Back in 1974, anthropologists spotted what would become the world’s most famous fossil in a shallow bed of a stream in Ethiopia. The approximately 40-percent-complete skeleton of a 3-foot-6-inch female who lived and died some 3.18 million years ago was identified as a member of Australopithecus afarensis, the oldest hominid species. Over the more than 40 years since that groundbreaking discovery, scientists have drawn a blank as to how exactly “Lucy” might have died. Now, researchers from the University of Texas at Austin say an in-depth analysis of the partial skeleton using high-resolution computer-imaging technology led them to conclude that Lucy likely fell from a tree, sustaining multiple bone fractures and other injuries that killed her.

The discovery of Lucy’s partial skeleton represented a major breakthrough in the study of ancient human ancestors, enabling scientists to establish that early hominids like Australopithecus afarensis learned to walk upright before their brains grew larger. Though Lucy’s feet, knees and hips resembled those of modern humans, she had a small head, with a brain similar in size to that of a chimpanzee. Also like chimps, early hominids matured at an earlier age than modern humans: Lucy’s skeleton and teeth show that she had reached maturity even though she was only around 15 or 16 years old when she died. She measured 3 feet 6 inches tall and weighed 60 pounds.

The new study, published this week in the journal Nature, originated in the mid-2000s, when Lucy’s skeleton was on temporary loan from the National Museum of Ethiopia for a traveling exhibition in the United States. After a show at the Houston Museum of Natural Science, researchers at the University of Texas at Austin spent 10 days examining the famous fossil—which includes bones from the skull, upper limb, hand, axial skeleton, pelvis, lower limb and foot—using high-resolution, high-energy Computed Tomography (CT) scanning equipment.

Model of the early hominin Lucy. (Credit: Schöning / ullstein bild / Getty Images)
Model of the early hominin Lucy. (Credit: Schöning / ullstein bild / Getty Images)

According to John Kappelman, a professor of anthropology at UT Austin who is the lead author of the new study, analysis of the scans revealed puzzling fractures in the upper arm and shoulders, with no signs of healing—indicating that the injuries may have occurred around the time of Lucy’s death. Kappelman and his colleagues consulted with orthopedic surgeons, who confirmed that the break in Lucy’s upper right arm appeared to be a compressive fracture, which can occur when someone falls from a great height.

With this in mind, the researchers inspected the rest of Lucy’s skeleton—including virtual 3D models they made from the scans as well as the original bones in Ethiopia—looking for other fractures that could have been caused by a fall. Though they found numerous breaks that likely occurred after she died (as the bones aged and were buried by sand), they also found more compression fractures and other breaks that appear to have been caused by a fall.

In the new study’s version of events, Lucy died quickly but not painlessly. The researchers believe she fell from a height of some 40 feet, hitting the ground at a speed of more than 35 mph. Because the area in which Lucy’s skeleton was found was low-lying, with no cliffs nearby, they believe she must have fallen from a tree. From the pattern of breaks in Lucy’s ankles, knees, hips and shoulders, the scientists hypothesized that she hit the ground feet first. Both of her shoulders were fractured in a way that suggests she stretched out her arms to break her fall, indicating she was conscious at the time of her death. But, the study’s authors concluded, “death followed swiftly” after the fall, as she likely suffered extensive injuries to her internal organs in addition to the broken bones.

“By understanding her death is how [Lucy] came alive to me,” Kappelman told USA Today. “Lucy was no longer simply a box of bones but in death became a real individual: a small, broken body lying helpless at the bottom of a tree.” He and his colleagues also explored alternate hypotheses about Lucy’s cause of death, including a flood, a seizure, a lightning strike or attack by an animal, but none matched with the pattern of fractures they found.

A full-scale replica of Lucy's skull. (Credit: Craig Hartley/Bloomberg via Getty Images)
A full-scale replica of Lucy’s skull. (Credit: Craig Hartley/Bloomberg via Getty Images)

The new study supports the much-debated theory known as “arborealism,” according to which human ancestors lived partially in trees and partially on land. Given her small size, Lucy would have had to deal with predators like hyenas, saber-tooth cats and jackals, and the scientists believe she and her fellow early hominids took to the trees to protect themselves.

Other experts, however, say that the new study’s authors didn’t do enough to rule out other causes for the fractures in Lucy’s bones. These critics include the paleoanthropologist Donald C. Johanson, who led the team that discovered Lucy’s skeleton in 1974. Johanson told the New York Times that the fractures were far more likely to have occurred long after Lucy’s death, as her bones were exposed to the elements and buried by sand. By the time Lucy lived, he contends, human ancestors were no longer skilled tree climbers, but had evolved to find their food on the ground. “Australopithecus afarensis was essentially a terrestrial animal,” Johanson said.

Despite such criticism, there’s no denying the intrigue surrounding a possible solution to such a famous cold case after more than 3 million years. Fortunately, armchair anthropologists will soon be able to judge for themselves, as the Ethiopian government has allowed Kappelman and his colleagues to publish their data online at eLucy.org.