(CNN) When Lucy, the world's most well-known fossil, was discovered sticking out of a shallow Ethiopian stream bed in 1974, she provided new insight about life for early human ancestors 3.18 million years ago. The image of her skeleton -- which is estimated to be 40% complete and considered the best representation of her species, Australopithecus afarensis -- became iconic.

But how did she come to rest in that shallow stream? Lucy just might be considered one of the world's oldest cold cases.

Forty-two years after the discovery, researchers from the University of Texas at Austin now believe that she was found in proximity to where she died, and that she fell from a great height to her death.

"When we think about any of the fossils that we work on, we know in every instance that that came from an individual who was born, they grew up, and then they died," said John W. Kappelman, a professor of anthropology and lead author of a new study on Lucy. "But it's rarely the case that the skeleton actually preserves evidence of how an individual died. What we're proposing here is the first hypothesis that's out there, and we've had her for 42 years now, about how she died. I am not aware that anyone else has ever [done that]."

The original fossil and CT scans of fractures to her skeleton paint a more vivid portrait of what happened in Lucy's final moments, and although it was probably swift, it wasn't without pain. The new study that proposes the hypothesis reads like a coroner's report, making Lucy's stone-like bones seem more lifelike than ever.

What happened to Lucy?

Lucy was small, about 3½ feet tall and 60 pounds. Analysis of her skeleton and teeth shows she had reached maturity, but not unlike chimpanzees, her species matured young. Kappelman estimates she was 15 or 16 years old.

Given her size, predators such as hyenas, jackals and saber-toothed cats would have posed a threat to Lucy. So Lucy most likely turned to the trees, Kappelman said. It's possible she scaled them only from time to time for safety or that she nested in them every night. Based on data on the nesting habits of chimps, an average of 46 feet above the ground makes them feel safe.

She stood up straight, with feet, knees and hips that are similar to ours. If you saw her walking from afar, you would think Lucy was human by her silhouette. But up close, she had a small head, a brain comparable in size to a chimpanzee's, longer arms and hair covering her body.

Bridging the gap between humans and chimps, Lucy had slightly curved fingers and toes, with mobile ankles and shoulders that provided more overhead range of movement. Even with those abilities, she would have been better at walking than climbing.

Perhaps Lucy was spooked by a predator, or perhaps she was asleep or settling in for the night. Maybe she spied some fruit and wanted to forage. Either way, this is what Kappelman believes happened next.

A sculptor's rendering of the hominid Australopithecus afarensis.

From 46 feet in the air, Lucy fell out of her tree, fully conscious. She fell toward the ground rapidly at 35 mph and hit feet-first, sending an impact punching through her body that created fractures in her ankles, knees, hip and shoulder. Internal organs were probably punctured by this "hydraulic ram effect." Lucy pitched forward and instinctively put out her arms to break the fall, creating fractures in the bone there as well. It would probably be her final conscious act.

She twisted to her right, landing primarily on that side. That twist fractured her neck and tilted her head. Unconscious, broken and bleeding, she lay on the stream bed. If there was water present at the time, it gently moved the body a short distance along, naturally carrying her to a final resting place since members of her own species didn't.

A diagram of Lucy's fall from a tree, showing the overlay of her skeleton.

Writing a death certificate for a fossil

Lucy's skeleton is represented by elements of her skull, upper limb, hand, axial skeleton, pelvis, lower limb and foot. Previously, researchers believed any damage or fractures within her bones simply happened over time as she became fossilized. The discovery of Lucy's skeleton has often been heralded as something of a miracle because as Kappelman points out, the geologic record isn't kind to bones over the years and it's difficult for them to last long enough to reach us.

But not all the fractures happened after death, Kappelman said. Using their High-Resolution X-ray Computed Tomography Facility, the researchers scanned her entire skeleton to look deeper within the bone. Previous CT scans were more medical, with less resolution, and weren't able to penetrate the rocky nature of the skeleton.

There are clean breaks, and there are also intact tiny bone slivers. Had those occurred once the bone was dry and exposed to the elements, they would have broken off and dispersed.

Lucy's humerus fractures also bear the unique signature that orthopedic surgeons associate with falling injuries in the elderly or those who fall from great heights. In both cases, the patients instinctively stretch out their arms to break the fall. Kappelman consulted with nine orthopedic surgeons, and they all agreed on the fracture signature.

Overall, the fractures and bone-to-bone impact associated with her death prove to be severe, and Lucy had to have fallen from a great height to receive such trauma. This also indicates that while Lucy had the ability to reach great heights, she may not have been an adept climber simply due to her features.

"The point we argue is that it may well be the evolution of these traits for bipedalism [walking upright] that compromised her ability to climb as safely and efficiently in the trees, and that may have meant that her species was more subject to a higher frequency of falls," Kappelman said. Researchers have found other fossils of her species with similar fractures, suggesting more falls.

As a professor of anthropology, Kappelman also received training in human anatomy and went through medical and surgical rounds. He and his team regarded falling as one hypothesis and researched other causes, such as a seizure, a flood, a lightning strike or even a violent animal. But none of these matched as well with the fractures.

"We've done what we thought we could do in evaluating alternative hypotheses," he said. "This is most consistent and common with the impact from a fall. This hypothesis is more or less being tested every day in thousands of ERs everywhere when people have these injuries."

John Kappelman with 3D printouts of Lucy's skeleton.

The Ethiopian government has also granted permission for 3-D files of the bone scans to be released to the public on Monday, so anyone can download them, look at the data and form their own hypothesis of what happened to Lucy.

After 30 years of working with fossils, Kappelman was eager to scan Lucy's iconic bones, but he wasn't prepared for how he would feel after discerning her cause of death when he looked over her skeleton laid out on his desk. Falling is easy to imagine for anyone, and he could picture the progression of her injuries as she plummeted to her death and lay at the bottom of the tree.

"It really hit me for the first time," he said. "Here she was in her death, reaching her arms out to try instinctively just to break her fall. It was at that moment that these broken bits of bone that I've taught for 30 years became a living individual. And it gave me this feeling of empathy, because I could identify with this.

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"It's like putting yourself there at someone's death and being able to picture that, almost as if understanding that drops us into a time machine and we fly back through 3 million years so we're there observing how this little individual died. It was in understanding her death that she became alive for me."