How did Lucy walk? Although the famous 3.2-million-year-old skeleton shows that she was undoubtedly an upright walker, our incomplete knowledge of her feet has fed a long-running debate about the mechanics of her stride. Now, thanks to the discovery of a single bone, scientists have found important similarities between Lucy’s feet and our own.

Reported Feb. 10 in the journal Science by University of Missouri anthropologist Carol Ward and co-authors William Kimbel and Donald Johanson of Arizona State University, the single foot bone was discovered in 2000 and comes from Hadar, Ethiopia. Of the same age as Lucy herself, this site contains the remains of many Australopithecus afarensis individuals who died under mysterious circumstances.

The bone is a fourth metatarsal; one of the bones of the mid-foot, just behind the toes. In modern humans, metatarsals create the distinctive arch of the foot and act as a shock absorber while keeping the foot stiff. Earlier research has shown, however, that the degree of the arch has varied during human history.

Like the same bone in our foot, but unlike that seen in chimpanzees or gorillas, the fourth metatarsal of this A. afarensis individual angled down to contact the ground, creating a strong, weight-bearing arch along the outside edge of the foot. Also telling were two facets on the bone’s end where it connected with the rest of the foot, trading the flexibility seen in ape feet for strength and stability.

This discovery may also resolve a long-running debate over the identity of prehistoric humans who left 3.6-million-year-old tracks in an ash bed found at Laetoli, Tanzania. The only human fossils found that locality are A. afarensis, but paleoanthropologists have disagreed about whether Lucy’s relatives truly could have left the tracks due to supposed differences in skeletal anatomy. As Ward and colleagues argue, the confirmation of an arched foot in A. afarensis removes this objection, meaning they were probably the true track makers, after all.

Despite the newfound correspondences between the A. afarensis foot and our own, their feet were significantly different from ours. According to American Museum of Natural History paleoanthropologist William Harcourt-Smith, who was not involved in the Science study, “If you look at the A. afarensis foot as a whole, it has a combination of human-like and ape-like features.” Much like the rest of their skeletons, the anatomy of these hominids’ feet is a mosaic of features seen in earlier and later humans. Some features of our own skeletons – such as the arch on the inner side of our foot – are not seen in A. afarensis.

The description of the metatarsal bone will inform paleoanthropologists about the way Lucy moved, but just how she walked and how that fits into the bigger picture of human evolution requires further study. “A. afarensis was a biped,” says Harcourt-Smith, “but not one like us.”

Image: The A. afarensis fourth metatarsal, and its analogue in the bones of a human foot. (Kimberly Congdon, Carol Ward and Elizabeth Harman)

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Citation: “Complete Fourth Metatarsal and Arches in the Foot of Australopithecus afarensis.” By C.V. Ward, W.H. Kimbel and D.C. Johanson. Science, Vol. 331 No. 6018, Feb. 11, 2011.