The tell-tale foot bone of A. afarensis is shown with a human foot to indicate where it would be positioned. It shows that Lucy's kin had arches stiffening their feet like modern people, as opposed to apes whose feet are more flexible for grasping tree branches. (Courtesy of Carol Ward)

The famous fossil, Lucy, a human ancestor who lived 3.2 million years ago, was more comfortable on the ground than in the the trees, a foot bone from her kin reveals.

Carol Ward of the University of Missouri and colleagues report in Friday's edition of the journal Science that ancient hominins of the species Australopithecus afarensis, including Lucy, had feet similar to modern humans.

Lucy, a partial fossil skeleton found in Ethiopia in 1974, was the first of her species ever discovered. Her other bones showed she was able to stand upright. But no foot bones were found with her skeleton, so researchers have puzzled over whether she walked like modern people or was a blend of ground-and tree-dweller.

An artist's life-sized model of Lucy, based on the 3.2 million year old Australopithecus afarensis skeleton was part of an exhibit at the Houston Museum of Natural Science in Houston that opened in 2007. (Michael Stravato/Associated Press)

The new discovery shows these relatives "were fully humanlike and committed to life on the ground," Ward said in a telephone interview from Africa. "It lays to rest the idea that they were a compromise."

The new bone, discovered with other A. afarensis bones at Hadar, Ethiopia, is a metatarsal, one of the long bones connecting the toes to the base of the foot.

It shows that Lucy's kin had arches stiffening their feet like modern people, as opposed to apes whose feet are more flexible for grasping tree branches.

'Not shuffling'

"This shows our early ancestor walked like we would walk. They were not shuffling, they were walking upright, which is a key feature of our branch of the family tree," Ward said.

The fact that the species walked upright affects where they lived, what they ate and how they avoided predators, added Ward, a professor of integrative anatomy.

"The development of arched feet was a fundamental shift toward the human condition, because it meant giving up the ability to use the big toe for grasping branches, signalling that our ancestors had finally abandoned life in the trees in favour of life on the ground."

Richard Potts, director of the human origins program at the Smithsonian's National Museum of Natural History, called the report "an impressive paper for just one bone."

"Every once in a while you do get one piece of the puzzle that helps you fill in something. This bone really fills in a missing piece," said Potts, who was not part of the research team.

"Where this article is a game-changer, to me, is that it correctly notes that there has been this discussion about whether Lucy's species had compromised two-legged walking. This shows that it wasn't compromised walking," said Potts.

That doesn't mean A. afarensis did not climb trees, he added. It was probably a very adaptable creature, using trees when they were available but being quite comfortable on the ground.

A. afarensis still retained the well-muscled arms that would have been useful in trees, Potts noted.