The following is an excerpt of Skeleton Keys: The Secret Life of Bone by Riley Black. Listen to his conversation on Science Friday here.

With the exception of the sesamoid bones, strung along tendons, the 206 or so bones in our bodies rely on contact with one another. Our skulls are composed of various elements fused together into a single unit, while our spines are a swerving stack of individual vertebrae separated by discs of squishy cartilage, with our ribs attaching to their own vertebrae. Our hips are flexible but fairly simple. This is our ball‑and‑socket joint, with the head of each femur fitting neatly into a little crater on each side of the hip, while our knees are simple hinges. There’s not much play for rotation since our legs evolved for the repetitive back‑and‑forth of walking wherever we desire to go. But the shoulder is the one that still confounds me. For as important as our arms are to us, in both the big picture of our history and day‑to‑day life, you’d think that our arms would be attached to our bodies with a really solid joint, something like a modified version of what’s at our hips. Instead our arms seem to float on the outside of our skeletons. The skeleton in that college classroom, as well as the model of “Stan” beside my desk now, requires special nuts and bolts and struts to make sure the arms stay attached to the rest of the body. So how does it work when we’re alive?

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Skeleton Keys: The Secret Life of Bone

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Your shoulder blades, or scapulae, are anatomically odd. They’re triangular bones that sit over your back, gliding back and forth just behind your ribs. That placement, dictated during the days when our ancestors were still fish lashing their way through Devonian seas, is one of those evolutionary happenstances that has become critical to our history. It’s a large part of why we can throw overhand, for example. If our shoulder blades were placed toward our sides, like on a dog or cat, we wouldn’t be able to rotate our arms to hurl a spear at a mammoth or throw a fastball. We’d be stuck throwing underhand, like some baboons do when irate at safari tourists, and we’d mark the end of summer with the World Series of softball rather than baseball. That is, if we ever got around to inventing such a thing at all.

Where the shoulder attaches to the rest of the body, though, things start to get strange. One tip of the scapula opens up into a cup that receives the head of your upper arm bone, and just above that is a little flange of bone that connects to the edge of your collarbone, which in turn anchors to the central bone of your chest, the sternum. The whole apparatus looks incredibly flimsy. The whole set of bones, from the little flattened phalanges at the ends of your fingers up your arms to the shoulder blade, is really connected to the rest of the skeleton only by the tips of your collarbones resting at the base of your throat. Yet this flimsiness translates to flexibility and is part of what has allowed us to make our livings by manipulating the world around us. It’s a gift from our ape ancestors, who couldn’t have foreseen that monkeying around in the trees for millions of years would result in upper bodies incredibly adept at shaping the world to the whims and wants of their descendants.

What those early apes didn’t have, however, was a foot suited for extended periods of walking upright. Their foot, so far as we can tell from the sparse human fossil record and our living ape relatives, was more like a hand, with curled toes and a sole that resembled a palm, the big toe jutting off to the side for opposable grasping. This allows great apes to walk upright for a time, but they have to shift their torsos from side to side to stay balanced and aren’t especially adept at staying upright for very long. While our ancestors were not just like chimpanzees—chimps have been evolving for just as long as our human lineage has—the fossil trail has shown that the early, pre‑Lucy human Ardipithecus ramidus had such a foot, and it would have required that “Ardi” amble around with a strange gait. Walking upright didn’t mark the beginning of humanity. We started off in the trees, not so different from our closest ape relatives.

This was one of the greatest evolutionary trade‑offs in our history, at least on the level of us all being preemies with squishy heads so that we might grow to have big brains as adults. Feet that were great at grasping branches and providing a firm grip for life in the trees just did not work for repetitive tromping around in the forest underbrush or out in the grasslands. The foot had to change. Hand‑like dexterity was lost in favor of a foot with shortened toes directed straight out in rank and file, with the big toe brought into line with the others instead of jutting out to the side. We can wiggle our toes, sure, but compared to our hands, our feet are rigid structures that move on a simple hinge from front to back. Just try to move one of your feet to this side or that—much of the rest of your leg has to move because your ankle just can’t contort that way. Our bones open possibilities and set firm limits.

Our evolution was constrained to give us such inflexible feet. Birds and the predatory dinosaurs they descended from, for example, required feet that could slash and catch and pin down food, and that’s why the foot of a chicken in the backyard or a raven out in the desert isn’t all that different from that of an Allosaurus. For our own lineage, though, moving out of the trees meant that there was no longer anything to grasp for support. A different foot shape was required to absorb the shock of step after step, not to mention a way for our feet to push off the ground at the end of each step and swing into position for the next. Doing this is the most natural thing in the world for most of us—like breathing. You don’t need to think about each step as you amble along. But take a moment to get up and stroll slowly. Concentrate on the heel strike, the way the ball of your foot hits the ground and pivots your big toe into position to lift your foot off the ground as you balance on your other leg. It feels strange. But that basic weird‑feeling motion is one of the most human things about us, and it’s something we’ve been doing for at least 3.7 million years.

Even though anthropologists can, and often do, debate the fine details of the way our ancestors and relatives moved, a trackway discovered by anthropologist Mary Leakey in Tanzania provides indisputable evidence that humans were walking in much the same way that we do now way back in the Pliocene. That’s because footprints, while not as sexy as bones and seeing the organism itself, are fossilized behavior. They are truly moments of time locked in the stone. And the Tanzania trackway, known as Laetoli, records the steps of at least three humans who walked over a pillowy bed of volcanic ash. Different researchers have interpreted them as a family, a couple followed by a child, a pair with a mother carrying a baby at her hip, or even unrelated people who passed by the same place within a broader window of time, and, frustratingly, there may never be a way to know any of that for certain. Out on a beach, for example, the footprints of various unrelated people cross and overlap and seem to follow each other yet were actually made at different times. All that can be said for certain is that the people of Laetoli were walking upright. And that’s the key to our biomechanical story. There are no knuckle marks. No sign that these people were going down on all fours. The ash recorded the anatomy of a foot very much like our own, which probably belonged to a close relative of Lucy. By 3.7 million years ago the feet of early humans already had a big toe brought into line with the rest, allowing them to literally walk tall over the ancient landscape.

The ebb and flow of such major anatomical changes happens in the course of evolutionary time. Natural selection and other evolutionary forces were critical in making our shape. And, provided that we survive our self‑destructive habits, they will continue to change us. We are still evolving. You can track the changes in our genes, as well as small tweaks to parts of our anatomy like our jaws and the microscopic structure of our bones as the nomadic lifestyle largely lost out to a sedentary and agricultural one. The way our ancestors moved, or didn’t, has left their marks on our skeletons, too.

From the book, Skeleton Keys: The Secret Life of Bone. Copyright ©2019 by Riley Black. Reprinted by permission of Riverhead Books, an imprint of Penguin Random House LLC