he evolution of a physique for long-distance running is what made humans look the way we do now - whether winning a marathon, nursing a strained Achilles' tendon or sitting on an ample gluteus maximus in front of the TV.

The apparently crucial role of running in human evolution, overlooked for the most part in previous research, is being proposed today in an article in the journal Nature by two American scientists.

While walking upright first set early human ancestors apart from their ape cousins, the scientists write, it may have been the ability to run long distances with springy step over the African savanna that influenced the transition to today's human body form.

Endurance running, unique to humans among primates and uncommon in all mammals other than dogs, horses and hyenas, apparently evolved at least two million years ago and probably enabled human ancestors to hunt and scavenge for food over large distances. And that, in turn, probably proved decisive in their pursuit of high-protein food for development of larger brains.

The scientists, Dr. Dennis M. Bramble of the University of Utah and Dr. Daniel E. Lieberman of Harvard, reported that their analysis of the fossil record showed striking anatomical evidence for the capability of prolonged running in the Homo genus as early as two million years ago.

'Today, endurance running is primarily a form of exercise and recreation, but its roots may be as ancient as the origin of the human genus," the scientists concluded in the article.

Dr. Bramble, a professor of biology and a specialist in the biomechanics of animal locomotion, said, "Running made us human, at least in an anatomical sense," adding that he and Dr. Lieberman were "very confident that strong selection for running was instrumental in the origin of the modern human body form."

Other paleontologists not involved in the research praised the hypothesis as an important insight into the apparent significance of long-distance running in human survival and evolution. But they raised questions over what stimulated the physical transition that led to this human capability.

By two million years ago, Dr. Bramble and Dr. Lieberman noted, early species of the Homo family, beginning at least with Homo erectus, had long, slender legs for greater strides. They had shorter arms and a narrower ribcage and pelvis. Their skulls included features to help prevent overheating. A ligament attached to the base of the skull kept their heads steady as they ran.

Although tissues do not fossilize, traces of muscle and tendon attachment points on bones of early species revealed an extensive network of springy tendons along the back of their legs and feet, including a well-developed Achilles' tendon that anchors the calf muscles to the heel bone. Tendons served to store and release elastic energy during running, but were not needed for ordinary walking.

And there was the gluteus maximus, the muscle of the buttocks. Earlier human ancestors, like chimpanzees today, had pelvises that could support only a modest gluteus maximus, nothing like the strong buttocks of Homo.

"Have you ever looked at an ape?" Dr. Bramble said. "They have no buns."

Dr. Lieberman, a paleontologist, explained: "Your gluteus maximus stabilizes your trunk as you lean forward in a run. A run is like a controlled fall, and the buttocks help to control it."

The two scientists speculated in the article and in interviews that running by early human ancestors was more than simply a natural step, so to speak, beyond upright walking. These apelike ancestors, the scientists noted, were walking almost three million years before they became runners and began to assume more modern physiques.

In such ancestors as the Australopithecus genus, famous for the 3.2 million-year-old Lucy skeleton, the physical characteristics favoring running were either absent or underdeveloped.

Somehow, the scientists continued, those early ancestors who developed primitive running attributes must have improved their chances of survival and reproduction. They were not as swift in a sprint as their contemporary four-legged competitors for prey. But their ability to run greater distances than other predators must have been an advantage in making a kill or at least scavenging the kills of their swifter rivals.

But running came to early Homo at a cost: the loss of physical traits useful for climbing trees for refuge and food. By this time, other scientists have noted, climate change had diminished the woodlands of East Africa, leaving more open grasslands to run long distances over.

Dr. Philip Rightmire, a paleoanthropologist at the University of Binghamton in New York, said the Bramble-Lieberman hypothesis was "a pretty compelling picture of the importance of running in human evolution, and, over all, I'm very impressed."