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Foot bone shows humans walked upright 3.2 million years ago

A fossilized foot bone from Ethiopia indicates that human ancestors had largely abandoned swinging from trees by 3.2 million years ago and were spending virtually all of their time walking upright, researchers said this week.

The metatarsal bone from Australopithecus afarensis — the species made famous by the 1974 discovery of the specimen known as Lucy — clearly shows that the species had stiff, arched feet just like modern humans.

Such feet are stiff enough to push off from the ground when taking a step and flexible enough to absorb shock when the foot touches down. But they have lost the ability to grasp branches and other objects — a distinguishing characteristic of Lucy’s predecessors, Ardipithecus ramidus.

Although fairly complete skeletons of A. afarensis had been obtained from the so-called First Family Site in Hadar in eastern Africa, researchers had not previously obtained a complete foot and thus could only speculate about Lucy’s method of locomotion.

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The new knowledge that she had arches in her feet affects much of what we know about the 3 1/2-foot-tall creatures, including where they lived, what they ate and how they avoided predators, said anatomist Carol V. Ward of the University of Missouri School of Medicine, one of the authors of the report in the journal Science. Her coauthors were William H. Kimbel of Arizona State University, who discovered the bone, and Donald C. Johanson of ASU, who led the team that discovered Lucy.

The development of an arched foot represented a fundamental shift toward the human condition, Ward said, because it meant giving up the ability to use the big toe to grasp branches, signaling that our ancestors had finally abandoned life in the trees for life on the ground. Lucy and her kin could leave the forest and roam the countryside to forage for food when necessary. With their powerful jaws, they could eat a variety of foods, including fruit, seeds, nuts and roots.

Researchers had previously believed that the first member of the human family tree to walk upright was Homo erectus, which flourished between 1.8 million and 70,000 years ago before giving way to modern humans. But the new discovery indicates that the transition from trees to grasslands actually took place one to 2 million years earlier.

The discovery is not a surprise to everyone.

A decade ago, anthropologist Bruce Latimer of Case Western Reserve University in Cleveland discovered fossilized footprints of three A. afarensis hominids who had walked over ash from a volcano in the Serengeti that had recently erupted. The 3.5-million-year-old footprints looked just like modern footprints, he said, indicating that the hominids had arched feet. But others did not consider that sufficient evidence.

Latimer, who specializes in studying walking, said that a major advantage of standing upright is that it allows us to carry things, including food, weapons and especially children.

The disadvantage is that two-legged creatures can be very slow. An injury to one leg “makes us leopard food,” he said.

thomas.maugh@latimes.com


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