Published: July 2006
The Downside of Upright
All those aching backs may be trying to tell us something: It's part of the price we pay for walking on two legs.
By Jennifer Ackerman

We humans are odd creatures: tailless bipeds with sinuous spines, long limbs, arched feet, agile hands, and enormous brains. Our bodies are a mosaic of features shaped by natural selection over vast periods of time—both exquisitely capable and deeply flawed. We can stand, walk, and run with grace and endurance, but we suffer aching feet and knee injuries; we can twist and torque our spines, and yet most of us are plagued by back trouble at some point in our lives; we can give birth to babies with big brains, but only through great pain and risk. Scientists have long sought to answer the question of how our bodies came to be the way they are. Now, using new methods from a variety of disciplines, they are discovering that many of the flaws in our "design" have a common theme: They arise primarily from evolutionary compromises that came about when our ancestors stood upright—the first step in the long path to becoming human.

A tight squeeze

In Karen Rosenberg's laboratory at the University of Delaware, a room packed with the casts of skulls and bones of chimpanzees, gibbons, and other primates, one model stands out: It's a life-size replica of a human female pelvic skeleton mounted on a platform. There is also a fetal skull with a flexible gooseneck wire. The idea is to simulate the human birth process by manually moving the fetal head through the pelvis.

It looks easy enough.

"Go ahead, try it," Rosenberg says.

Turn the little oval skull face-forward, and it drops neatly into the pelvic brim, the beginning of the birth canal. But then it jams against the protrusions of the ischial bones (those that bear the burden during a long car ride). More shoving and rotating, and it's quickly apparent that the skull must traverse a passage that seems smaller than itself, cramped not only by the ischial bones but also by the coccyx, the bottom of the tailbone, which pokes into the lower pelvic cavity. Only by maneuvering the skull to face sideways in the middle of the canal and then giving it a firm push, does it move a centimeter or two—before it gets hung up again. Twist it, jostle it: The thing won't budge. Rosenberg guides my hand to turn the skull around to face backward, and then, with a hard shove, the stubborn cranium finally exits the birth canal.

"Navigating the birth canal is probably the most gymnastic maneuver most of us will ever make in life," says Rosenberg, chair of the university's department of anthropology. It's a trick all right, especially if there's no guiding hand to twirl and ram the skull. And the neat two-piece model doesn't even include the broad, rigid shoulders of the human infant, a legacy from our apelike ancestors who, some 20 million years ago, evolved wide clavicles that allowed them to hang suspended from branches and feed on fruit. To follow the head, a baby's shoulders must also rotate two times to work through the birth canal; they sometimes get stuck, causing injury to part of the spinal nerves that control the arms.

Suddenly I understand as never before why it took 36 hours, two doctors, and three shifts of nurses to safely deliver my firstborn.

Birth is an ordeal for women everywhere, according to a review of birthing patterns in nearly 300 cultures around the world by Rosenberg and colleague Wenda Trevathan, an anthropologist at New Mexico State University. "Not only is labor difficult," Rosenberg says, "but because of the design of the female pelvis, infants exit the birth canal with the back of their heads against the pubic bones, facing in the opposite direction from the mother. This makes it tough for her to reach down and guide the baby as it emerges without damaging its spine—and also inhibits her ability to clear the baby's breathing passage or to remove the umbilical cord from around its neck. That's why women everywhere seek assistance during labor and delivery."

Compared with humans, most primates have an easier time, Rosenberg says. A baby chimpanzee, for instance, is born quickly: entering, passing through, and leaving its mother's pelvis in a straight shot and emerging faceup so that its mother can pull it forward and lift it toward her breast. In chimps and other primates, the oval birth canal is oriented the same way from beginning to end. In humans, it’s a flattened oval one way and then it shifts orientation 90 degrees so that it’s flattened the other way. To get through, the infant’s head and shoulders have to align with that shifting oval. It’s this changing cross-sectional shape of the passageway that makes human birth difficult and risky, Rosenberg says, not just for babies but also for mothers. A hundred years ago, childbirth was a leading cause of death for women of childbearing age.

Why do we possess a birth canal of such Byzantine design? "The human female pelvis is a classic example of evolutionary compromise," Rosenberg answers. Its design reflects a trade-off between the demand for a skeletal structure that allows for habitual walking on two feet and one that permits the passage of a baby with a big brain and wide shoulders. Its unique features didn't come about all at once, but at different times in our evolutionary history, in response to different selective pressures. "The result of these different pressures is a jerry-rigged, unsatisfactory structure," Rosenberg says. "It works, but only marginally. It's definitely not the type of system you would invent if you were designing it. But evolution is clearly a tinkerer, not an engineer; it has to work with yesterday's model."

Yesterday’s model

Humans come from a long line of ancestors, from reptile to mammal to ape, whose skeletons were built to carry their weight on all fours. Our ape ancestors probably evolved around 20 million years ago from small primates that carried themselves horizontally. Over the next several million years, some apes grew larger and began to use their arms to hold overhead branches and, perhaps, to reach for fruit. Then, six or seven million years ago, our ancestors stood up and began to move about on their hind legs. By the time the famous Lucy (Australo-pithecus afarensis) appeared in East Africa 3.2 million years ago, they had adopted walking as their chief mode of getting around.

It was a radical shift. "Bipedalism is a unique and bizarre form of locomotion," says Craig Stanford, an anthropologist at the University of Southern California. "Of more than 250 species of primates, only one goes around on two legs." Stanford and many other scientists consider bipedalism the key defining feature of being human. "Some may think it's our big brain," Stanford says, "but the rapid expansion of the human brain didn't begin until less than two million years ago, millions of years after we got upright and began using tools. Bipedalism was the initial adaptation that paved the way for others."

Evolutionary biologists agree that shifts in behavior often drive changes in anatomy. Standing upright launched a cascade of anatomical alterations. The biomechanics of upright walking is so drastically different from quadrupedal locomotion that bones from the neck down had to change. The skull and spine were realigned, bringing the head and torso into a vertical line over the hips and feet. To support the body's weight and absorb the forces of upright locomotion, joints in limbs and the spine enlarged and the foot evolved an arch. As for the pelvis: It morphed from the ape's long, thin paddle into a wide, flat saddle shape, which thrust the weight of the trunk down through the legs and accommodated the attachment of large muscles. This improved the stability of the body and the efficiency of walking upright but severely constricted the birth canal.

All of these architectural changes, seen clearly in the fossil record, did not happen overnight. They came gradually, over many generations and over long periods of time, in small steps favored by natural selection.

Upright citizens

Consider the simple human act of walking or running. At his laboratory in the anthropology department at Harvard University, Dan Lieberman does just that, using biomechanical studies to see how we use our body parts in various aspects of movement. As a volunteer subject in one of his experiments last fall, I was wired up and put through paces on a treadmill. On my feet were pressure sensors to show my heel and toe strikes. Electromyographic sensors revealed the firing of my muscles, and accelerometers and rate gyros on my head detected its pitching, rolling, and yawing movements. Small silver foam balls attached to my joints—ankle, knee, hip, elbow, shoulder—acted as reflectors for three infrared cameras mapping in three-dimensional space the location of my limb segments.

These biomechanical windows on walking and running illuminate just how astonishing a feat of balance, coordination, and efficiency is upright locomotion. The legs on a walking human body act not unlike inverted pendulums. Using a stiff leg as a point of support, the body swings up and over it in an arc, so that the potential energy gained in the rise roughly equals the kinetic energy generated in the descent. By this trick the body stores and recovers so much of the energy used with each stride that it reduces its own workload by as much as 65 percent.

The key lies in our human features: the ability to fully extend our knees; the way our lower back curves forward and our thighbone slopes inward from hip to knee so that our feet straddle our center of gravity; and the action of the gluteal abductors, the muscles attached to the pelvis that contract to prevent us from toppling over sideways mid-stride when our weight is on a single foot.

In running, we shift from this swinging pendulum mode to a bouncy pogo-stick mode, using the tendons in our legs as elastic springs. Lieberman's recent studies with Dennis Bramble of the University of Utah suggest that running—which our ancestors mastered some two million years ago—was instrumental in the evolution of several features, including our extra leg tendons, our relatively hairless skin and copious sweat glands (which facilitate cooling), and our enlarged gluteus maximus, the biggest muscle in the body, which wraps the rear end and acts to stabilize the trunk, preventing us from pitching forward. Now Lieberman is studying the role in upright locomotion of a tiny slip of muscle in the neck called the cleidocranial trapezius—all that remains of a massive shoulder muscle in chimps and other apes—which steadies our head during running, preventing it from bobbling.

Watching the graphs from the experiment on a computer screen, one can't help but marvel at the effectiveness of the system, the little cleidocranial portion of the trapezius steadying the head; the regular pumping action of arms and shoulders stabilizing the body; the consistent springlike rhythms of our long-legged stride.

"Compare this with the chimp," Lieberman says. "Chimps pay a hefty price in energy for being built the way they are. They can't extend their knees and lock their legs straight, as humans can. Instead, they have to use muscle power to support their body weight when they're walking upright, and they waste energy rocking back and forth."

Chimps are our closest living evolutionary relatives and, as such, are especially well suited to teach us about ourselves. Almost every bone in a chimp’s body correlates with a bone in a human body. Whatever skeletal distinctions exist are primarily related to the human pattern of walking upright—hence the keen interest in parsing these distinctions among those who study the origins of human bipedalism.

Two-legged walking in a chimp is an occasional, transitory behavior. In humans, it is a way of life, one that carries with it myriad benefits, perhaps chief among them, freed hands. But upright posture and locomotion come with a host of uniquely human maladies.

Achilles' back

An old friend of mine, a former politician from West Virginia, has difficulty remembering names. He saves himself from embarrassment with a simple trick: He delivers a hearty handshake and asks, "So how's your back?" Four times out of five he strikes gold. Names become unnecessary when the acquaintance, flattered by the personal inquiry, launches into a saga of lumbar pain, slipped disk, or mild scoliosis.

Back pain is one of the most common health complaints, accounting for more than 15 million doctor visits each year. That most of us will experience debilitating back pain at some point in our lives raises the question of the spine's design.

"The problem is that the vertebral column was originally designed to act as an arch," explains Carol Ward, an anthropologist and anatomist at the University of Missouri in Columbia. "When we became upright, it had to function as a weight-bearing column." To support our head and balance our weight directly over our hip joints and lower limbs, the spine evolved a series of S curves—a deep forward curve, or lordosis, in the lower back, and a backward curve, or kyphosis, in the upper back.

This change took place at least four million years ago, probably much earlier. Ward and her colleague Bruce Latimer, director of the Cleveland Museum of Natural History, recently analyzed the vertebral column of Lucy, along with two Australopithecus africanus skeletons from more than two million years ago. They found that the spines of all three possess the same S curves present in the human spine, confirming that Australopithecus walked on two legs.

"This system of S curves is energetically efficient and effective for maintaining our balance and for bipedal locomotion," Ward says. "But the lower region of the column suffers from the excessive pressure and oblique force exerted on its curved structure by our upright posture."

Lean back, arching your spine. You're the only mammal in the world capable of this sort of backbend. Feel a cringing tightness in your lower back? That's the vertical joints between your vertebrae pressing against one another as their compressive load increases. The curvature in your lower spine requires that its building blocks take the shape of a wedge, with the thick part in the front and the thin part in the back. The wedge-shaped vertebrae are linked by vertical joints that prevent them from slipping out from one another.

"These joints are delicate structures and very complex," Ward says. "They allow our spines to move with great flexibility, to twist and bend and flex, pivoting on the disks between the vertebrae."

But in the lower back region, where the load is heaviest and the wedging most dramatic, strains such as heavy lifting or hyperextension (say, from doing the butterfly stroke or cleaning the gutters) can cause your lowest vertebrae to slip or squish together. When the vertebrae are pressured in this way, the disks between them may herniate, or bulge out, impinging on spinal nerves and causing pain. Or the pressure may pinch the delicate structures at the back of the vertebrae, causing a fracture called spondylolysis, a problem for about one in twenty Americans.

No other primate experiences such back problems—except, Ward and Latimer say, our immediate ancestors. The two scientists have found fossil evidence that back trouble likely plagued our bipedal forebears. The bones of the Nariokotome boy, a young Homo erectus (a species preceding our own Homo sapiens) who lived some 1.5 million years ago, reveal that the youth suffered from scoliosis, a potentially devastating lateral curvature of the spine.

The cause of most scoliosis cases remains a mystery, Latimer says, but like spondylolysis, it appears linked to the spinal features associated with upright posture, particularly lordosis, the deep forward curvature and flexibility of our lower spine. "Because scoliosis occurs only in humans and our immediate bipedal ancestors, it appears likely that upright walking is at least partially to blame," he says.

Considering the pressures of natural selection, why are such seriously debilitating diseases still prevalent? Latimer suspects the answer lies in the importance of lordosis for upright walking: "Selection for bipedality must have been so strong in our early ancestors that a permanent lordosis developed despite the risk it carries for spondylolysis and other back disorders."

Disjointed

Liz Scarpelli's postural orientation is at the moment horizontal. Her leg is elevated in a surgical sling as Scott Dye, an orthopedic surgeon at California Pacific Medical Center, examines her knee with an arthroscope. The ghostly image of the joint—femur, tibia, and patella—appear magnified on a flat screen above the gurney. An athletic woman of 51, a former gymnast and skier, Scarpelli is a physical therapist who works with patients to rehabilitate their joints after surgery. While demonstrating to one patient a technique for leg-strengthening knee squats, Scarpelli blew out her own knee for the third time. Dye's arthroscopic camera shows healthy bone and ligaments, but large chunks of cartilage float about like icebergs in the fluid spaces around the joint. Dye expertly scrapes up the pieces and sucks them out before sewing up the holes and moving on to the next five surgeries scheduled for the day.

To hear Scott Dye speak of it, the knee joint is among the greatest of nature's inventions, "a 360-million-year-old structure beautifully designed to do its job of transferring load between limbs." But it is also among the most easily injured joints in the human body; medical procedures involving knees total a million a year in the United States.

"In standing upright, we have imposed unprecedented forces on the knee, ankle, and foot," Bruce Latimer says. When we walk quickly or run, the forces absorbed by our lower limbs may approach several multiples of our own body weight. Moreover, our pelvic anatomy exerts so-called lateral pressure on our lower joints. Because of the breadth of our pelvis, our thighbone is angled inward toward the knee, rather than straight up and down, as it is in the chimp and other apes. This carrying angle ensures that the knee is brought well under the body to make us more stable.

"But nothing is free in evolution," Latimer says. "This peculiar angle means that there are forces on the knee threatening to destabilize it. In women, the angle is greater because of their wider pelvis, which explains why they are slower runners—the increased angle means that they're wasting maybe ten percent of their energy—and also why they tend to suffer more knee injuries."

Unlikely feat

And where does the buck finally stop? What finally bears the full weight of our upright body? Two ridiculously tiny platforms.

"The human foot has rightfully been called the most characteristic peculiarity in the human body," says Will Harcourt-Smith, a paleontologist at the American Museum of Natural History. "For one thing, it has no thumblike opposable toe. We're the only primate to give up the foot as a grasping organ."

This was a huge sacrifice. The chimp's foot is a brilliantly useful and versatile feature, essential to tree climbing and capable of as much motion and manipulation as its hand. The human foot, by contrast, is a hyper-specialized organ, designed to do just two things, propel the body forward and absorb the shock of doing so. Bipedality may have freed the hands, but it also yoked the feet.

Harcourt-Smith studies foot bones of early hominins with the new technique of geometric morphometrics—measuring objects in three dimensions. The variations in foot structure he has discovered in Australopithecus and Homo habilis (a species that lived 2.5 to 1.6 million years ago) suggest that these early hominins may have walked in different ways.

"We have a desire to see the story of bipedalism as a linear, progressive thing," he says, "one model improving on another, all evolving toward perfection in Homo sapiens. But evolution doesn't evolve toward anything; it's a messy affair, full of diversity and dead ends. There were probably lots of ways of getting around on two feet."

Still, in all the fossil feet Harcourt-Smith studies, some type of basic human pattern is clearly present: a big toe aligned with the long axis of the foot, or a well-developed longitudinal arch, or in some cases a humanlike ankle joint—all ingenious adaptations but fraught with potential problems. "Because the foot is so specialized in its design," Harcourt-Smith says, "it has a very narrow window for working correctly. If it's a bit too flat or too arched, or if it turns in or out too much, you get the host of complications that has spurred the industry of podiatry." In people with a reduced arch, fatigue fractures often develop. In those with a pronounced arch, the ligaments that support the arch sometimes become inflamed, causing plantar fasciitis and heel spurs. When the carrying angle of the leg forces the big toe out of alignment, bunions may form—more of a problem for women than men because of their wider hips.

And that's not all.

"One of the really remarkable aspects of the human foot, compared with the chimp and other apes, is the relatively large size of its bones, particularly the heel bone," Bruce Latimer notes. "A 350-pound male gorilla has a smaller heel bone than does a 100-pound human female—however, the gorilla bone is a lot more dense." While the ape heel is solid with thick cortical bone, the human heel is puffed up and covered with only a paper-thin layer of cortical bone; the rest is thin latticelike cancellous bone. This enlargement of cancellous bone is pronounced not just in the heel, but in all the main joints of our lower limbs—hip, ankle, knee—and has likely marked the skeleton of our ancestors since they first got upright; it has been found in the joints of 3.5-million-year-old hominin fossils from Ethiopia.

"The greater volume of bone is an advantage for dissipating the stresses delivered by normal bipedal gait," Latimer says. However, it's not without cost: "The redistribution in our bones from cortical to cancellous means that humans have much more surface exposure of their skeletal tissue. This results in an accelerated rate of bone mineral loss—or osteopenia—as we age, which may eventually lead to osteoporosis and hip and vertebral fractures."

What do we stand for?

We humans gave up stability and speed. We gave up the foot as a grasping tool. We gained spongy bones and fragile joints and vulnerable spines and difficult, risky births that led to the deaths of countless babies and mothers. Given the trade-offs, the aches and pains and severe drawbacks associated with bipedalism, why get upright in the first place?

A couple of chimps named Jack and Louie may offer some insights. The chimps are part of an experiment by a team of scientists to explore the origin of bipedalism in our hominin ancestors.

Theories about why we got upright have run the gamut from freeing the arms of our ancestors to carry babies and food to reaching hitherto inaccessible fruits. "But," says Mike Sockol of the University of California, Davis, "one factor had to play a part in every scenario: the amount of energy required to move from point to point. If you can save energy while gathering your food supply, that energy can go into growth and reproduction."

Paleogeographical studies suggest that at the time our ancestors first stood upright, perhaps six to eight million years ago, their food supplies were becoming more widely dispersed. "Rainfall in equatorial East Africa was declining," Sockol says, "and the forest was changing from dense and closed to more open, with more distance between food resources. If our ape ancestors had to roam farther to find adequate food, and doing so on two legs saved energy, then those individuals who moved across the ground more economically gained an advantage."

To test the theory that the shift to two feet among our ancestors may have been spurred by energy savings, Sockol and his colleagues are looking at the energy cost of locomotion in the chimp. The chimp is a good model, Sockol says, not just because it's similar to us in body size and skeletal features and can walk both bipedally and quadrupedally, but also because the majority of evidence suggests that the last common ancestor of chimps and humans who first stood upright was chimplike. By understanding how a chimp moves, and whether it expends more or less energy in walking upright or on all fours (knuckle-walking), the scientists hope to gain insight into our ancestors' radical change in posture.

Jack and Louie and several other young adult chimps have been trained by skillful professional handlers to walk and run on a treadmill, both on two legs and on four. One morning, Jack sits patiently in his trainer's lap while Sockol's collaborators, Dave Raichlen and Herman Pontzer of Harvard University, paint small white patches on his joints—the equivalent of those silver balls I wore on Dan Lieberman's treadmill. Only occasionally does Jack steal a surreptitious lick of the sweet white stuff. Once he's marked, he jumps on the treadmill and runs along on two legs for a few minutes, then drops to four. Every so often, his trainer hands him a fruit snack, which Jack balances on his lower lip, thrust out as far as it will go, before rolling the fruit forward and flicking it into his mouth. For a set time, Jack breathes into a small mask connected to equipment that gathers information on how much oxygen he consumes—a measure of energy expenditure—while the movements of his limbs (marked by those white dots) are monitored with cameras to help the scientists understand how the energy is being used.

Once the scientists have refined their model for how things work in the chimp—for what limb movements are used in the two types of locomotion and how each consumes energy—they hope to apply this model to the fossils of our ancestors. "We use the biomechanical data to determine the types of anatomical changes that would have reduced energy expenditure," Raichlen explains. "Then we look at the fossil record and ask, Do we see these changes? If we do, that’s a pretty good clue that we're looking at selection for reduced energy costs in our ancestors who became bipedal. That's the dream."

Scientists are the first to admit that much work needs to be done before we fully understand the origins of bipedalism. But whatever drove human ancestors to get upright in the first place—reaching for fruit or traveling farther in search of it, scanning the horizon for predators or transporting food to family—the habit stuck. They eventually evolved the ability to walk and run long distances. They learned to hunt and scavenge meat. They created and manipulated a diverse array of tools. These were all essential steps in evolving a big brain and a human intelligence, one that could make poetry and music and mathematics, assist in difficult childbirth, develop sophisticated technology, and consider the roots of its own quirky and imperfect upright being.