New research suggests that developing the ability to run — long thought to be a byproduct of walking for early man — was actually an instrumental step in the evolution of people from ape-like creatures into what we know today as humans.

In a study published in Thursday's edition of the journal Nature, University of Utah (search) biology professor Dennis Bramble and Harvard anthropologist Daniel Lieberman conclude that natural selection favored early humans who had genetic mutations that accommodated running.

Over time, that included a narrowed waist, shorter forearms, larger buttocks and skull modifications allowing the body to cool itself.

The research suggests that primitive man's evolution from walking to running was fundamental to human evolution — forsaking the capacity to live in trees, but vastly improving the species' ability to find food and ultimately becoming anatomically similar to the way we are today.

They also hypothesize that running preceded brain enlargement — and may have precipitated brain development by allowing primitive man to locate and consume more protein.

"The structure of specializations required for hunting are sort of counter to those that are useful for climbing," Bramble said. "We sort of favor the notion that humans may have evolved running initially to increase their intake. But not through hunting — probably through scavenging."

Bramble estimates early man walked for 2.5 million to 3 million years before finally developing running ability.

Previously, researchers hadn't much focused on running, instead considering it simply a byproduct of walking. But Bramble points to specific adaptations — like the Achilles tendon (search) — as evidence that man wasn't capable of running until evolution accommodated it.

"It turns out that [the Achilles tendon] is a terrific spring, and it makes a huge difference in the economy of running, but it makes no difference in walking," he said.

Bramble said scientists might have overlooked the importance of running because humans aren't considered particularly fast in comparison to four-legged animals.

However, Bramble says endurance, more than speed, was the key for early man — enabled by adaptations allowing the body to cool itself and function over long distances.

Furthermore, Bramble suggested that early runners could even have been faster than our contemporaries, perhaps in part because projectile hunting weapons made running less necessary as humans evolved further.

"Some of our ancestors very likely could have been better runners than some of the best runners today," he said.

Christopher Ruff, director of the Center for Functional Anatomy and Evolution (search) at Johns Hopkins University, said the theory is valuable, but won't necessarily change the way scientists think about human evolution.

Ruff said it's difficult to tell which genetic modifications uniquely accommodated running — because so many of them also aid in walking.

"It's more of a hypothesis than a real test of the theory," he said. "It's interesting to think about the data in this way, and it's a good idea to consider some driving forces, but I don't consider this proof."