"I didn't buy it at all," Bramble says. Like most of his peers, Bramble's first reaction to Carrier's hypothesis was that "humans are pitifully slow." From the perspective of a vertebrate morphologist, humans lack one of the most obvious features of animals adapted for serious speed: a tail. In creatures that cover ground bipedally, such as kangaroos, kangaroo rats, and roadrunners, "the tail is the major balance organ," Bramble says. "In the whole history of vertebrates on Earth—the whole history—humans are the only striding biped that's a runner that's tailless."
Still, Bramble eventually came to realize that people turn in remarkable performances. He once filmed a horse cantering, with Carrier running alongside at the same pace. The movie showed that Carrier's legs were churning more slowly than the horse's, which meant that the student's strides had to be spanning more distance per step than the horse's.
Although Carrier moved on to other research, Bramble grew convinced that his student had discovered something. During a visit to Harvard in 1991, Bramble encountered Daniel Lieberman, then an anthropology Ph.D. student, making a pig trot on a treadmill. To glean insights into how bones grow—and thus to better interpret fossilized human jaws and skulls—the student wanted to see whether the repeated impact of running would spur a thickening of the pig's skull. "You know," Bramble said, "that pig's not holding its head still." He went on to explain that adept runners like horses, dogs, and rabbits keep their noggins remarkably steady as they lope, thanks to an obscure bit of anatomy called the nuchal ligament. It's a tendonlike band that links the head to the spine. People, he said, have a version of this band.
Rummaging through a collection of replicas of fossilized primate bones in a nearby lab, Bramble pointed out that the nuchal ligament leaves a trace—a delicate ridge—where it attaches at the base of the human skull. Then the scientists noticed the ridge in a pitted, yellowed skull of our 2-million-year-old relative Homo erectus—but not in older hominids known as australopithecines, who walked the earth as far back as 4.4 million years ago. "Holy moley!" Lieberman thought. "There's something going on here, and what's more, we might be able to study it in the fossil record."
"Once the idea is in your head, then you start thinking about things differently," Lieberman says. A short 41-year-old with a receding hairline, a slight paunch, and disarming dimples, Lieberman doesn't look athletic, but he has been a jogger since his teens. I joined him for his morning run with his dog, Vashti, a border collie mix, whom he easily proved he could outlast. Lieberman says it's wrong to assume, as many do, that running is like walking. The two motions are strikingly different. He demonstrates that during walking his heel hits the ground first, the leg straightens, and then the body vaults over it.
"Your center of gravity, which is basically near your belt buckle, r-i-i-i-ses"—he takes a slow-motion step forward with his right leg and pauses, now up on the ball of his right foot—"so that it's over your leg." The body has now stored potential energy. The arch of the foot stiffens, and Lieberman pushes off against it. As he tips forward, potential energy converts to kinetic energy, and he swings his left foot ahead to complete the stride.
But in running, he says, the legs become springs. You land on and squash the entire arch and bend your knee. So initially the body's center of gravity falls. "You go down—and then you go up," Lieberman says. Kinetic energy from the crash landing is stored in the many stretchy tendons of the arch and the leg, most notably the huge Achilles tendon connecting calf muscles to the heel bone. Like rubber bands, the tendons extend and then recoil—boing!—to launch you onto the next step.
"So why do we have all these tendons in our legs?" Lieberman asks. "You don't evolve big tendons unless you're a runner." Kangaroos, antelope, and other serious animal runners all have a great set of springs, which do nothing for walking. So our tendons can't be explained as being necessary for walking.
Part by part, Bramble and Lieberman have reinterpreted the hominid physique by juxtaposing bits of fossil evidence with what's known about the physiology and biomechanics of jogging. Although much of the anatomy that lets us lope is the same equipment that humans first evolved for walking, the researchers say many of our physical traits seem tailor-made for sustained running.
To test their ideas, they conduct biomechanical studies in Lieberman's lab—Room 53 in Harvard's small redbrick Peabody Museum. The space looks more like a meld between a sports medicine clinic and an untidy engineering workshop than an anthropologist's sanctum of precious old bones. In addition to a gray-and-black running treadmill, there are wall shelves and counters cluttered with boxes of cotton applicators and latex gloves, containers of antiseptic, a small toolbox, tangles of electric cords, and plastic models of human parts, including a gigantic ear.
In one recent experiment, a volunteer named Jeff, dressed only in dark Lycra shorts and white socks, looked like a guinea pig trapped in a bad sci-fi flick. To track the electrical activity of key muscles, Lieberman and a postdoc, David Raichlen, had taped circular force-detecting pads to the bottoms of Jeff's feet and carefully rigged other parts of him with electrodes. Wires from those sensors ran through a small preamplifier box strapped to his lower back and then to a nearby computer.
To capture an image of his movement, they attached little silvery gray reflective balls onto Jeff's shoulders, hips, knees, and other joints. Three infrared cameras would track the balls' motion and record a stick-figure animation of him as he moved. Finally, to measure forces acting on his skull, the researchers mounted an inch-long accelerometer and gyroscope onto a small round tin and tied it all on top of Jeff's head with a black mesh do-rag that knotted securely under his chin.
Then Lieberman and Raichlen put Jeff on the treadmill and started it up. "Focus on the gazelle on the savanna," Lieberman instructed over the hum of the machine, indicating a big black X on a sheet of paper taped to a shelf straight ahead. The researchers gradually cranked up the pace until Jeff was pounding along at a hard run, sweaty and breathless. "Your gluteus is getting a serious workout," Lieberman said cheerfully.
The goal of the exercise was to understand how joggers stabilize their heads and torsos—part of the distinctive human balancing act that puzzled Bramble years ago. Without the balancing help of a tail, how do we avoid falling over when we run? The butt, it turns out, is crucial—right up there with the chin among traits that make us uniquely human. Chimps and other primates have little buns. Our own rear ends are huge; the upper part of the gluteus maximus is greatly expanded. Although few scholars have studied its role in running, the butt is, according to Bramble, "basically a substitute for a tail."