The treadmill studies support that idea. A few months later, Lieberman showed the results from Jeff and 15 other joggers, partly summarized in a conference presentation ("Why Is Our Gluteus So Maximus?"). Sitting at his computer in shorts and flip-flops, Raichlen pulled up Jeff's data: a rainbow-colored series of 16 synchronized electrical recordings from all the sensors. Looking like an EKG signal, the electromyograph, or EMG, reading from the gluteus maximus, in red, showed little activity when Jeff strolled. But once he broke into a jog, boom!—the red line tightly zigzagged. The faster the pace, the bigger the spikes, like an earthquake signal on a seismograph.
What that shows, says Lieberman, is that the butt isn't much involved during walking. In running, however, the body leans forward so that each time the leading foot strikes the ground, the trunk wants to topple forward. The gluteus maximus prevents that: It fires just before the foot slams into the floor, creating a braking action that keeps the torso from falling down.
Meanwhile, the way we pump our arms back and forth in a trot helps steady us too. And based on their experiments, the researchers suspect that the motions of our shoulders and arms actually help counterbalance the head, preventing it from pitching forward on each landing. Simultaneously, with each heel strike, certain shoulder muscles contract and put tension on the nuchal ligament, pulling up the skull and keeping it level.
Our long neck is also important for running, Bramble says, because it allows the shoulders to twist freely of the head as we gaze forward. Chimps, in contrast, have hulking muscles anchoring the skull to the shoulders, which appear permanently shrugged—an orientation ideal for reaching overhead to dangle from tree branches. Controversial fossil evidence hints that australopithecines also had chimplike shoulders. But by the time of 2-million-year-old Homo erectus, says Bramble, hominids had lowered their shoulders, losing the thick, muscular connections to the head.
These running features, the researchers argue, are unmistakably obvious once you look for them. What's really hard to pin down, they admit, is when these adaptations emerged. How do you figure out when the first human butt appeared on the savanna? Muscles, tendons, and sweat glands don't fossilize, and old bones can't reveal precisely how their owners moved. Still, between biomechanical studies and bone analyses, it's possible for researchers to infer whether a fossil hominid was a jogger.
For example, most scientists reasoned that the 3.2-million-year-old hominid Lucy, with her chimplike build, couldn't have been a good endurance runner. She was squat, with short legs, a wide waist, long arms, and long, curving fingers and toes that suggest a tree-climbing lifestyle. Although researchers disagree on Lucy's gait while walking upright, nobody thinks she could have strolled like a human. Yet more than a million years later, Homo erectus roamed Africa with a much longer, leggier build, sporting a dramatically different set of physical changes that made it harder to climb trees but easier to jog, Bramble says. "All the running equipment's already there."
What biomechanics and paleontology studies cannot reveal is why these transitional hominid types forsook life among the boughs to become earthbound marathon runners. Archaeological studies at hominid sites offer one strong clue—animal bones. About 2.6 million years ago, our forebears started eating meat and marrow, rich sources of protein and fat that perhaps eventually fueled the growth of larger brains. Bramble and Lieberman find it conceivable that endurance running helped hunters pursue prey to exhaustion.
Back in the 1980s, Carrier had read ethnographers' accounts of indigenous peoples who chased deer, antelope, and kangaroos to exhaustion under the scorching sun. The Tarahumara of the mountainous desert of northwestern Mexico, for example, were legendary runners. But by modern times, their running tradition had turned to sport: Men wearing simple tire-tread sandals bound with leather thongs compete in a 24-hour footrace that involves kicking a ball over about 100 miles of mountainous road. So Carrier, a triathlete in college, took it upon himself to prove his case. He and his younger brother, Scott, went to the desert in Utah and Wyoming to chase pronghorn antelope. The beasts ditched them every time. The sleek, bouncy animals would join up with others, and soon the men would be huffing after a dozen of them. "You wouldn't know which were the animals you started with," Carrier says.
For direct evidence of endurance hunting, Bramble and Lieberman point to the observations of Louis Liebenberg, author of The Art of Tracking: The Origin of Science, who has spent time on the traditional hunts of the Bushmen hunter-gatherers in the central Kalahari Desert in Botswana. Liebenberg ran with them when they chased down kudu antelope on two occasions. For eight other hunts he trailed them in his Land Cruiser, sometimes with a GPS device. The men attempted to run prey to exhaustion only when temperatures neared 100 degrees F, says Liebenberg. Three men would gulp a lot of water and head out together. Two initially did the hard work of tracking and pursuing over the arid grassland and woodland terrain, while the other held back. Eventually, the leaders dropped behind, leaving the third man to hound and spear the antelope when it reached its limit. "The animal will either just completely collapse, or it will actually slow down to a point where it just stands there . . . with sort of glazed-over eyes," Liebenberg says. "Essentially, you're pushing the animal to overheat." The hunters would then walk home with the meat, enough to share—in small portions—with the tribe.
During a chase, Liebenberg noted that the men maintained speeds of around 4 to 6 miles per hour, for anywhere from two to six and a half hours, and traversed up to 22 miles of terrain. These stats fall well within the performance range of the world's fastest competitive marathoners, who set a pace of roughly 12 miles an hour to cover 26 miles, albeit under far less harsh conditions.
Although Liebenberg's observations support the runner-as-hunter hypothesis, Bramble and Lieberman think early Homo would more likely have first run to scavenge prey killed by other carnivores—a strategy the Hadza people of East Africa are known to use. When leopards or hyenas bring an animal down, the hunters "can spot these fresh kills at a distance from the vultures circling above," Bramble says. A carcass is an ephemeral treasure, picked clean within hours, so the Hadza quickly head off running, chase away the carnivores, and take what's left.
Of course, no one knows whether scavenging reaped enough caloric and nutritional returns to make it worthwhile for our forebears. But Bramble and Lieberman feel that the collective evidence, fielded from so many different angles, makes a compelling case for the running hypothesis. Even ordinary studies of human physiology, for example, suggest that humans are so adapted for intense physical activity that a sedentary lifestyle spawns modern-day scourges like diabetes and heart disease. Additional support could come from the chimpanzee genome, which may allow researchers to clock when the genes for slow-twitch muscle fibers—crucial for running long distances and plentiful in people but not chimps—diverged in the common evolutionary history of humans and apes. Other clues could come from tracing the genes involved in our abundant sweat glands and loss of body hair.
Meanwhile, other researchers are looking for holes in the argument. Functional morphologist Brigitte Demes, at the State University of New York at Stony Brook, notes that the gluteus maximus is absolutely essential for rising from a squatting posture at rest or during foraging, so it might not have evolved just for running. Stony Brook anatomist Jack Stern, famed for analyses of how Lucy walked, says it's a tough call to classify the Achilles tendon as an adaptation for jogging. Longer legs evolved in many animals through the extension of lightweight tendons rather than heavier muscle, thus producing a limb that took less effort to swing—a change that would save energy during walking, Stern says.
Nonetheless, Stern credits Bramble and Lieberman for a clever, coherent theory. "In essence, this is what Darwin did when he wrote On the Origin of Species: He took a series of facts and wove a beautiful, elegant story," Stern says. His own hunch is that the running hypothesis is correct, but it will take decades of scientific squabbling to confirm it. "This is just the beginning," he says. And even Demes, although skeptical of the running hypothesis, can't think of an alternative explanation. "That might actually be their strongest argument," she admits. "Why do we have this skill to go long distances for extended periods of time? Because you really don't see that in other primates." Bramble and Lieberman have received a flood of enthusiastic e-mail messages (and even poems) about their work, mostly from runners. "Sometimes you come across an idea that for some reason has been kind of overlooked. But it's just obvious. And those are the ideas, I think, that touch a nerve," Lieberman says. The evolution of walking was unquestionably fundamental to becoming human, he says, but running played a pivotal role too. "If we can get people to agree on that, I'll go to bed happy."
