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Afarensis, Upright: A breakthrough discovery sheds light on human origins.

when superstar anthropologists Tim White and Donald Johanson discovered partial skeleton AL 288-1 in Ethiopia’s Afar Desert back in 1974, a spirited debate broke out among scientists who studied human origins. Was the 3.2 million-year-old specimen, soon to be known to the world as “Lucy,” a curious but otherwise unimportant evolutionary dead-end? Or was she instead a new species of Australopithecus, and thus a newly discovered human ancestor?

White and Johanson argued for the latter interpretation, labeling Lucy as Australopithecus afarensis. Eventually their colleagues agreed. But new questions and controversies emerged. White and Johanson’s forensic investigations indicated that Lucy walked upright. But did this bipedalism mean she spent most of her day walking tall across the African savannah, or might she have spent part of her time gamboling about in the jungle?

The answer is of more than just esoteric interest. Walking upright on the plains would suggest Lucy represents a milestone in Homo sapiens’ transition from animal to human. Ultimately, by piecing together Lucy’s physiology and lifestyle, scientists hoped to better understand how, over many thousands of years, she, in effect, became us.

Since the 1970s, subsequent investigations have won the majority of anthropologists over to the idea that Lucy and her peers had indeed liberated themselves from arboreal ambling. But a lack of clear fossil evidence has vexed efforts to declare the issue settled.

No longer. An examination of a recently discovered A. afarensis foot bone has determined that Lucy and her relations had arched feet, a trait that Carol Ward, the MU professor of pathology and anatomical sciences who led the investigation, says demonstrates conclusively that A. afarensis spent most of its time upright. It was a mode of movement, she says, that represented nothing short of “a fundamental shift toward the human condition.”

Arched feet, Ward continues, “meant giving up the ability to use the big toe for grasping branches, signaling that our ancestors had finally abandoned life in the trees in favor of life on the ground.”

Ward collaborated on the project with Johanson, professor and founding director of the Institute of Human Origins at Arizona State University, and Professor William Kimbel, the institute’s current director. She based her analysis on a rare fossilized metatarsal bone from a specimen discovered near Hadar, Ethiopia. Their results were published in the February 11 edition of the journal Science.

Arched feet, the scientists say, would have allowed Lucy to leave the forest to forage on the open ground for fruit, seeds, nuts and roots — all edible thanks to powerful jaws. The upright posture and strong jaws represent a clean evolutionary break from earlier, less-humanlike species such as Ardipithecus ramidus. These creatures, like modern apes, alternated between moving on all four feet and upright, and probably spent much of their time in the trees.

Ward says filling in gaps in our evolutionary history is not the only benefit Lucy’s bones may confer.

“Understanding that the arch appeared very early in our evolution shows that the unique structure of our feet is fundamental to human locomotion,” Ward says. “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.”

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Afarensis, Upright. Illustration by Bob Dob

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