this way of getting around.
With such a heavily specialized set of forelimbs, using them as part of your method of getting around on the ground becomes impractical, but there are still times when moving around on the ground might be necessary- not every tree is close enough to every other tree to jump from one to the other, naturally. This is usually accomplished by our relatives by a knuckle-walking, loping gait made possible by their relatively long forelimbs and short hindlimbs, but a lot of our relatives can display bipedalism in short bursts or when grabbing something, just due to the way their body is set up, in a way animals like dogs and cats really can’t.
We aren’t really exactly sure why the lineage that led to humans became SO bipedal. A common working theory is that we were subject to a severe biome shift due to climate change in our formative evolution around 7 mya, when significant bipedalism starts showing up in our ancestors in the fossil records. This would have led to a much less treed habitat, and forced our ancestors to reckon with traversing a lot more flat ground. Since there also would likely have been evolutionary pressure to keep our grasping appendages around for very primitive tool use (think throwing a stick or a rock) evolution simply used what it had and made us fully bipedal.
Latest Answers