Yes, You Were Born to Run

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 journalNatureput the idea on its cover, with the headline "Born to Run." And in his bookWhy 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 genusHomo, 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.
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