BY RICHARD CONNIFF, APRIL 16, 2008, Men’s Health

Millions of years of genetic mutation and adaptation have produced a singular animal whose body, mind, and spirit are primed to sprint as if life depended on it. That animal is you. So why are you just standing there?

During his first full-throttle “persistence hunt,” the South African biologist Louis Liebenberg was working with bushmen in the Kalahari Desert in the early 1990s. Armed with handmade bows and arrows, the hunters had been stalking kudu — a nimble antelope, slightly smaller than an elk. When a young stag split off from the herd, the bushmen ran flat-out after it.

The kudu moved quickly out of sight in the brushy Kalahari landscape. But keeping up was more than just a matter of running; the hunters also needed to pick up footprints in the sand on the fly. Liebenberg, then age 30, hadn’t done the conditioning to be a long-distance runner, and he was wearing heavy leather boots as a precaution against poisonous snakes. And this was shaping up to be a hard run.

In persistence hunting, the trick is to trot almost nonstop in the heat of the midday sun, pushing the animal along so that it never has time to recover in the shade of an acacia tree. The Kalahari hunters have figured out how to play one critical advantage in a deadly game that pitches their survival against that of animals: Humans have an evaporative cooling system, in the form of sweat; antelope don’t. When conditions are right, a man can run even the fastest antelope on earth to death by overheating.

But after 10 or 12 miles, Liebenberg was overheating, too, and by the time he reached the kill, he was so dehydrated he’d stopped sweating. The only liquid in sight was the stomach water of the dead animal, but his companions stopped him from drinking it, because kudu eat a leaf that’s toxic to humans. If one of the hunters hadn’t run back to camp for water, Liebenberg figures he would have died. He also figures the experience taught him the answer to an ancient question.

What makes people run?

Why do 11 percent of Americans and tens of millions of people around the world tie on running shoes and clock their weekly miles? The three most recent presidents of the United States have put in time as runners (and earlier this year, one candidate, Mike Huckabee, trained for the Boston Marathon while campaigning for the U.S. presidency). The president of France, Nicolas Sarkozy, is a runner. And beyond the vast army of ordinary joggers, it can sometimes seem as if the entire planet is trembling beneath the footfalls of ultramarathoners, Ironmen, and other endurance athletes.

Runners also make the news by dying while running–two died in the 2006 Los Angeles Marathon, another during unseasonably hot weather at October 2007’s Chicago Marathon, and yet another a month later, when 28-year-old Ryan Shay died of heart failure during the Olympic Marathon trials. So the question is asked not just in puzzlement but sometimes in anger and sorrow: What makes us run?

The answer, according to a controversial body of research, is that our passion for running is natural. A small group of biologists, doctors, and anthropologists say our bodies look and function as they do because our survival once depended on endurance running, whether for long-distance hunts like the one Liebenberg witnessed or for racing the competition across the African savanna to scavenge a kill. The prominent science journal Nature put the idea on its cover, with the headline “Born to Run.” And in his book Why We Run, the biologist and runner Bernd Heinrich, Ph.D., argues that something exists in all of us that still needs to be out chasing antelopes, or at least dreaming of antelopes. Without that instinct, “we become what a lapdog is to a wolf. And we are inherently more like wolves than lapdogs, because the communal chase is part of our biological makeup.”

Daniel Lieberman, Ph.D., first started to think about whether humans evolved for running as he was running a pig on a treadmill. A colleague, the University of Utah biologist Dennis Bramble, happened to look in. “That pig can’t keep its head still,” he remarked.

This was an observation Lieberman admits he never made in months of running pigs. Bramble invited him next door, where a dog running on a treadmill was holding its head “like a missile.” The conversation turned to the nuchal ligament, a sort of shock cord stretching from the back of the skull down the neck. It keeps the head from pitching back and forth during a run. Dogs have one because they’ve evolved for running. Pigs don’t.

Lieberman and Bramble were soon digging through bone collections. The skulls of chimpanzees, our closest primate relatives, showed no evidence of a nuchal ligament. But skulls of the genus Homo, which includes modern humans, did. “We had one of those epiphany moments that happen occasionally in science,” says Lieberman. Much as chimps were built for life in the treetops, the two scientists began to ask if humans were built for life on the run.

Almost 20 years later, I’m the pig on Lieberman’s treadmill. A postdoctoral fellow, Katherine Whitcome, has me trussed around my hips, chest, neck, and forehead with gyroscopes and accelerometers for measuring angles and speed of movement. The insoles of my running shoes have been fitted with inserts laced with devices that will measure my heel strikes and the way I roll off my fifth metatarsal. Wires run through a duct-tape collar to an assortment of electronic boxes on a nearby shelf and from there to Whitcome’s computer.

Lieberman starts the treadmill. “Pretend that the piece of yellow paper on the wall is your antelope,” he says. The speed kicks up to 6.7 miles per hour, and as my stride lengthens to keep pace, a dismal, office-worker thought passes through my mind: I salivate for Post-it pads.

I have never been a hunter. But as a journalist, I have been in on chases after real animals and close enough to witness a kill. Once I was following a fox hunt on foot through hilly country in Ireland’s County Meath. The riders came thumping down a muddy lane, shaking the earth with the staccato of metal horseshoes clattering on the occasional rock. They paused as the hounds searched a stand of woods. Flocks of blackbirds fled in alarm from the bare treetops. Then a hound let out the first strangled cry as he caught a hot scent, and a moment later a fox made a beeline out of the woods and up a hill. After a moment of confusion, the hounds also burst into the open. The horses took off. I followed, leaping from hummock to hummock to traverse a wet section and then sprinting up a slope, feeling as fleet and sure-footed as the 9-year-old who was running beside me. On another hunt, I saw the hounds chase a fox into a wetland, cascades of water kicking up around their feet. Then the distance closed and the fox vanished in a bloody cloudburst.

I suppose I should have felt remorse. But what I honestly felt was exhilaration at the close connection to the hunt, with life and death in the balance. The sudden power of forgotten urges astonished me. Had they been my kills, I would have smeared my face ritualistically with the blood.

Anyone who has put in some miles knows how good running can feel, once it stops feeling bad. But beyond the way it feels, medical evidence also suggests that humans are built for endurance exercise. In response to a good training program, for instance, the left ventricular chamber of the heart can increase as much as 20 percent in volume. The chamber walls thicken, too. So the heart fills up faster and pumps more blood to the rest of the body. The coronary arteries also change, dilating more rapidly to meet the body’s demand for oxygen. Endurance exercise won’t make anyone live forever. But it seems to make the cardiovascular system function the way the owner’s manual intended.

In the skeletal muscles, increased blood pressure causes new capillaries to emerge. The mitochondrial engines of the cells ramp up to consume energy more efficiently, helped along by an increase in the production of various antioxidants. These changes in the heart and extremities together typically boost the maximum amount of oxygen the body can consume each minute by 10 to 20 percent. For men who used to become short of breath slouching to the fridge for a beer, VO2 max can increase even more. Lapdogs start to function like wolves.

More surprisingly, the brain responds as if it was built for endurance exercise, too. Everybody knows about the runner’s high, that feeling of euphoria thought to be triggered by a rush of endorphins to the reward centers of the brain, usually near the end of a good, long workout. (Running for dinner, as part of a hunt, could very well amplify that effect; in essence, a love of running could lead to more ample dining opportunities.) But researchers have discovered lately that exercise affects the function of 33 different genes in the hippocampus, which plays a key role in mood, memory, and learning. By stimulating growth factors, exercise also produces new brain cells, new and enhanced connections between existing cells, new blood vessels for energy supply, and increased production of enzymes for putting glucose and other nutrients to work.

People who exercise regularly perform better on some cognitive tests: Run more, think better, hunt smarter, eat better. Exercise also seems to buffer the brain against neurological damage, reducing the effects of stress and delaying the onset of Alzheimer’s and other diseases. Most significant, exercise helps prevent and alleviate depression, which afflicts one in six Americans and costs $83 billion a year. In fact, studies suggest that exercise works as well as pharmaceutical antidepressants, and that the effect is “dose dependent”–that is, the more you exercise, the better you feel.

Running may also be the forgotten reason for many of the movements — the turn of a shoulder, the sway of a hip — we think of as most gracefully human. The lines of a Theodore Roethke poem come to mind: “My eyes, they dazzled at her flowing knees; / Her several parts could keep a pure repose, / Or one hip quiver with a mobile nose / (She moved in circles, and those circles moved).”

To put it in the less romantic language of anatomy, it’s the reason we are sweaty, hairless, elongated, and upright. It’s also the reason, Lieberman and Bramble say, for the exaggerated size of the human gluteus maximus. Their studies show that our big buttocks don’t matter much in walking on level ground, but they are essential for staying upright when we run.

Our legs have evolved for running, too, says Lieberman, and not merely in length. “Human legs are filled with tendons.

Chimpanzees have only a few, very short tendons. Tendons are springs. They store up elastic energy, and you don’t use elastic energy when you walk — at least not much of it.” But when you run, storing up the force of impact and releasing it as you kick off is essential. Smart runners know they can release that force more efficiently by using a springier gait, says Lieberman. “It’s really about the jump.”

Other scientists have begun to incorporate the “endurance-running hypothesis” into their research. Timothy Noakes, M.D., a South African physician whose book The Lore of Running is the bible of technical running, argues that misunderstanding human evolution can pose a deadly hazard to endurance athletes. British and American runners in particular have fallen prey to the notion that it’s essential to stay heavily hydrated during a race. Runners have died of hyponatremia brought on by drinking too much liquid while sweating profusely, which diluted their blood sodium to a lethal level.

“Humans evolved not to drink much at all during exercise,” says Dr. Noakes, chairman of exercise and sports science at the University of Cape Town. “If they had to stop every 5 minutes to drink, they would never have caught the antelope.” The secret for modern runners, he says, is to drink just enough to minimize thirst. “The best runners in any culture are the ones who run the farthest and drink the least, and the bushmen are the classic example. Humans are built to become dehydrated. That’s the point.”

But other researchers have attacked the endurance-running hypothesis, mainly on cultural grounds. Writing last year in the Journal of Human Evolution, Travis Pickering and Henry Bunn, anthropologists at the University of Wisconsin, argued that persistence hunting was too rare to have played a large role in our evolution. Bunn also calls endurance-running proponents “incredibly naive” in failing to consider alternate explanations of how early humans secured meat. They may have banded together as “power scavengers,” for instance, to steal kills from ambush predators. In any case, he says, meat was a relatively minor, though coveted, part of their diet.

Lieberman all but rolls his eyes at their arguments. Early humans didn’t have fire to cook meat and release its nutrients until 250,000 years ago. They didn’t have the bow and arrow until 20,000 years ago. “But we know that people have been hunting for 2 million years. The best weapon they had available to them was a sharpened wooden stick. I’m not exaggerating. How the hell are you going to kill an animal with a sharp wooden stick? It’s incredibly dangerous. You have to move close to the animal, which means the animal can kick you or gore you.”

And the alternative? Simply run the animal for 5 or 10 miles until it’s dying of heatstroke, and then knock it over with a feather. “That’s it. It’s amazing. It’s so easy.”

So if humans evolved for distance running, does that mean we should all be out notching up marathons now? Even ardent runners generally don’t think so.

On a winter afternoon, Walter DeNino, a medical student at the University of Vermont, is doing his regular training run along the Lake Champlain shoreline. Back in high school, he says, he logged so many miles that he ended up on crutches at the age of 15, with multiple stress fractures. He started to think that maybe some people really aren’t built for long-distance running after all, or at least not for the distances we’re tempted to run by the addictive nature of the sport.

Eventually, DeNino took up the triathlon, with a training emphasis on swimming and cycling. He also founded a coaching and sports-nutrition company, Trismarter.com, which aims, among other things, to lure lapdogs and couch potatoes back to the active life. The triathlon is a much newer sport than the marathon, he says, and it’s more welcoming to different body types.

That seems to be how nature works, too. Heinrich points out that humans have hunted with weapons long enough for natural selection to favor survival talents other than running. The rise of agriculture also may have changed the shape of the human animal. So some people have the light, lean, almost birdlike build of the ideal long-distance runner, and others are built squat and strong, for moving earth. According to one line of research, the cultures of our ancestors may even give some people a genetic predisposition to or away from long-distance running.

And yet as I ran on the treadmill that day in Lieberman’s Harvard laboratory, it seemed to me that the proponents of endurance running were onto something persuasive and appealing about human nature. There were moments when I forgot about the Post-it-pad antelope. Instead, I imagined a real antelope racing out ahead of me. I imagined my distant ancestors on the African savanna, hunting not quite beside me, but somewhere within. And just the thought of that connection lifted me out of this mundane world and away to someplace wild and even a little sacred.

Later, Heinrich told me about feeling that same connection when he was doing research in Zimbabwe’s Matobo National Park. As he looked under a rock overhang, he suddenly found himself staring at a wall drawing made thousands of years ago by bushman hunters. It showed a series of stick figures, bows and arrows in hand, arms pumping, legs extended at full stride in the heat of the chase. Big, horned wildebeests loomed in the background. And off to the right, one hunter was raising both arms in an unmistakable gesture of triumph. It was the same gesture Heinrich had instinctively made the first time he won a marathon, the same one countless other runners still make as they cross the finish line. “Looking at that African rock painting,” Heinrich later wrote, “made me feel that I was witness to a kindred spirit, a man who had long ago vanished yet whom I understood as if we’d just talked.”

And, he concluded, “There is nothing quite so gentle, deep, and irrational as our running — and nothing quite so savage and so wild.”