November 18, 2004

By JOHN NOBLE WILFORD, NY Times

Running Extra Mile Sets Humans Apart in Primates' World

If walking upright first set early human ancestors apart from their ape cousins, it may have been their eventual ability to run long distances with a springing step over the African savanna that influenced the transition to today's human body form, two researchers are reporting today.

The evolution of physiques for distance running 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.

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 let human ancestors hunt and scavenge over great distances. That was probably decisive in the pursuit of high-protein food for development of large brains.

The apparently crucial role of running in human evolution has been largely overlooked in previous research. But today, the two scientists, Dr. Dennis M. Bramble of the University of Utah and Dr. Daniel E. Lieberman of Harvard, report in the journal Nature that their analysis of the fossil record found striking anatomical evidence for the capability of prolonged running in the Homo genus, beginning about 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 conclude 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."

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 about 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, notably Homo erectus, had long, slender legs for greater strides. They had shorter arms and a narrower rib cage 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 the legs and feet, including a well-developed Achilles tendon that anchored 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 scientists compiled a list of 26 traits connected with running that early Homo specimens exhibited. It was a result of 13 years of research that started with watching pigs running on a treadmill.

"Dennis and I noticed how the pigs can't hold their heads still while running," Dr. Lieberman recalled. "Any good human runner keeps his head still because of the nuchal ligament, a tendon in the back of the neck."

The scientists learned that all accomplished running animals, modern or fossil, had a mark in the skull where the nuchal ligament had been. They found it in early Homo specimens, but not in Australopithecus, the genus that lived more than three million years ago and included the 3.2-million-year-old Lucy skeleton. "That was an epiphany for us," Dr. Lieberman said.