AS THE RUNNER MOVES IN FOR THE KILL, his most effective weapon -- his feet -- splash into the deep white sands of Africa's Kalahalari Desert. The footprints that he leaves behind are like fossils dating back to the origins of human speed. For several hours, under a suffocating sun that has pushed temperatures toward 120 degrees, the runner, a member of the !Xo San tribe named Karoha, has been engaged in a persistence hunt. He's chased an animal, literally, to death.

Dating back millions of years, the persistence hunt is the most ancient form of hunting known to man. It's also a theoretical key to both our modern-day athletic makeup and our obsession with speed.

Gliding through the razor-sharp thornbush, the shirtless Karoha, lean and muscular, focuses his deep-set eyes up ahead on his prey, an African kudu antelope. Unable to rest and cool itself because of Karoha's relentless pace, the beast is overcome by heat exhaustion. It stands helpless, then collapses, appropriately, at the feet of the hunter.

More than 10 years after filming this scene, Craig Foster, co-director of "The Great Dance: A Hunter's Story" and one of the few outsiders ever to witness a persistence hunt, is still moved by the rare anthropological glimpse back in time. "It was an extraordinary, life-changing experience to be thrown backward like that into the original human design," he says. "It provided this massive realization of who we are and what our body, mind and soul had been designed to do."

To run. And do it in a way that distinguishes us from every other species on the planet: at a moderate pace over long distances. It's not nearly as mesmerizing, or as provocative, as conventional displays of ambulatory velocity like, say, the explosive sprinting power of Usain Bolt. But the truth is that our raw speed is rather unremarkable when compared with the significance and uniqueness of our relative speed. Persistence hunting helps prove that when it comes to human movement, speed doesn't kill. Stamina does.

Captured on film for the first time in "The Great Dance", man's extraordinary success as an endurance predator has helped give rise to the endurance running evolutionary hypothesis -- a.k.a. the Running Man theory. The basic premise is that we evolved into the species we are today thanks to one key but long-overlooked

attribute: the ability to run long distances. The sports implications are both elemental and monumental, telling us that running isn't just a whim of humans; it's why we are human.

Popularized by author Christopher McDougall in his best-selling book "Born to Run", the Running Man theory has moved from the fringes of the

scientific community toward the athletic mainstream, challenging nearly everything we know about movement, sports and the human body. "It's fascinating the way this is all connected," says Louis Liebenberg, one of the leading experts on persistence hunting. "By exploring our past, we are rediscovering new limits to our endurance that we were never fully aware of."

The realization that we are all natural-born runners may play some role in what the organization Running USA dubs marathon "mania." From 2008 to 2009, the number of participants finishing the 26.2-mile race jumped 10 percent to an

estimated 467,000 runners, the largest percentage increase in 25 years. Meanwhile, the number of people completing half-marathons has grown an astronomical 131 percent since 2000, to

1.1 million runners. (The 5K, by comparison, has increased 40 percent during the same time frame.) What's more, the awakening of our ancient,

running self seems to have transformed the ultramarathon -- a race up to 100 miles long or 24 hours in duration -- from a death-defying fringe activity into one of the fastest growing segments of endurance sports.

More than a million people worldwide now compete in marathons. Pierre Verdy/AFP/Getty Images

If you believe the Running Man theory (and not everyone does), ultrarunners aren't nuts; they're doing exactly what our bodies were designed to do. It's the couch potatoes who

are going against nature. Which is why the International Association of Ultrarunners claims we could soon see a 100-mile race in the Commonwealth Games and the Olympics. "It might sound silly, but I do feel a connection to some ancient, Neolithic, warrior spirit in these

races," says American Scott Jurek, a world-class ultramarathoner who won the 100-mile Western States Endurance Run seven consecutive times. "That connection to our primal self is drawing people to ultrarunning. I mean, it's pretty damn cool, if you think about it, that we adapted as humans to a point where we could run down animals."

AND TO THINK, our running renaissance started with a pig on a treadmill.

In the 1990s, Dennis Bramble, an evolutionary biologist at the University of Utah, became

intrigued by the theories of his former grad student David Carrier, the first person to publish a connection between evolution, our unique human physiology and running. Carrier's conjecture: That our ancestors, the semisimian Australopithecus, pulled their knuckles off the dirt and became bipedal not to free their hands to carry weapons

(as hunters) or to carry supplies (as gatherers) but to open their lungs in order to breathe -- and thus, run -- better.

The idea gained little traction in the scientific community. For starters, it went against every anthropology textbook ever printed. Years later, when the theory was first mentioned to

C. Richard Taylor of Harvard, one of the world's leading experts on animal locomotion, he thought the whole thing sounded, in a word, "stupid." Evolution suggests that animals who have adapted best to their environment develop an advantage as a species. What advantage could running hold for humans who, over short distances, are among the slowest mammals? Bolt, the fastest man on the planet, would get caught by a lion in less than 20 seconds. Looking at raw speed instead of relative speed or endurance, it was hard to see a competitive upside for primitive man to run.

As a runner, though, Bramble was intrigued enough by Carrier's theory

to determine if humans shared any unique attributes with running mammals, like cheetahs, rather than with walkers, like chimps. As part of his research, he visited Harvard evolutionary anthropologist Dan Lieberman, who was in the midst of

studying animals that run poorly. Inside his lab at Harvard's Peabody Museum, Lieberman had a pig on a treadmill ("Not a pretty sight," jokes Bramble) when his guest pointed and said, "Dan, look, that pig can't hold his head still."

That was the first of several aha! moments for the researchers, one that led the two men to discover that pigs and other nonrunning mammals lack what humans, horses and dogs have in their necks: the little-known nuchal ligament that stabilizes the cranium at high speeds. This is one of the human body's 26 unique morphological markers that Bramble and Lieberman revealed in a breakthrough 2004 paper, "Endurance Running and the Evolution of Homo," which ended up on the cover of Nature. "From head to toe in humans, it all just works," says Lieberman. "We evolved to run."

ONE DAY WHILE WATCHING A PIG JOG ON A TREADMILL, BRAMBLE HAD AN AHA! MOMENT -- HUMANS EVOLVED AS RUNNERS IN ORDER TO HUNT.

Need more proof? Even though we still share 95 percent or more of our DNA with chimps, only human legs and feet are loaded with springlike ligaments and tendons that are essential for running. Chimps, if you've noticed, also don't have much of a gluteus maximus. In contrast, our robust behind is not only our biggest muscle, it's also primarily used for running, firing at the moment of foot impact as a counterbalance to the chest that prevents us from falling on our faces. In fact, this system gives us a relative stride length that's longer than a galloping horse's. Our narrow waist, meanwhile, allows us to swing our arms and run in a straight line. Even our short toes are perfect for running. If they were just 20 percent longer, we'd need twice the amount of energy just to push and lift our feet off the ground.

As described in "Born to Run", while conducting his research, Lieberman became intrigued by the behavior of his dog, Vashti. The pair would often go for jogs on hot summer afternoons. After a short distance, though, the dog usually plopped down in the shade and refused to move, panting like crazy in order to cool down. Quadrupeds like dogs might have an impressive initial burst of speed, but their lack of sweat glands means they must stop moving relatively quickly

to pant and cool themselves. Humans, meanwhile, with sweat glands located all over our furless bodies, can run, breathe and cool

ourselves simultaneously, a physiological and evolutionary royal flush. Perhaps, Lieberman thought, as he looked at his weary pet, the evolutionary advantage that spurred early humans to become runners was predatory in nature. He

recalled the stories that he'd heard about African hunters using an overheating technique to run down antelope across the scorching-hot savannas. Was this the keystone to the entire Running Man hypothesis? All he needed was proof.

ENTER LOUIS LIEBENBERG, the South African scientist who had studied persistence hunting firsthand for 25 years in the Kalahari. On one hunt in the late 1990s, Liebenberg saw men who ran down a kudu in three hours 35 minutes, traveling 35km to do so. As the hunt progressed, the trackers seemed to enter what the scientist calls a "trancelike state." The experience left

Liebenberg certain that "the full picture of why we became human must include running, it absolutely must."

Liebenberg's observance of persistence hunting helped close the logic gap on the Running Man theory. Roughly two million years ago, Australopithecus was short, stout and hairy with a small brain and a big jaw. For its descendants to transform into the taller, skinnier, brainier Homo erectus, Australopithecus would have required a steady, heavy diet of calorie- and protein-rich meat. But how was it getting that meat? Fossil records show that the stone-tipped spear is only 200,000 years old, and the bow and arrow dates back just 50,000 years. That leaves a span of at least 1.8 million years when our primitive ancestors were chowing down on game before hunting tools had even been invented. Because our prey could run only short distances, to gain access to that essential form of food, we developed a body that was more efficient on two feet over long distances than our faster four-legged prey. Over millions of years, those beneficial traits were selected, passed on and can be found in the dozens of morphological markers for running in modern man. All of which has led some scientists to

rethink the birth of -- and the limits to -- human speed and endurance.

OVER SHORT DISTANCES, HUMANS RANK AMONG THE SLOWEST MAMMALS. EVEN WORLD-RECORD HOLDER USAIN BOLT WOULD GET CAUGHT BY A LION IN LESS THAN 20 SECONDS.

As a result, new theories on the equipment and nutritional requirements of runners have begun to take root. In fact, the current barefoot and minimalist-shoe running craze was inspired by research into the way our hominid forebears once moved while in pursuit of prey. Organizers of the New York City Marathon say they have seen a noticeable increase in the barefoot trend. Membership in the Barefoot Runners Society, meanwhile, has doubled since 2009, though this group still represents a tiny fraction of the total running population.

What's more, since the original runners were persistence hunters in the extreme heat of Africa, physician and renowned author Tim

Noakes has developed a theory that could put a major dent in the sports-drink industry. For

millions of years, hunters typically ran for hours while drinking only what they could

carry inside an ostrich egg, usually less than

1.5 liters of water. Likewise, Noakes found that the best modern-day runners on the planet,

like marathon world record-holder Haile

Gebrselassie, aren't the ones hyperhydrating with fancy sports drinks; they're the ones who can go the farthest while drinking the least.

"We have to explain all of these amazing

features that make us good at running, and

persistence hunting is the most obvious," says Harvard's Lieberman. "Nobody had ever really thought that running was all that important in human evolution. But no one else was making this argument, so we kept asking ourselves, Are we crazy? Are we missing something?"

Skeptics of the theory say yes on both counts despite substantial ethnographic proof that persistence hunting was used by many cultures all over the planet, including the Papago Indians of the American Southwest, the Tarahumara

of Mexico and the Australian Aborigine. Opponents suggest that millions of years ago, the African plains were too lush to run through, that the theory overemphasizes the importance of meat in primitive man's diet and development, that most of the physiological markers could have been for walking, not running, and that the energy cost for chasing an antelope would have far exceeded the calories gained

by consuming the meat. "Utter rubbish," says

Noakes. "Scientists learn one way, and the cost and energy of changing that thinking becomes so high it is not feasible for them. When you take all this data in with an open mind, there's just no other explanation: We evolved as runners."

It's a conclusion that gets even harder to argue while watching the predatory runners

of the !Xo San tribe in action. For hours, the running men float effortlessly, like ghosts, across one of the harshest climates on earth. And when they see erratic tracks in the sand indicating that the kudu is tiring, they lock on to their prey like a guided missile. They call themselves "sons of the first people," and their focus seems almost primordial.

The hunt ends when Karoha walks up to the animal and plunges a rudimentary wood spear into its chest -- an act that is largely ceremonial. Karoha then kneels and quietly honors the kudu by ritualistically spreading sand over its body and transferring saliva from the antelope's mouth onto his own exhausted and burning legs. On Karoha's face, and especially in his dark eyes, there is a solemn sense of relief and joy. As it has for millennia, this successful persistence hunt means his children will eat and grow, as will his standing in the tribe.

In the end, what the filmmakers thought would be the most disturbing part of the documentary, the graphic death of an animal, turned out to be the most touching and transcendent. At the movie's debut, the scene with Karoha and the kudu left many audience members in tears. Foster believes that witnessing a persistence hunt evokes an undeniable connection to our primal, predatory running past.

"People were overcome," he says, "because they were seeing a deep, important part of themselves that they never knew existed."