Researchers say they have found a fossilized foot bone that proves human ancestors were walking on two feet 3.2 million years ago.
The fossil of the midfoot bone (fourth metatarsal) shows that Australopithecus afarensis had a permanently arched foot and walked essentially the same as modern humans, according to researcher William Kimbel, of the Institute of Human Origins at Arizona State University, and colleagues.
The authors of the report said their finding resolves debate between experts who believe A. afarensis was bipedal and those who think the species used a form of locomotion that was a stepping stone between the four-limbed (quadrupedal) tree-climbing of chimpanzees and full bipedalism, BusinessWeek reports.
The fossil's size and shape allowed scientists to determine that the foot it had belonged to was stiff and had a well-defined arch-two features that help modern humans spring forward and that cushion the shocks of bipedal walking.
Scientists had already known, from pelvis fossils and other remains, that A. afarensis could walk on two legs and no longer had the apelike "foot thumbs" used by other human ancestral species for grasping and climbing.
Before the discovery of the new A. afarensis metatarsal, though, it had been hard to say for sure whether Lucy and her kin had left the trees for good, National Geographic informs.