One of the best-known ancestors of humans, the hominin known as Lucy, lived 3.18 million years ago, and there are over 300 known individuals from her species, Australopithecus afarensis. But Lucy’s fossil is one of the most complete ever found, and now, researchers have a convincing idea of how she may have died.

In a paper titled “Perimortem fractures in Lucy suggest mortality from fall out of tall tree,” published in the journal Nature on Monday, researchers said they analyzed Lucy’s fossil using CT scans and based on “fractures in multiple skeletal elements,” they concluded “that her cause of death was a vertical deceleration event or impact following a fall from considerable height.”

A. afarensis were early humans — they were bipeds and walked on land — but scientists still debate whether Lucy and her kind spent at least some of their time in trees as well. They were much smaller than modern humans — Lucy was about 3.5 feet tall and weighed close to 60 pounds — and so could have climbed trees to escape larger predators. While they used tools, their fingers were suited to climb trees, also suggesting they led a partly arboreal existence.

John Kappelman, an anthropologist at the University of Texas at Austin and the lead author of the paper, said in a statement: “It is ironic that the fossil at the center of a debate about the role of arborealism in human evolution likely died from injuries suffered from a fall out of a tree.”

Looking at the over 35,000 slices of CT scans the researchers created, Kappelman noticed unusual fractures in Lucy’s right humerus — the bone that runs from the shoulder to the elbow. He concluded: “This compressive fracture results when the hand hits the ground during a fall, impacting the elements of the shoulder against one another to create a unique signature on the humerus.”

LucyHominin The 3.2 million-year-old fossilized remains of 'Lucy,' the most complete example of the hominid Australopithecus afarensis, is displayed at the Houston Museum of Natural Science, Aug. 28, 2007. Photo: Dave Einsel/Getty Images

Consulting orthopedic surgeons confirmed his analysis about the humerus as well as other fractures in the skeleton. In the absence of any healing marks, he also surmised that they happened perimortem — close to the time of death.

But to account for a fall that caused all those fractures, Lucy had to have been at a significant height.

According to the statement, “in comparing her with chimpanzees, Kappelman suggested Lucy probably fell from a height of more than 40 feet, hitting the ground at more than 35 miles per hour. Based on the pattern of breaks, Kappelman hypothesized that she landed feet-first before bracing herself with her arms when falling forward, and ‘death followed swiftly’.”

The fossil was found about 40 years ago in the Afar region of Ethiopia, near the village of Hadar. The Ethiopian National Museum has put online a set of 3D files on Lucy’s skeleton for the public; they can be accessed on eLucy.org.