First Steps

Jeremy DaSilva

• Publisher: William Collins
• Year: 2020
• Print ISBN: 9780008342876
• eBook ISBN: 9780008342852
• Topic: Physical Sciences
• Subtopic: Evolution

Humans are the only mammals to walk on two, rather than four, legs. From an evolutionary perspective, this is an illogical development, as it slows us down. But here we are, suggesting there must have been something tremendous to gain from bipedalism.

PART I

The Origin of Upright Walking

WHY THE FAMILIAR CHIMPANZEE-TO-HUMAN IMAGERY OF BIPEDAL EVOLUTION IS WRONG

All other animals look downward; Man,
Alone, erect, can raise his face toward Heaven.

—OVID, METAMORPHOSES, AD 8

CHAPTER 1

How We Walk

Walking is falling forward. Each step we take is an arrested plunge, a collapse averted, a disaster braked. In this way, to walk becomes an act of faith.

—Paul Salopek, journalist, at the start of his ten-year, 20,000-mile journey in the footsteps of our early ancestors from their African homeland to the ends of the Earth, December 2013

Let’s face it: humans are weird. Although we are mammals, we have comparatively little body hair. While other animals communicate, we talk. Other animals pant, but we sweat. We have exceptionally large brains for our body size and have developed complex cultures. But, perhaps oddest of all, humans navigate the world perched on fully extended hind limbs.

The fossil record indicates that our ancestors started walking on two legs long before they evolved other uniquely human features including large brains and language. Bipedal walking on the ground started our lineage on its unique path shortly after our apelike ancestors split from the chimpanzee lineage.

Even Plato recognized the uniqueness and the importance of upright walking, defining the human as a “two-footed, featherless animal.” According to legend, Diogenes the Cynic was not pleased with Plato’s description and, with a plucked chicken in hand, he disparagingly revealed “Plato’s man.” Plato responded by tweaking his definition of humans to include “with flat nails,” but held fast to the biped part.

Bipedalism has since made its way into our words, expressions, and entertainment. Think of the many ways we describe walking: we stroll, stride, plod, traipse, amble, saunter, shuffle, tiptoe, lumber, tromp, lope, strut, and swagger. After walking all over someone, we might be asked to walk a mile in his shoes. Heroes walk on water while geniuses are walking encyclopedias. To humanize animated television characters, cartoonists draw them standing and walking on two legs. Mickey Mouse, Bugs Bunny, Goofy, Snoopy, Winnie the Pooh, SpongeBob SquarePants, and Brian the dog from Family Guy all walk bipedally.

In a lifetime, the average, nondisabled person will take about 150 million steps—enough to circle the Earth three times.

But what is bipedalism? And how do we do it?

Researchers often describe bipedal walking as a “controlled fall.” When we lift a leg, gravity takes over and pulls us forward and down. Of course, we don’t want to fall on our faces, so we catch ourselves by extending our leg forward and planting our foot on the ground. At that point, our bodies are physically lower than they were at the start of our journey, so we need to raise ourselves upward again. The calf muscles in our legs contract and raise our center of mass. We then lift the other leg, swing it forward, and fall again. As primatologist John Napier wrote in 1967, “Human walking is a unique activity during which the body, step by step, teeters on the edge of catastrophe.”

The next time you look at a person from the side as he or she walks, notice how the head dips and then rises with each stride. This wavelike pattern characterizes our controlled-fall form of walking.

Of course, walking is not this clunky, and it’s not this simple. To get technical for a moment, when we raise our center of mass by contracting our leg muscles, we store potential energy. When gravity takes over and pulls us forward, it converts the stored potential energy into kinetic energy, or motion. By taking advantage of gravity, we save 65 percent of the energy we would use otherwise. This ticktocking of potential energy to kinetic energy is how pendulums work. Human walking can be thought of that way—as an inverted pendulum that resembles a metronome.

Is this any different from how other animals walk when they rear up on two legs? It turns out, the answer is yes.

As a Ph.D. student, I spent a month with wild chimpanzees in Kibale Forest National Park in western Uganda. There, I met Berg. He was a large male in the Ngogo community of chimpanzees that numbered about 150—an unusually large group of apes. He was on the older side, his head hair receding a bit and his black coat flecked with patches of gray on his lower back and calves. Berg was not a high-ranking male, but occasionally he experienced a surge of testosterone, his hair puffed out, and he gave a booming pant-hoot that echoed through the forest. When he did this, it was best for humans to step out of his way.

Berg would grab a branch from the forest floor or tear one from a nearby tree, stand upright, and walk through the understory on just two legs. But he didn’t move like I do. Instead, his knees and his hips were bent—the crouched kind of walk comically performed by Groucho Marx in A Day at the Races and other Marx Brothers films. Unable to balance on a single leg, Berg wobbled from side to side as he gracelessly crashed through the forest. It was an energetically expensive form of travel, and he tired quickly, dropping to all fours after about a dozen steps.

Humans, in contrast, are not crouched over. We stand with extended knees and hips. Our quadriceps muscles do not have to do as much work as a chimpanzee’s when they walk on crouched legs. Muscles positioned on the sides of our hips allow us to balance on a single leg without tipping over. We walk gracefully and with much more energetic efficiency than Berg did.

But why did these changes to our anatomy happen? Why did this unusual form of locomotion evolve?

Let’s start our journey by considering bipedalism in the fastest human on the planet. In 2009, Jamaican sprinter Usain Bolt set the men’s world record in the one-hundred-meter dash at 9.58 seconds. Between the sixty- and eighty-meter mark, he maintained a peak speed of nearly twenty-eight miles per hour for about 1.5 seconds. But by the standards of other mammals in the animal kingdom, this human speed demon is pathetically slow-footed.

Cheetahs, the fastest land mammals, exceed sixty miles per hour. Cheetahs do not typically hunt humans, but lions and leopards, who occasionally do, top out at fifty-five miles per hour. Even their prey, including zebras and antelopes, can flee snapping jaws at fifty to fifty-five miles per hour. In other words, the predator-prey arms race in Africa currently stands at no less than fifty miles per hour. That’s how fast most predators run, and how fast most prey try to escape. Except for us.

Usain Bolt not only could not flee from a leopard, he couldn’t catch a rabbit. The fastest among us runs at half the speed of an antelope. By moving on two legs rather than four, we’ve lost the ability to gallop, making us exceptionally slow and vulnerable.

Bipedalism also makes our gait somewhat unstable. Sometimes our graceful “controlled fall” is not controlled at all. According to the U.S. Centers for Disease Control and Prevention, more than 35,000 Americans die annually from falling—nearly the same number who die in car accidents. But when’s the last time you saw a four-legged animal—a squirrel, dog, or cat—trip and fall?

Being slow and unstable seems like a recipe for extinction, especially given that our ancestors shared the landscape with the large, fast, hungry ancestors of today’s lions, leopards, and hyenas. Yet here we are, so surely there must be advantages to bipedalism that outweigh the costs. The great film director Stanley Kubrick thought he knew what these were.

IN KUBRICK’S 1968 film 2001: A Space Odyssey, a group of hairy apes gather around a watering hole on a dry African savanna. One of them looks inquisitively at a large bone lying on the ground. He picks it up, holds it like a club, and gently taps the scatter of bones around him. Strauss’s 1896 Also sprach Zarathustra, Op. 30, begins to play. Horns: dah, dahhh, dahhhhh, DAH-DAH! Bass drum: dum-dum, dum-dum, dum-dum, dum. The ape imagines wielding the bone as a tool—a tool to kill. The furry beast rises on two legs and slams the weapon down, shattering bones and symbolically clubbing a meal, or an enemy, to death. That’s how Kubrick imagined the Dawn of Man. He and his cowriter Arthur C. Clarke were dramatizing what was then a widely accepted model for human origins and the beginning of upright walking.

This model is still with us, and it is almost certainly wrong. It postulates that bipedalism evolved in a savanna environment to free the hands to carry weapons. It asserts that humans are, and always have been, violent. These ideas go all the way back to Darwin.

Charles Darwin’s On the Origin of Species (1859) is one of the most influential books ever written. Darwin didn’t invent evolution; naturalists had been discussing the changeability of species for decades. His great contribution was to present a testable mechanism for how populations changed and continue to change over time. He called this mechanism “natural selection,” although most of us know it as “survival of the fittest.” More than 150 years later, there’s ample evidence that natural selection is a strong driver of evolutionary change.

Almost from the beginning, skeptics howled at the implication that human beings descended from apes, but in Origin, Darwin had written almost nothing about the evolution of his own species. He simply wrote on the penultimate page of the book that “light will be thrown on the origin of man and his history.”

Nevertheless, Darwin was thinking about humans. Twelve years later, in The Descent of Man (1871), he hypothesized that humans possess several interrelated traits. He asserted we are the only apes that use tools. We know now he was wrong, but Jane Goodall’s observation that chimpanzees at Gombe Stream National Park in Tanzania make and use tools was still ninety years away. However, Darwin correctly posited that humans are the only fully bipedal ape, and that we have unusually small canine, or fang, teeth.

To Darwin, these three human attributes—tool use, bipedalism, and small canines—were linked. As he saw it, individuals who moved on two legs could free their hands for tool use. Thanks to tools, they no longer needed large canine teeth to compete with rivals. Ultimately, he thought, this suite of changes led to an increase in brain size.

But Darwin was working with a handicap. He had no access to firsthand accounts of wild ape behavior, data that didn’t start trickling in until a century later. Furthermore, in 1871 there wasn’t a single known early human fossil from the African continent—the place of origin for our lineage as we understand it now, and even as Darwin predicted a century and a half ago. The only premodern human fossils known to Darwin were a few Neandertal bones from Germany misidentified by some scholars at the time as diseased Homo sapiens.

Without the benefit of a fossil record or accurate behavioral observations of our closest living ape relatives, Darwin did the best he could in proposing a testable scientific hypothesis for why humans walk on two legs.

Data required to test his idea started surfacing in 1924 when a young Australian professor named Raymond Dart, a brain expert at the University of the Witwatersrand in South Africa, obtained a crate of rocks from a mining operation near the town of Taung, nearly three hundred miles southwest of Johannesburg. He opened the crate and noticed that one of the rocks contained the fossilized skull of a juvenile primate. Dart used his wife’s knitting needles to extract the skull from the surrounding limestone. As he did, he saw that the skull belonged to a strange primate. For one thing, the Taung child, as it would come to be known, had tiny canine teeth quite unlike those in baboons and apes. But the real clues were lurking in the child’s fossilized brain.

My primary research interests are the foot and leg bones of our ancestors, but historically and aesthetically, no other fossil can match the Taung child’s skull. In 2007, I traveled to Johannesburg, South Africa, to examine it. The curator there is my friend Bernhard Zipfel, a former podiatrist who became a paleoanthropologist after he “grew tired of fixing people’s bunions.” One morning, he retrieved a small wooden box from the vault. It was the same box Dart used to house his precious Taung nearly a century earlier. Zipfel carefully removed the fossilized brain and placed it in my hands.

After this little hominin died, the brain decayed and mud filled the skull. As millennia passed, the sediment hardened into an endocast, a replica of the brain. It faithfully duplicated the size and shape of the original brain and even preserved details of the folds, fissures, and external cranial arteries. The anatomical detail is exquisite. I carefully turned the fossil brain over to reveal a thick layer of sparkling calcite. Light reflected from it as if it were a geode, not an ancient human fossil. I hadn’t expected Taung to be so beautiful.

The preservation of the folds and fissures of the brain was a remarkable stroke of luck because Dart knew brain anatomy as well as anyone in the world. He was, after all, a neuroanatomist. His studies revealed that the Taung child’s brain was about the size of an adult ape’s but had lobes organized more like a human’s.

The endocast fit perfectly, like a puzzle piece, into the backside of Taung’s skull. I turned the skull slowly to peer into this 2.5-million-year-old child’s eye sockets, the closest I could come to seeing an ancient hominin eye to eye. When I rotated the skull to examine the underside, I saw what Dart had observed in 1924. The foramen magnum—the hole through which the spinal cord passes—was located directly under the skull as it is in humans. When alive, little Taung held its head atop a vertical spine.

In other words, Taung was bipedal. In 1925, Dart announced that the fossilized skull was from a species brand-new to science. He called it Australopithecus africanus, meaning “southern ape from Africa,” following the traditional way in which scientists classify and name animals by genus and species. Domestic dogs, for instance, are all members of the same species, but they are also part of a larger group, or “genus,” of related animals including wolves, coyotes, and jackals. All the members of that genus are part of a still larger and more distantly related group, or “family,” tha...

Table of contents

• Cover
• Title Page
• Copyright
• Praise for First Steps
• Dedication
• Contents
• One Interpretation of the Human Family Tree
• Author’s Note
• Introduction
• Part I: The Origin of Upright Walking
• Part II: Becoming Human
• Part III: Walk of Life
• Conclusion: The Empathetic Ape
• Notes
• Index
• Acknowledgments
• About the Book
• About the Author
• About the Publisher