Researcher studies how we learned to walk like man

Researcher studies how we learned to walk like man
By William Mullen
Chicago Tribune staff reporter
Published February 28, 2006

Dan Gebo believes that if we could go back millions of years and see hominids, the early, small-brained humans from which modern humans inherited the ability to move around on two legs, we would see some pretty peculiar styles of walking.

The hominid nicknamed "Lucy," a 3 1/2-foot-tall adult female who lived in what is now Ethiopia 3.2 million years ago, tired easily and would have had to run at a dead trot to keep up with a strolling modern human. And Australopithecus robustus, a toothy, dim, broad-faced early human that went extinct 1.4 million years ago, was hopelessly knock-kneed.

But these walks, however imperfect, were the most important steps ever taken for humankind, said Gebo, a Northern Illinois University anthropologist and co-author of a new study of the biomechanics of early human locomotion.

"The origin of upright walking--bipedalism--is perhaps the most singular evolutionary event in human history," said Gebo. "It clearly separates us from our African ape ancestry."

Gebo and Arizona State University anthropologist Gary Schwartz make a case in an upcoming issue of the American Journal of Physical Anthropology that runs counter to prevailing theories of hominid locomotion.

Current thinking is that the first hominids to use two legs had feet similar to those of modern humans and therefore walked in the same way. Gebo and Schwartz say hominids' foot anatomy evolved in subtle but significant ways over millions of years before they walked like modern humans.

The duo came to their conclusions, which some of their colleagues do not entirely accept, after reassessing the hominid fossil record, comparing them to modern humans and apes.

"I study how animals move," said Gebo. "I want to know the exact sequence of changes leading to bipedalism. What happens first? Do we lose the graspable big toe of apes, or do we get long legs or a different pelvic structure first?"

Humans and the modern great apes that still primarily walk on all fours--chimpanzees, bonobos, gorillas and orangutans--are thought to have an unknown, hominid common ancestor.

The emergence of bipedalism in some of its descendants was a profound change because walking on two feet freed up the hands for other uses.

Early humans had small, chimp-sized brains. Being able to use the hands for making and using tools may have triggered growth of the brain and its cognitive powers, giving rise to language, reasoning, music and all the distinctive traits of modern humans, who didn't show up until about 130,000 years ago.

Gebo and Schwartz's findings in the journal also challenge the belief that various early hominid species, many of which lived at the same time and in proximity to one another, developed bipedalism independently.

"We believe the origins of bipedalism arose once," said Schwartz. In other words, they think all bipedal hominid species descended from that first one, including modern humans, the only surviving hominids.

Early hominids did walk somewhat differently, however, said Schwartz, "perhaps adapting to unique lifestyles or environmental conditions."

Reaction to Gebo and Schwartz's research in paleontological and anthropological circles has been mixed. University of Chicago evolutionary biologist Russell Tuttle, who had not seen their research, said he agrees that "it would be very likely that a lot of variations in walking would show up" in early bipedal hominids.

Others, like anthropologists Carol Ward at the University of Missouri and Bruce Latimer at the Cleveland Museum of Natural History, said the study was based on too few fossil samples for Gebo and Schwartz to draw their conclusions. "I am not convinced that there is really a different mode of walking based on these hominid examples they are using," said Ward.

Gebo, however, said he "looked up all the fossil foot bones that I could find" for the study.

"They don't all look alike," Gebo said of the hominid foot and ankle bones he looked at. "There are some very different shapes, with implications of how these fossils walked."

Australopithecus afarensis, a fossil nicknamed Lucy when 40 percent of her skeleton was found intact in Ethiopia in 1974, in life was a wimp compared to humans in terms of standing for long periods or walking long distances, said Gebo.

She had the same bones in her feet as modern humans. The difference is that many of the human foot bones are fused, but not Lucy's. She had to rely on muscle to stabilize the bones for efficient walking and standing, causing her fatigue humans don't experience.

"Lucy had a sort of kiddie-stride, because her legs were so short," said Gebo. "It's like comparing the horse versus the pig stride. Pigs can move just as fast, but they have to move their legs much faster. Horses are great long-distance runners, but pigs are not."

Australopithecus robustus lived later--2.5 million years ago--in Africa's Rift Valley. The biomechanics of its walk would have been similar to those in modern humans, Gebo said.

"We walk with a heel strike, then the weight of our body moves over the ankle," he said. "For a period of time with each step we are actually balancing on one leg."

But there was one noticeable difference, he said. Australopithecus robustus "had these very short ankles," with an inward curvature of their lower legs, so that "their knees wobbled inward when walking. They wouldn't have made it as Olympic runners."

If their findings stand up, the information will help answer questions of how early humans stood and walked on two feet, but not the reason why, a matter of much speculation.

Until 14 million years ago, Africa was largely low and flat, covered almost completely by humid rain forests populated in part by tree-dwelling, four-footed, ape-like primate ancestors.

Then the forests began to recede, replaced by grassy savannas. "There may have been competition with other apes over resources in the shrinking forest," said Gebo.

But being two-legged meant early hominids, who had brains about the size of chimps' brains, were less adept at living in trees and slower on the ground than when they had four legs, making them more vulnerable.

"The reason we stood up is the million-dollar question in anthropology," said Gebo.

In the end, two-footedness did help the hominids adapt as they moved into the savannas.