The book begins with a scene that is both startling and instructive: a chimpanzee hunt. We see our closest living relatives, with whom we share over 98% of our DNA, not as the gentle, intelligent cousins of our popular imagination, but as a whirlwind of acrobatic violence. They tear through the forest canopy with a speed and agility that is, from a human perspective, simply superhuman. The scene forces us to confront a foundational paradox of our own existence. If we are the Earth’s dominant species, why are we so physically unimpressive? A human is no match for a chimp in speed, power, or dexterity in the trees. We are, as the text puts it, “athletically pathetic.” This physical weakness is not a minor footnote in our story; it is the central mystery we must solve to understand how we became who we are. The answer, it turns out, begins not with our big brains or clever hands, but with the simple, and surprisingly awkward, act of standing up.
The classic story of human origins, first sketched by Darwin, is a neat and satisfying causal chain: our ancestors stood up, which freed their hands from the business of locomotion. These newly liberated hands could then make and use tools, which in turn created an evolutionary pressure for bigger, cleverer brains to manage these new skills. It’s an elegant narrative, but as the fossil record has grown richer, it has become clear that this story has the sequence of events wrong. The first stone tools don’t appear in the archaeological record until millions of years after our ancestors first became bipedal. For a vast stretch of our history, our ancestors were walking around on two legs, with their hands free, but doing very little with them that a chimp couldn’t do. The trigger for bipedalism must have been something else—something more immediate and practical than a distant, unforeseen future of technology and civilization.
The real impetus seems to have been a changing world. Between 10 and 5 million years ago, around the time the human and chimpanzee lineages diverged, the global climate was cooling. In Africa, this meant the vast, lush rainforests began to shrink, giving way to more open woodlands and savannas. For a fruit-eating ape, this was a disaster. The fruit that formed the core of their diet became scarcer, more dispersed, and more seasonal. This created a new and intense selective pressure: the need to travel longer distances between food sources, and the need to rely more heavily on “fallback foods”—tougher, lower-quality items like stems, bark, and leaves—when fruit was unavailable. The solution to this problem was not to become a better climber, but to become a better traveler on the ground. This is where the surprising biomechanics of walking come into play.
One of the book’s most compelling points is just how inefficient other apes are at walking. When a chimpanzee walks on two legs, it lurches from side to side, its body a study in instability. It must walk with bent hips and bent knees, requiring constant, energy-draining muscle contractions just to keep from toppling over. Human walking, in contrast, is a model of efficiency. Our anatomy is built for it. Our S-shaped spine acts like a spring, positioning our torso directly above our hips for stability. The large, broad bones of our pelvis face sideways, not backward like in an ape, allowing muscles on the side of our hip to keep our upper body stable with each step, rather than letting it fall to one side. The result is that our skeleton, not our muscles, does most of the work. We essentially just fall forward, catching ourselves with each step. The energy savings are staggering: lab studies show that a chimpanzee uses four times as much energy to walk a given distance as a human does. For an early hominin in a world of dwindling resources, being able to travel four times as far on the same “tank of gas” would have been a monumental advantage. Bipedalism wasn’t an adaptation for a future of tool use; it was a fuel-efficiency upgrade for the present.
This story is not just a theory; it is written in the bones of our ancestors. The fossil known as “Ardi” (Ardipithecus ramidus), dating to about 4.4 million years ago, provides a remarkable snapshot of this transition in progress. Ardi’s skeleton is a mosaic of old and new. Her pelvis, though fragmented, shows the beginnings of the broad, sideways-facing shape needed for stable walking. Yet, her feet still had a divergent, grasping big toe, much like an ape’s, indicating she was still an adept climber. She was a creature adapted for two worlds: capable of moving through the trees, but also capable of walking on the ground with a stability and efficiency a chimpanzee could never match. She was not lurching like a chimp, but she was not striding like a modern human either. Ardi embodies the intermediate step, a creature who became a biped not by giving up climbing, but by becoming better at walking when she had to.
But evolution is a story of trade-offs, and the shift to bipedalism, while solving one problem, created a host of others. These new challenges, in turn, became the selective pressures that shaped the next stages of our evolution. One of the most profound is the engineering problem of being a pregnant biped. In a four-legged mammal, the extra weight of a fetus is slung between the limbs, like cargo in a hammock. The animal’s center of gravity remains stable. For a biped, however, that extra weight is all out front, pulling the center of gravity forward, away from the base of support provided by the feet. This creates a constant risk of falling over and places immense shearing stress on the lower back as the vertebrae try to slide past one another. The solution, visible in the fossil record, was for females to evolve more wedge-shaped lumbar vertebrae than males, allowing for a more pronounced curve in the lower back to shift the center of mass back over the hips. This anatomical quirk, a direct consequence of being a pregnant biped, is a beautiful and painful illustration of evolutionary compromise, and the ultimate reason for the chronic lower back pain that afflicts so many human mothers.
The downsides didn’t stop there. By committing to walking, our ancestors surrendered the ability to gallop. They became slow and unstable, making them easy targets for the many predators of the African savanna. Giving up their powerful climbing arms and grasping feet also meant they were less agile in the trees, their only reliable escape route. This loss of speed and power, this embrace of being “athletically pathetic,” was an enormous gamble. Yet it was this very vulnerability that set the stage for everything that followed. Being slow, weak, and ground-bound created a new, desperate need for solutions—for cooperation, for endurance running, for better communication, and, eventually, millions of years later, for the stone tools that Darwin had originally placed at the beginning of the story. Bipedalism was not the first step in a pre-ordained march toward humanity. It was an accidental, contingent, and risky adaptation to a changing climate, a path that, through a series of trade-offs and new challenges, inadvertently led to us.