One of the oddest, most improbable things about humans is that we are habitual bipeds. That is, we walk most of the time on two legs.
Lots of mammals can and do walk on two legs: bears, lemurs, capuchin monkeys. But humans are the only mammals who do it habitually — all the time, everywhere we go. Once you learn about the complexity and difficulty of bipedal walking you start to realize how truly out of the ordinary this habit is.
For every step you take, your brain has to make an estimated billion calculations. The vestibular system in your inner ear feeds your brain information about distance, balance, orientation within the environment you’re traversing, and — the detail that always blows my mind — the gravitational pull of the planet. No matter where you’re heading, you’re doing it on a ball in space spinning at about 1,000 miles per hour. Your brain has to account for that motion.
At the same time, your proprioception feeds back other information about the territory underfoot — icy, rocky, flat, bumpy — allowing you to respond automatically to changing terrain. There are infinite variations on the kinds of encounters your body has with the world, and your brain has to accommodate all of them to keep you upright.
You step forward, your foot bends to balance precariously on the big toe and ball of the foot while your opposite knee swings forward and somehow, miraculously, you stay upright and finish the step. And then you do it again and again, your mind and body working in tandem to keep you erect and moving.
This mode of locomotion took millions of years to develop — at a minimum six million if we go back to the earliest current evidence of hominin bipedalism, a skull of the species Sahelanthropus tchadensis whose placement of the foramen magnum (the hole in the skull through which the spine exits) shows possible evidence of at least part-time bipedalism.
Now that Homo sapiens is the only species of hominin left of the many that tangled our family tree, habitual bipedalism is a trait unique to us — though it’s not, it’s important to note, a habit that sets individuals apart. Many of the most famous fossils studied by paleoanthropologists, like 3.2-million-year-old Lucy of the species…