Nov. 17, 2004  -- Not everyone may feel this way, but new research argues that humans evolved to become natural runners.

From our spring-loaded ligaments to our muscular behinds to our ability to sweat, the human body took the ideal shape of a long-distance runner starting some 2 million years ago, the researchers say. The long, lean build helped us scavenge widely scattered kills and could also have been an advantage when hunting down prey over long distances.

"We're lousy sprinters, but we're really great long-distance runners," said Daniel Lieberman, an anthropologist at Harvard University. "Anyone who jogs regularly can tell you that it feels good."

How can two legs be better than four when it comes to striding for long distances? Consider the fact that some 334,000 people ran marathons in the United States last year, and then try getting an antelope to run 26 miles, or a chimp, for that matter.

"You'd never beat a chimp in a 100-meter dash, but you could never get them to run a marathon," he said. "And they wouldn't like trying."

Evolutionary biologists have generally credited humans' ability to run as an offshoot of our ability to walk on two feet.

"How can anyone even conceive of an animal evolving a walking strategy that was entirely decoupled from a running strategy?" asks C. Owen Lovejoy of Kent State University in Kent, Ohio.

But in a study appearing in this week's issue of "Nature," Harvard's Lieberman and Dennis Bramble of the University of Utah argue that endurance running may have been so critical in early humans' survival that it played a role in shaping many aspects of our bodies.

Runners From Head to Toe

The pair studied both modern human anatomy and the fossil record of early human ancestors to look for characteristics that would have specifically enhanced people's ability to run for long distances. They say most, if not all of these key features seem to have emerged 2 million years ago with the first appearances of the genus Homo -- the same group as modern living humans.

The peroneus brevis tendon, for example, made famous by Red Sox pitcher Curt Schilling's injury during the 2004 World Series, is one of several elongated tendons in the human body that the authors argue provides critical spring as a person runs. In apes and chimpanzees, the same tendons are much shorter, says Lieberman, and don't offer the same kind of spring-loading action.

Then there is the gluteus maximus -- the unusually large muscle humans carry at their rear. Why such bulk in back? Lieberman says it's for running and, again, this feature is less pronounced in our evolutionary ancestors.

"When we walk, we barely use the gluteus maximus," he said. "As soon as you start running, it plays a vital role to keep you from falling -- it stabilizes your trunk."

Other features the authors list that help us run include the arches in our feet, which offer spring in our step, and broad surface areas of our joints, which help distribute the shock of impact from running -- at least enough for ancient man, who didn't run on pavement and who never lived much longer than 40 years.

The upper body, meanwhile, carries its own made-for-running designs, including wide shoulders -- good for swinging arms from for balance as we stride -- and lighter forearms that are easy to move back and forth. Even our heads are equipped for running, they say, as a large ligament stretching from our spines to the back of our heads acts to dampen the oscillation of our heads as we plod along.

Finally, our ability to sweat is unmatched with our estimated 3 million sweat glands. Couple that with the fact that we aren't very furry and you have a cool, running machine.

Jogging for Supper

Bernd Heinrich, a world record holder in the ultra marathon and biologist at the University of Vermont in Burlington, says the authors' points make sense.

"Most of us don't do much running so it may not feel natural, but it feels natural to me," he said. "Not much is new here, but I think they bring together a lot of evidence so it all fits into a pattern."

While few anthropologists argue with the fact that humans evolved to become good runners, Lovejoy remains skeptical we were specifically designed for endurance running.

"There is little doubt that many of the bony features that are mentioned … are adaptations to running and walking, but there is no evidence that they are specifically adapted to endurance running," he said.

Lovejoy points out that our arms and legs could also be considered well designed for swimming, but that doesn't necessarily mean we evolved specifically to be elite swimmers.

But Lieberman and others counter that endurance running, unlike swimming, could have been a key part of early man's survival. It may have helped them during long hunts and in scouting out abandoned carcasses first, for example.

"Being fast would have been a huge premium," said Heinrich. "Vultures can come in and devour a dead cow in an hour or two. So ideally, the humans would get there first."

Patricia Kramer of the University of Washington, points out there may be a small glitch in that theory. According to most research, early female humans likely did not participate in long hunts, but stayed behind to care for the young. If this is the case, Kramer asks, why would women also have evolved to be good long-distance runners?

"If endurance running was a male activity, then why do women have small waists and hypotrophied gluteus maximae?" she asks. "I think that understanding how we moved through our environment is critical to understanding who we are as evolved primates … but as with all good research this causes us to ask a new set of questions."