Her last living act is a familiar one: She reaches out with her hands to break her fall before hitting the unforgiving earth. Bones shattered, she dies alone.
Some researchers say they now know how Lucy died: by falling out of a tree. But the study has brought out fierce debate among the men and women who study our species' origins.
Lucy's skeleton has been studied at length since she was discovered in Ethiopia in 1974. As one of the oldest and most complete hominid fossils ever found, Lucy's skeleton provides quite a bit of information on how she lived. She walked upright, showing an intriguing mix of modern and ancestral skeletal traits that would have made her bipedal while other primates swung from the trees. We know that she was a fully mature adult when she died, although she was no bigger than a 4- or 5-year-old of our own species. She was very likely a vegetarian.
But in a study published Monday in Nature Communications, researchers did something new — they tried to puzzle out how she died.
The research comes out of a marathon of X-ray scanning. Lucy's skeleton is usually kept in Ethiopia, but during a 2008 United States museum tour, researchers at the University of Texas were able to borrow her for 10 days. They used the opportunity to scan her with the Jackson School of Geosciences' High-Resolution X-ray Computed Tomography Facility (UTCT) — a machine designed to scan through materials as solid as a rock (as old as it is, Lucy's skeleton is totally mineralized) and at a higher resolution than medical scans.
"We scanned nonstop, 24/7, for 10 days," lead author John Kappelman, a University of Texas at Austin anthropology and geological sciences professor, told The Washington Post. "We were exhausted. I was happy to see her come, but I was happy to see her go."
When bones sit around for upward of 3 million years, they tend to break. A lot. But in studying Lucy's remains in new detail, Kappelman and his colleagues thought they saw evidence of breaks that had occurred before death. They believed they could see compression fractures — caused when force presses bones against one another so hard they break — and greenstick fractures, where the bone partially bends and partially breaks (much as a fresh, living twig will splinter and twist instead of snapping in two). The former is usually a sign of a traumatic impact, not the wear and tear of life as a fossil, and the latter only occurs on fresh, living bone.
To Kappelman, this was a clear sign that the traumatic fractures occurred during Lucy's life. And when he went looking for possible causes of these fractures in the medical literature, he found what many orthopedic surgeons have told him is a perfect match: The injury to her shoulder looked just like a four-part proximal humerus fracture.
When people fall and put their arms out to break the impact, force is transmitted along the long axis of the arm, compressing the components of the shoulder against one another. If that force is high enough, the head of the shoulder blade acts as an anvil, shattering and compressing the components of the proximal humerus, or top of the upper arm bone.
"Orthopedic surgeons see these breaks day in and day out all over the planet," Kappelman said. He's had something like 10 specialists take a look at Lucy's big break.
"To the person, it's not like, 'Oh, you know, there's a chance.' They say, 'This is what it is; we see it in our practice all the time.' We have been able to demonstrate that these are matches to what is widely seen in the literature in patients recovering from a fall," he said.
Once his team went looking for fractures in Lucy's skeleton that could have resulted from a devastating fall, they found loads: An ankle fracture known as a pilon, usually caused by falls or motor vehicle accidents; high-energy traumatic fractures to both knees; signs that bones in her legs dislocated and rammed up into the joints above them; a fracture to the first rib, which is well protected by the collar bone and usually not broken except in cases of traumatic impact.
But other paleontologists aren't so sure.
"I've worked in Eastern Africa at these sites for many years, and there's hardly a fossil out there that doesn't have damage like Lucy has," said William Kimbel, director of the Institute of Human Origins at Arizona State University. Kimbel, who wasn't involved in the new study, doesn't agree that the fractures on Lucy's bones are greenstick — and without this distinction, he doesn't think the paper does a sufficient job of proving they came from an injury before death.
"If you roam the area, you’ll see this type of damage on all types of bones from all kinds of sources, from hares to hippos," he scoffed.
Kimbel's ASU colleague Donald Johanson — a member of the team that discovered Lucy — agreed. "The only green fracture I see on the skeleton is a small indented pit on Lucy’s pelvis in the pubic region," he said — a fracture already identified as a possible sign that a carnivore took a swipe at Lucy, perhaps just after she died (as there is no other evidence that she was attacked).
As the University of Texas study hinges on the idea that these fractures are clearly signs of trauma — something Kimbel and Johanson vehemently disagree with and that can't really be proven — they point out that the "research" that follows is just a story.
"I just don't buy it," Kimbel concluded.
"It's an elaborate story," Bernard Wood, a professor of human origins at George Washington University who was not involved in the study, said. "It's not science."
The research team's conclusion that Lucy likely fell from a tree, specifically — a conclusion they came to by estimating the force of a fall from the heights at which chimpanzees and other primates typically nest arboreally — only makes the study harder to swallow for some paleontologists. It is, once again, impossible to prove or disprove. And Lucy's relationship with trees is somewhat controversial.
"From the waist down, Lucy's bones are clearly adapted to a terrestrial, bipedal walking. But from the waist up, Lucy has an upper body that looks like that of any tree-climbing ape," said Dartmouth anthropologist Nathaniel J. Dominy, who wasn't involved in the study. "Everyone agrees that Lucy walked upright, but did she climb trees occasionally or never at all?"
This debate was once so acrimonious, Dominy said, that researchers used to have to pick sides and stick to them. Dominy (and many other paleontologists) now take the moderate view: Of course Lucy could climb trees, just as humans can climb them today. And if she had that skill, as a tiny herbivore living in a world of big predators, she no doubt would have hoisted herself up into the trees once or twice for protection or to find food. But we have no way of knowing whether she and her ilk regularly climbed to great heights.
"The a priori assumption that Lucy spent much time in the trees is not supported," Johanson said.
None of this criticism necessarily means that the study is wrong. Some experts are intrigued by the research: David Green, a researcher from Midwestern University, told The Washington Post that while the conclusions seemed "dramatic" compared with the typical assumption of postmortem damage, he thinks the study makes "a very compelling case."
Rebecca Ackermann of the University of Cape Town, who has a background in paleontology and forensic science, agreed with the critique that the study failed to disprove alternative causes for the breaks but thought that the explanation presented seemed quite plausible.
"In my opinion, this is a nice study that tells us something interesting about an individual who has played an important role — both scientifically and historically — in our understanding of human evolution," she said.
But the fact remains this new story may be impossible to prove. It's a case that's colder than cold, full of circumstantial evidence and he-said-she-said fracture diagnoses.
Even if Kappelman could prove that Lucy had greenstick fractures, he wouldn't be able to prove they occurred before death. A well-timed stampede while her corpse was still fresh could have caused the still-fresh bones to break this way. Even if there was some way to prove definitively that the injuries took place while Lucy was alive — or at least show that they were very unlikely to occur after death — the fall itself would be impossible to puzzle out. Maybe Lucy fell from a cliff instead of a tree. Maybe she was gathering honey and got stung by a bee, not sleeping in a nest she returned to every night as Kappelman's team suggests. Maybe she did climb a tree and fall but was the first of her species to ever attempt such a feat.
All of this means that Lucy's fall will likely never be part of the scientific consensus. But it will also be hard to discredit completely and will no doubt enter into the hallowed halls of Internet lore.
Maybe that's not such a bad thing. In the story of her death — however unproven it may remain — Lucy has become more of a person than ever before.
"I’ve taught about her for years, and up until the moment I pictured her death, she was just a box of bones to me," Kappelman said. At the moment when all of her fractures were laid out as evidence before him, Kappelman could see her final moments in his mind's eye. A terrified scramble. A fall. An unavoidable urge to reach out her hands, bracing against the impact that would kill her. And finally, a tiny, broken body lying alone in the dirt.
"In her death, she became a living individual," Kappelman said. "I'm not a philosopher, I'm a scientist. But to me, it's such a beautiful juxtaposition that by understanding her death, I now feel that I understand her life."
This post was originally published on August 29. It has been updated.