"She stretched out her arms at the moment of impact in an attempt to break her fall."
Lucy, an ancient ape-like human relative, met a brutal end when she plummeted from a tall tree, new analysis of the famous fossil suggested Monday -- offering a solution to a decades-old mystery.
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.
"That tells us that Lucy was conscious at the point of impact, and that instant in time right before her death," he told AFP.
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.
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.
"For me, understanding her death brought her to life for me for the first time," Kappelman said of arguably the world's best-known hominin.
"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.
The bones, discovered in 1974, make up nearly 40 percent of a full skeleton and filled a major gap in the human evolutionary tree.
Lucy in the sky
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.
Monday'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.
The team found that Lucy, standing a mere three feet, six inches (just over a metre) tall, must have fallen from a height of more than 40 feet to suffer such horrific injuries.
She would have hit the ground at more than 35 miles (56 kilometres) per hour.
It follows that Lucy must have climbed trees, the team said, possibly seeking refuge in the forest canopy at night.
But this versatility may also have precipitated her demise.
Physical features adapted for walking upright "may have compromised her ability to climb trees, predisposing her species to more frequent falls," said the researchers.
From her injuries, the team concluded that she crashed to the ground feet-first, then braced with her arms.
Death "followed swiftly".