For decades, anthropologists have considered upright walking, or bipedalism, a defining characteristic of the human lineage. Knuckle-walking was thought to have evolved uniquely in apes after humans had taken a separate evolutionary path.
But in an article in Thursday's issue of the journal Nature, researchers said they found fossil evidence that two species of early humans descended from knuckle-walkers.
"Instead of coming down out of the trees and walking upright, the ancestors of early upright walkers were already adapted to a life on the ground," said Brian G. Richmond, an anthropologist at George Washington University in Washington, D.C., and a co-author of the study.
In the two fossils studied, both of hominids that walked upright more than 3 million years ago, the researchers found structures in the wrist bones that once could have supported knuckle-walking by restricting movement of the wrist.
The researchers believe those wrist structures were traits left over from distant ancestors, much like the tail bone or appendix.
The study's findings also challenge theories of a special chimpanzee-gorilla relationship, which was based on the fact that they are the only primates to exhibit the unusual way of walking, Richmond said.
"We showed the ancestor that gave rise to upright walkers and humans was a knuckle-walker," he said. "That means it's shared between the ancestors of humans and chimpanzees and gorillas."
The newly analyzed fossils belong to Australpithecus anamensis and Australopithecus afarensis, known as Lucy. Both lived in Africa between 4.1 million and 3 million years agoat least a million years after the evolutionary split from apes.
Richmond and colleague David Strait, who compared existing fossils with the bones of today's apes, predict a hominid from 5 million years ago will show evidence of actual knuckle-walking.
"We didn't actually find fossils from this critical time period," Richmond said. "We found for the first time echoes of the earlier ancestor in the earliest human fossils that we currently have."
Today's chimps and gorillas as well as the early human fossils have a bony projection from their forearms that the wrist locked into, preventing it from moving back more than 30 degrees.
"The reason that chimpanzees and gorillas have this very stiff wrist is that if you're walking on your knuckles, you don't want your wrist to collapse from the weight of the body," Richmond said.
Other scientists praised the research, which appears to resolve a long-standing debate over genetic tests that show a close evolutionary relationship between chimps and humans even though it was not supported by fossils.
"We are more closely related to chimps than either of us ito gorillas, yet they share the same specialization that doesn't appear in our ancestry," said John Fleagle, a professor of anatomical science at State University of New York at Stony Brook. "The simple solution is that a common ancestor was a knuckle-walker, but the fossil record doesn't seem to jibe with that."
The new research shows that at least some early humans had that unique trait.
"If the anatomical analyses are shown to be correct, I think it's a hugely important breakthrough in our understanding of early bipedalism," said Craig Stanford, a University of Southern California biological anthropologist.