by Chip Walter
This was remarkable and puzzling. What, after all, would have caused these creatures to stand upright so quickly? And how did it happen?
…
In Gray’s Anatomy the curious appendage we carry at the end of our foot is called the hallux magnus. Most of us know it as our big toe. It is an odd-looking thing that we generally take for granted. But we shouldn’t, because had our predecessors never developed their big toes, they would never have stood upright. And had they never stood upright, we would not be here to ask how such a thing could have happened in the first place.
A comparison of the shape of a chimp foot (left) and a human foot (right). Ape feet look remarkably similar to human hands. In a human’s foot, weight travels along the outside and then shifts across the ball of the foot to the big toe. (Used by permission of WGBH in Boston, Massachusetts.)
Like modern chimps, gorillas, and orangutans, Lucy’s predecessors had no experience of big toes, at least not our variety. Their inside toes would have been more like crude thumbs made for gripping branches rather than for pushing off hard ground.
Gorillas and chimpanzees can walk upright, but they don’t do it very well. Their pelvises and legs push the weight to the outside of their flat feet, and they tend to roll from side to side. Five million years ago, when the first savanna apes and their jungle-dwelling cousins began to head off in separate evolutionary directions, they very likely shared the same bowed legs and big, square pelvises. Their feet would have looked nothing like human feet, but remarkably similar to human hands. And the four outer toes would have been considerably longer than ours.
But the real difference between these predecessors and modern humans would have been that inner toe, which remained set apart from the others like an unwanted dinner guest. At its base it would have turned outward and then run back in at the top, adapted for grabbing and holding.b
The traditional view is that the ancestral apes from which we sprung began to benefit from the evolution of a big toe about five million years ago. It was then that their crescent-shaped, thumblike digit would have begun to drift inward, where it grew knobbier and less fingerlike, a trait that eventually enabled it to support 40 percent of its owner’s body’s weight.
At least that’s the way Darwin would have imagined it. For him all evolutionary change came gradually. But the fossil record doesn’t always support Darwin’s views. Stephen Jay Gould famously brought this to the world’s attention with his theory of “punctuated equilibrium”—the idea that big genetic changes can occur in relatively short periods of time. Gould pointed out that occasionally species seemed to make sudden, apparently inexplicable leaps in their looks or anatomy, as if some evolutionary switch had been flicked.13 When it comes to the evolution of the big toe, that may be precisely what happened.
The most dramatic example of punctuated equilibria in all of evolution is the Burgess Shale, an ancient ridge of rock about a city block long discovered one day in 1909 by a young paleontologist name Charles Doolittle Walcott when he was horseback riding in British Columbia. Walcott realized he had found something amazing, and between 1910 and 1925 he mined 80,000 specimens from the ridge. Among the rock he found no fewer than 140 new species of ancient creatures. But more importantly, his discovery revealed that about 500 million years ago, at the dawn of the Cambrian period, life suddenly blossomed into a shocking number of forms—trilobites, brachiopods, ancestral starfish, sea urchins, and alien creatures like Opabinia, with its five eyes and fire hose nose. Whatever their forms, the creatures seemed to have emerged from nowhere. In one layer of the fossil record they didn’t exist. In the next they were everywhere.
Gould put it this way: “The Burgess Shale included a range of disparity in anatomical designs never again equaled, and not matched today by all of the creatures in the world’s oceans.” The foundations for the body design of every living animal (and then some) that ever lived afterward, Gould concluded, had evolved in this evolutionary blip. The evidence sat right there in the ancient, hardened muck of the Burgess Shale.
Though scientists now know that all of these creatures did not instantly appear as if by magic, they did arrive rather suddenly, considering that for the better part of the previous three billion years all that life on Earth had been able to manage were single-celled bacteria, plankton, and a few species of multicelled algae. Gould, and others who have studied these sudden evolutionary leaps, could not explain how they happened, only that they apparently did. But then in 1984 American scientists Mike Levine and Bill McGinnis were studying fruit-fly embryos when they discovered what are known as HOX genes, and a possible explanation began to unfold.
HOX genes are “master switches,” which toggle other strings of genes on and off. Since McGinnis’s and Levine’s discovery, other scientists have found them in every existing animal, including humans. In the fly embryo, HOX genes control the number and length of body sections, including where, precisely, features such as wings, legs, and antennae end up. In humans and horses the same genes ensure that appendages like toes, heads, and feet take shape in the right location. But more importantly HOX genes, in whatever form of life they are found, control whether other, more specific genes switch on at all to form various appendages—arms or antennae or wings.
Since HOX genes control squads of other genes, the results can be dramatic when they mutate. Recently scientists have found that several human malformation syndromes can be caused by an alteration in a HOX. When a developing embryo’s Hoxd 13 gene mutates in a certain way, for example, a condition can result in a baby being born with more than the normal number of fingers or toes. Doctors call this polydactyly syndrome.
But while in the medical world afflictions such as polydactyly may be considered “malformations” and “conditions,” an evolutionary biologist might just as easily call them genetic mutations. But the thing is, they don’t develop gradually. They appear instantly.14
Some scientists believe that HOX genes can help explain Gould’s punctuated equilibria. And in particular some believe that their existence may help explain how we managed to go from knuckle-walking jungle apes to upright-walking savanna apes in a few hundred thousand years or less.15
Scientists generally agree that most evolutionary adaptations are gradual, just as Darwin theorized, but some believe that they can show up inside of a single generation. If those adaptations improve a creature’s chances of survival, they would tend to spread through the rest of the gene pool. And their spreading would more likely happen when an environment has radically changed because that would make a dramatic mutation more likely to serve a purpose.
In the case of our ancestors, perhaps their DNA shifted and a gene like Hoxd 13 mutated and reshaped the feet of a particular family of savanna apes more than four million years ago that enabled them to more easily walk upright. In another time and place an australopithecine doctor might have written the alteration off as an unfortunate syndrome, but given that our ancestors found themselves struggling to survive in open grasslands that were nothing like the dense jungles in which they had originally evolved, a differently shaped toe might have been precisely what the doctor ordered. And it may explain how we got up and running so soon after we found ourselves orphaned among East Africa’s expanding savannas.16
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Once our ancestors’ knobbier big toes emerged, nature clearly favored other mutations that enabled walking and running. The remaining eight toes contracted, for example. Heels grew longer and thinner, and an intricate system of small bones and muscled arches developed that absorbed the constant shock of body weight being transferred incessantly from left to right and back.
In time fully one quarter of the bones in the human body came to be located in our feet, and our big toe developed enough strength to support 40 percent of our weight.c When we run or jump, this system, woven together by a web of 141 tendons, muscles, and ligaments can handle 6,000 pounds of pressure without a sprain, all of which helps explain the twists, turns, leaps and vaults
that amaze us when we have watched athletes and dancers seemingly fly through the air and land like feathers. Other primates may swing through the trees with the greatest of ease, but none can run, stride, or leap as gracefully as we do.
Nevertheless, our smooth gaits are not solely the result of our finely engineered feet. Other anatomical rearrangements also followed, bringing still shinier clusters of metamorphoses in tow. These did not likely happen neatly in a row. They probably arrived in fits and starts, with one adaptation feeding back on another. Some species of early savanna apes may have been more upright than others. Some may have developed knobbier toes, while cousins evolving two mountain ranges to the east or west did not. We can’t know all the details. There was evolution and coevolution and reevolution, mystifying exchanges between changing environments and the accidentally transformed strands of DNA that arose from one troop to the next.
These environmental conversations led to four other broad changes in the hominid line that eventually enabled savanna apes to walk and run very much the way we do today. First, our bowed legs straightened. Lucy’s remarkably preserved skeleton shows us that. Her legs, though not as long as ours, had already begun to bend inward from the thighs to make her the most knock-kneed primate the world had seen yet. She also developed stronger gluteal abductors, muscles on the sides of her hips that contracted to prevent her body from toppling left or right when all her weight shifted from one foot to the other in midstride.
Second, the pelvis and hip joints morphed. A chimp’s pelvic saddle, the circle of bones that connect the legs to the torso, is longer and straighter than ours, and the legs are hinged to its side at something like a ninety-degree angle. While this is fine if you are bent over at the waist most of the time walking on all fours, it is a problem when standing upright. Simian hip bones have the shape of shoehorns, and their angle is perfectly vertical. But to balance their upper bodies less precariously on top of their legs, the pelvises of hominids like Lucy had to grow shorter and spread out so the hip joints came to connect at something closer to a forty-five-degree angle. Lucy’s pelvis, strangely enough, is even more splayed than ours. Her hips are shorter and more open. This may be because she was lighter and didn’t require the same amount of bone to support her upper body. But whatever the reason, Lucy’s pelvic girdle was nothing like a modern chimpanzee’s. Nor was it fully human, but it was getting there.
Third, as weight was hoisted up on top of their pelvis, our predecessors’ spines realigned. The spines of chimps and gorillas are straight. They can afford to be because most of the time they are bent over with their backs parallel to the ground. This means that they are carrying about half their weight on their feet and half on their knuckles. Our spines (and the spines of Australopithecus afarensis and africanus), on the other hand, bend in the shape of an S, inward at the bottom and then outward as the vertebrae run up to the neck, a piece of evolutionary engineering that more effectively handles the weight that arms and hands had once supported.
Inevitably these reconfigurations shifted the location of our heads, something that eventually had immense consequences. When you are at the zoo and a gorilla walks toward you on all fours, his head is tilted back so he can look in front of him. If you had X-ray vision and could see the vertebrae of the gorilla’s neck, you would notice that they enter at the back end of the base of his skull. This makes anatomical sense if you normally walk on all fours, but doesn’t work very well if you stand upright because then you would find yourself staring at the sky. For an upright-walking ape, this arrangement had to change.
The place where the head and neck connect is called the foramen magnum. If you sat a gorilla and a human side-by-side and looked straight down through the tops of their heads, you would see that while the spine of a gorilla enters at the back of the cranium, ours enters at the middle, so that our heads tilt forward and sit squarely on top of our bodies, making it the final piece of skeletal statuary to be stacked in one straight line from the tips of our toes to the tops of our necks.
This was a lot for a readapted big toe to accomplish, but fortunately for us it did because there were enormous advantages to standing upright in Africa’s new open and dangerous woodlands.17 When the sun fell away on a moonless night, the darkness that followed would have been more black and limitless than anything we can imagine today in our artificially brighter world. At any moment our ancestors could have been snatched away by a nighttime predator. Often they probably were. No wonder we still worry about things that go bump in the night.
Life in broad daylight wouldn’t have been much easier. In the dry season, with the equatorial sun beating down, temperatures would often have reached triple digits (Fahrenheit). Troops of apes would have been kept constantly busy gathering food, finding water, and taking care of the young. On the savanna they would have had to contend with new predators that were evolving just as they were, early versions of jackals and hyenas and the megantereon, a lion-sized, saber-toothed cat with teeth the size of daggers. Though australopithecines could run faster than their knuckle-walking cousins, they could never hope to match the speed of predators like these.
Considering the circumstances, the remaining trees that clustered here and there along riverbanks and lowland mountain ridges must have been welcome sights, a kind of hominid security blanket they could recall from their simian days. Apparently they made good use of them. The fossil record reveals that Lucy and her cousins were still first-rate climbers, even if they walked upright. They had long, apelike arms, and wrist sockets that snapped into place for swinging from tree branch to branch when the situation called for it.
But standing on two feet was the adaptation that helped most. It vastly improved any savanna ape’s chances of survival. Knuckle-walking may have been ideal for short forays through the dense jungle undergrowth, but in East Africa’s expanding grasslands it would have been slow, tiring, and ultimately deadly.18 Studies show that knuckle-walking chimpanzees use up to 35 percent more energy than we do when we walk upright. That would never do in the open woodlands of the late Pliocene and early Miocene epochs.
Today’s surviving tribes of hunter-gatherers, like the bushmen of the Kalahari, are living proof. Studies have shown that each day they typically cover six to eight miles tracking down the food they need to survive. Had savanna apes been forced to do this walking on their knuckles, not only would they have used a third more energy (and a lot more time) to locate their food, they also would have had to eat a third more calories to replenish the additional calories their knuckle-walking required. Standing upright was the best way to survive. Maybe the most convincing proof is that today there is not a single knuckle-walking ape still roaming Africa’s grasslands.
Though food in the savannas was undoubtedly tougher to come by than it was in the jungle, there would have been some sources. Fresh prey, as well as nasty predators, were evolving on the grasslands: the deinotherium, for example, a thirteen-foot-high elephantlike creature with down-curved tusks; a giant buffalo called a pelorovis with a horn span six feet from end to end; and an early form of today’s giraffe that was short-necked and a mere seven feet tall. These would have supplied our upright-walking ancestors significant supplies of protein as they scavenged carcasses left behind by the grassland’s big cats.19
It is even possible that early hominids did some hunting and killing of their own. Standing upright, after all, not only meant getting around faster, it also created a taller creature that could now throw objects with more force and accuracy. Chimpanzees have been shown to hunt small game; perhaps A. afarensis and A. africanus did as well.
We can’t say for certain how many of these evolutionary scenarios are truly accurate, but paleoanthropologists have invoked them all to help explain how and why the first savanna apes did the supremely odd thing of developing toes that enabled them to rise on their hind legs. All of them, to some degree or another, are probably true, and the last possibility might be the most compelling because it means our predecessors
could not only avoid becoming a meal, they were also able to improve their chances of finding one. One way or another, standing upright would have helped solve the number one challenge every creature must solve if it hopes to avoid death and successfully pass along its genes: live to eat another day. For our ancestors, bipedalism became their version of sharp claws or deadly fangs, a radical adaptation that enabled them to survive in their equally new environment.
But upright walking accomplished even more than that. It changed the way we looked, too, and it changed how we saw the world and behaved, mostly with one another, in deeply fundamental ways. In other words, it not only shaped a new kind of body, it also shaped a new kind of mind.
Chapter 2
Standing Up: Sex and the Single Hominid
The evolution of sex is the hardest problem in evolutionary biology.
—John M. Smith
Animals avoid extinction two ways. First, they adapt to their environment. Second, they compete among their own kind for the affections of the opposite sex. Darwin called this intraspecies competition “sexual selection,” and he wrote extensively about it in The Descent of Man. His point was basically this: To survive in their environment, all animals have to first outflank predators, disease, dangerous weather, and whatever else nature throws at them. Those that develop the right physical equipment to live another day pass their well-adapted genes on to the next generation.
But to pass their genes along, they also have to successfully procreate, and to manage that, they have to gain the attention of the opposite sex. In the case of males they may evolve brightly colored feathers, a lion’s mane, or an elk’s fourteen-point crest of antlers. Each is a way of saying, “Look here! I have great genes.” These features, Darwin pointed out, “acquired their present structure not from being better fitted to survive in the struggle for existence, but from having gained an advantage over other males.”1
