3D reconstruction of lower limb muscles of Australopithecus afarensis fossil AL 288-1, aka "Lucy." Credit: Ashleigh Wiseman

One of the most famous fossils in human evolutionary history is known as "Lucy," who belonged to an extinct species called Australopithecus afarensis--an early relative of Homo sapiens who was among the first hominins to walk upright. But scientists have long debated the extent of her bipedalism. Now a 3D digital re-creation of Lucy's muscular anatomy, combined with computer simulations, has reaffirmed that she was quite capable of walking fully erect. The results appeared in a new paper published in the journal Royal Society Open Science.

"Lucy's ability to walk upright can only be known by reconstructing the path and space that a muscle occupies within the body," said author Ashleigh Wiseman, an archaeologist at Cambridge University. "We are now the only animal that can stand upright with straight knees. Lucy's muscles suggest that she was as proficient at bipedalism as we are, while possibly also being at home in the trees."

Lucy's remains were found in 1974 in Ethiopia at a site called Hadar. Several paleoarchaeologists--including Donald Johanson, Mary Leakey, and Yves Coppens--began surveying the site for signs of fossils relating to the origin of humans. The first interesting find occurred in November 1971, when Johanson found a fossilized upper shinbone and, nearby, the lower end of a femur. Now known as AL 129-1 and dating back more than 3 million years, the angle of the knee joint indicated this was a hominin (now known as Australopithecus afarensis) capable of walking upright.

Once all the pieces were assembled, scientists were able to reconstruct Lucy, revealing that she stood about 1.1 meters (3 feet, 7 inches) tall and weighed about 29 kilograms (64 pounds). Her brain was small, like a chimpanzee's, but her pelvis and leg bones (including a valgus knee) looked almost identical to modern humans, indicating that Australopithecus afarensis was fully bipedal, i.e., stood upright and walked erect.

How Lucy died is a matter of heated scientific debate. A controversial 2016 paper suggested that a careful analysis of her bones reveals how she died--by falling to her death from a very tall tree--although other scientists (including Johanson) thought the evidence was thin at best. As we reported at the time, University of Texas-Austin anthropologist John Kappelman and his team did a complete X-ray CT scan on Lucy's bones, allowing them to create high-resolution 3D renders and 3D printouts of her skeleton.