Published online 10 February 2011 | Nature | doi:10.1038/news.2011.85

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These bones were made for walking

Human-like foot arches strengthen argument that Australopithecus 'Lucy' was not a climber.

foot bone connected to the...Bone that resolved contention: a fourth metatarsal from Australopithecus afarensis.Image courtesy of Carol Ward and Elizabeth Harman (deceased)

One of the earliest human ancestors, Australopithecus afarensis, the most famous skeleton of which is commonly known as 'Lucy', seems to have had human-like foot arches that would have allowed it to walk around effectively on two legs.

The finding, published today in Science1, centres on the discovery in Hadar, Ethiopia, of a 3.2 million-year-old fourth metatarsal bone from an A. afarensis. The analysis was done by a team led by Carol Ward, a palaeontologist at the University of Missouri in Columbia.

The fourth metatarsal, a small bone that makes up the inner part of the fourth toe, is useful to palaeontologists because of the way that it differs in shape between tree climbers and land walkers. Ward's team found that Lucy's metatarsal was more like that of a modern human than a chimpanzee.

"This paper presents the most convincing skeletal evidence yet that A. afarensis had well-developed, modern human-like, arches," says Jeremy DeSilva, a functional morphologist at Boston University in Massachusetts.

For the tree-dwelling chimpanzee, the fourth metatarsal lies flatter against the ground, and the middle of the foot is mobile. This flat-footed structure grants chimpanzees tremendous flexibility and allows them to grasp branches in trees. Human feet are very different. The fourth metatarsal is twisted along its long axis and sits at an angle to the ground, because the bones of the foot form an arch from front to back and side to side. The presence of these arches robs humans of the ability to grasp with their feet but makes the foot rigid enough to function as a solid lever as it pushes off the ground, and to absorb shocks during walking and running.

An arch look

It has long been questioned whether A. afarensis had a flexible foot like a chimpanzee or a more human-like arched one. Because fossilized foot bones are rare and fourth metatarsals of A. afarensis have never before been found, many had speculated that the species had feet that were something of a compromise between those of chimpanzees and humans. Now Ward and her colleagues are putting this matter to rest.

As in the human foot, the ends of the A. afarensis fourth metatarsal are angled and twisted relative to one another, reflecting the presence of stiff arches. This strongly suggests that the early hominin had the ability to walk and run in much the same way as humans.

The discovery shows that A. afarensis was not dividing its time between trees and open land. "I'm sure they went into trees sometimes, but they would not have been able to do this much better than you or I could," says Ward.

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"Based upon this discovery I think if you saw Lucy in clothing walking across a soccer field you would think she was a child, not an australopithecine," says Bruce Latimer, a palaeoanthropologist at Case Western Reserve University in Cleveland, Ohio.

The finding could also resolve a long debate surrounding some of the most famous hominin footprints ever discovered. At Laetoli, Tanzania, some hominins walked across a bed of wet volcanic ash 3.6 million years ago. "When I saw those footprints being excavated, I thought, gosh, you'd lose these on a modern day beach, they have an arch and a totally human gait," recalls Latimer. However, the movements were so close to human that many palaeontologists doubted they could have possibly belonged to the ancient A. afarensis.

"This work certainly puts a nail in the coffin of that argument," says Latimer. 

  • References

    1. Ward, C. V., Kimbel, W. H. & Johanson, D. C. Science 331, 750-753 (2011). | Article | ChemPort |
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