Lucy's upper-arm bones were shattered by the impact of the fall some 3.2 million years ago - a type of trauma also common in car crash victims, researchers from the United States and Ethiopia wrote in the journal Nature.
Her injuries suggested "she stretched out her arms at the moment of impact in an attempt to break her fall," said study co-author John Kappelman of the University of Texas at Austin.
Until now, there has been no official theory on how Lucy, whose bones were unearthed in Ethiopia in 1974, met her demise. Previous studies had suggested the bone breaks happened after death.
The new study, based on high-resolution 3D scans, said the fractures were rather consistent with a traumatic impact such as a fall from "considerable" height, said the team.
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They showed that Lucy had also suffered a broken ankle, knee, pelvis and at least one fractured rib - suggesting she must have suffered severe internal organ damage, the researchers concluded.
"When I better understood the potential cause of her death, I could picture her broken body lying there at the foot of the tree. I could empathise with her."
The team had performed 10 days of computed tomography (CT) scans on Lucy, one of the most complete hominin fossil skeletons ever unearthed.
Lucy was an Australopithecus afarensis that died in Ethiopia - an extinct member of the hominin family which includes modern humans and all our ancestors.
While Lucy had an ape-like skull, jaws and teeth, as well as long, dangling arms, she walked upright like us.
There is ongoing debate as to whether she was a direct human ancestor - the "Mother of Mankind" - or a relative further removed.
Today's findings added evidence to the theory that Lucy and her ilk spent at least some of their time in trees.
The analysis is an important contribution to the scientific tracking of our forebears' evolutionary journey from tree-dwelling foragers to tool-wielding shapers of nature.