PHILADELPHIA—Imagine running 100 miles — barefoot. Christopher McDougall believes that’s what the human body is built to do.

In his best-selling new book, “Born to Run,” McDougall argues that humans evolved for persistence hunting—chasing game animals for hours until they keel over from overheating or exhaustion.

“We didn’t evolve as weight lifters, yoga gurus, or swimmers — we evolved as long-distance runners,” he said after an early-morning barefoot jog along paved Kelly Drive.

Can’t imagine yourself outlasting an antelope on the open savannah? Neither could McDougall before he began working on the book. In fact, he wasn’t sure if he would ever be able to run again.

Born and reared in Philadelphia, the 47-year-old freelance writer and former war correspondent had moved to Lancaster County, Pa., to enjoy a quieter life with his wife and two daughters. He had taken up running on local dirt roads, two or three miles every other day.

But five years of what should have been healthy exercise had left him with a catalog of ailments — torn hamstrings, aching arches, sprained ankles, strained Achilles tendons and, finally, dislocation of the small, wedge-shaped cuboid bone, which felt like “an ice pick driving straight up through the sole of my foot.”

Like many runners, he saw pain as an unavoidable part of the sport. That is, until he began reading up on the Tarahumara. Members of the reclusive tribe live in the Copper Canyons of north-central Mexico, where they routinely run hundreds of miles through the harsh, rocky desert in flimsy sandals made from strips of tire rubber.

What really got McDougall, though, was that they do it smiling.

As he learned more about the Tarahumara and other long-distance runners from around the world, McDougall became convinced that these amazing athletes were not just interesting aberrations. They were living — and running — as nature intended.

‘Running man hypothesis’

Several intriguing lines of evidence, laid out in a 2004 Nature paper by University of Utah biologist Dennis Bramble and Harvard University biological anthropologist Daniel Lieberman, support McDougall’s argument. Unlike other animals, they wrote, humans can dissipate excess heat on the run by sweating. And our breathing rhythm is independent of our stride, allowing us to breathe faster and get more oxygen into our bloodstream during exertion.

Other anatomical features — short toes, the Achilles tendon, a head-stabilizing ligament absent in other primates — also appear to be adaptations for running.

The “running man hypothesis” also helps explain a seeming paradox: Although young men leave women and older runners in the dust in short races and even marathons, when it comes to ultramarathons — competitions that typically are 30 to 100 miles long — the playing field becomes surprisingly level.

Since primitive humans hunted in packs, different individuals would have needed to maintain the same pace over a long distance in order to stay together, said McDougall. So it makes sense that everyone — old, young, male and female — would have similar long-distance running ability.

Too much structure?

For natural-born runners, though, we hurt ourselves pretty often. According to a 2007 review paper published in the British Journal of Sports Medicine, up to 80 percent of runners suffer an injury each year. And, despite major advances in running- shoe technology over the last three decades, injury rates have held steady or even increased.

McDougall now thinks that modern running shoes may actually be the problem.

Their raised heels encourage people to run with a higher-impact, heel-first strike, and their cushioned soles make such a strike less painful — but not less damaging.

And, by preventing the foot’s arch from bearing weight, support features in the shoe may weaken it over time and increase the likelihood of injury.

In contrast, feet that are in direct contact with the ground get constant sensory feedback. They adjust to reduce impact. Except for falls, running injuries are almost unheard of among the Tarahumara.

“In bare feet, you can’t overpronate, overtrain, or overstride,” McDougall said. “The bare foot is the best coach of all.”

Nike spokesman Derek Kent acknowledged that running barefoot could help strengthen the feet and improve performance. This insight, he said, led to the development of the Nike Free, a lighter, less supportive shoe with grooves in the sole that “allow for a more natural range of motion.” Wearing the shoe, he said, protected the foot from cuts, abrasions and other hazards.

Asked about McDougall’s assertion that traditional running shoes weakened the feet and predisposed people to injury, Kent said he had no information.

Podiatrist Howard Palamarchuk, director of sports medicine at Temple University’s Gait Study Center, said he believed that running shoes might not always be beneficial. “We encase our feet in a box, and those muscles never develop,” Palamarchuk said.

But he added: “As a medical practitioner who specializes in the foot, I’d say there are some people who should not even attempt to run barefoot.”

These include diabetics and others who do not have full feeling in their feet (peripheral neuropathy), as well as people with certain conditions — including club foot, very high arches, or legs that differ in length—that prevent their feet from absorbing shock well.

Stephen Pribut, the former president of the American Association of Podiatric Sports Medicine, concurred. “Too much structure [in running shoes], for many people, is not a good thing,” he said. “But I don’t think everyone is made to run barefoot.”

Like McDougall, Pribut believes that humans evolved not just to walk, but to run.

Now that he’s not getting hurt all the time, McDougall is finally able to reap the benefits of running. He has lost 30 pounds, worked his way up to 50-mile runs and lowered his stress level.

He attributes the last benefit to a simple fact of barefoot running: If you tense up, your feet hurt more.

And, like the elusive Tarahumara, he runs smiling.