A prehistoric foot bone found in Ethiopia shows that human ancestors took a giant step forward in their evolution as much as 3.2 million years ago, U.S. researchers say.

The fossil is a fourth metatarsal, one of the long bones in the foot that connect to the toes, according to a report in the journal Science. The bone, found 11 years ago, indicates the foot had an arch, providing evidence that the species walked on two feet, instead of swinging from the trees like chimpanzees. Apes don’t have permanent arches on their feet, which gives the feet more flexibility than humans’ and makes it easier to climb tree trunks.

The foot bone is from a member of the species A. afarensis, whose most famous representative is “Lucy,” a 3.2 million- year-old female whose partial skeleton was found in Ethiopia in 1974. The oldest known human ancestor, a 4.4 million-year-old member of Ardipithecus ramidus, had feet that were like those of primates who lived in trees.

“Understanding that the foot arches appeared very early in our evolution shows that the unique structure of our feet is fundamental to human locomotion,” said Carol Ward, an author of the report, who’s an anatomist at the University of Missouri in Columbia, in a statement. “Arches in our feet were just as important for our ancestors as they are for us.”

To contact the reporter on this story: Elizabeth Lopatto in New York at elopatto@bloomberg.net.