Lucy's feet were much like ours, according to analysis of an arched fossilized foot bone.
An arch in the foot would provide leverage to push off the ground at the start of a stride.
It also helps absorb shock when the foot meets ground again.
An arched fossilized foot bone found in Ethiopia shows that human ancestors walked upright 3.2 million years ago and were no longer tree dwellers, said a study Thursday in the journal Science.
The bone belongs to a cohort of the famed hominid Lucy, whose species Australopithecus afarensis roamed eastern Africa, and is the first evidence to address the question of how they got around.
"This fourth metatarsal is the only one known of A. afarensis and is a key piece of evidence for the early evolution of the uniquely human way of walking," said study co-author William Kimbel of Arizona State University.
An arch in the foot would provide leverage to push off the ground at the start of a stride and then help absorb shock when the foot meets ground again, suggesting Lucy's feet were much like ours.
Apes have flatter, more flexible feet with a big toe for grasping tree branches, attributes that do not appear in the A. afarensis.
"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 co-author Carol Ward of the University of Missouri.
An older species, Ardipithecus ramidus of Ethiopia, is the earliest kin of modern man of whom paleontologists have uncovered significant skeletal remains. It lived 4.4 million years ago but its more ape-like feet indicate it walked upright only some of the time.