At some point in the past five million years or so, our human ancestors traded in an arboreal existence for a dedicated two-legged life on the ground. A patchy fossil record, however, has frustrated researchers hoping to pinpoint the emergence of more modern human upright walking

Even the gait of the well-studied "Lucy" species, Australopithecus afarensis, has been the subject of some debate. Most of her 3.2-million-year-old skeleton suggests that Lucy was a fairly obligate biped, but without crucial foot bones, researchers could not confidently reconstruct the hominin's stride.

Now, a newly described fossil foot bone might help solidify the case for A. afarensis's bipedal abilities. The fourth metatarsal, a pivotal bone for distinguishing human feet from ape feet, is the first of its kind to be described for the species. 

Uncovered in 2000 in Hadar, Ethiopia, the fossil is "nearly perfectly preserved," according to the researchers behind the new study, published online February 10 in Science. And the slim bone, less than 60 millimeters long, suggests the species had stiff arched feet—much more like those of Homo habilis and H. sapiens than like those of African apes or earlier hominins—better adapted to hoofing it on the ground than seeking refuge in the trees.

"Arches in the feet are a key component of human-like walking because they absorb shock and also provide a stiff platform so that we can push off from our feet and move forward," Carol Ward, of the pathology and anatomical sciences department at the University of Missouri, Columbia, and lead author of the new paper, said in a prepared statement. "Arches in our feet were just as important for our ancestors as they are for us."

Such a rigid foot stands in contrast to that of Ardipithecus ramidus, the 4.4-million-year-old hominin described in 2009. Although scientists are still debating how often "Ardi" would likely have walked upright, some of the specimens' foot bones—including a large grasping great toe—point to a highly capable climber with flatter, more flexible feet.

The new analysis also supports the theory that A. afarensis was indeed the primate to have left the 3.6-million-year-old two-legged tracks in Laetoli, Tanzania.

"Now that we know Lucy and her relatives had arches in their feet, this affects much of what we know about them, from where they lived to what they ate and how they avoided predators," Ward said. "If we can understand what we were designed to do and the natural selection that shaped the human skeleton, we can gain insight into how our skeletons work today."

Image of foot placement courtesy of Kimberly A. Congdon/Carol Ward/Elizabeth Harman; image of side views of fossil courtesy of Science/AAAS