It has long been a mystery as to how Lucy the Australopithecus died.
But now, some 3.18 million years later, scientists think they know what really happened.
The ancient hominin fell out of a tree, according to a new study published Monday in the journal Nature. And if the researchers are right, this could support the idea that Lucy's species, Australopithecus afarensis, were frequent tree-climbers.
"This was a species that was a relative of ours that led a lifestyle that had her both on the ground and up in the trees," John Kappelman, the lead author of the paper and a paleobiologist and anthropologist at the University of Texas at Austin, tells The Christian Science Monitor in a phone interview. And understanding Lucy could help scientists better understand the suite of species that came before our own.
The forensic evidence lies directly in Lucy's bones. A team led by Dr. Kappelman used CT scans to examine every bump and crack in her bones in an effort to reconstruct the individual hominin's life – and death.
The researchers compared breaks in Lucy's bones with the injuries sustained by humans today and found that "a number of fractures are consistent with the sorts of injuries seen in patients who have suffered a fall from a considerable height," Kappelman says.
Given that there were no skyscrapers to fall off, and Lucy's skeleton was unearthed far from any steep cliffs, Kappelman and his colleagues hypothesized that Lucy had been in a tree just before plummeting to her death.
"This interpretation really explains a lot," Randall Susman, the chair of the department of anatomical sciences at Stony Brook School of Medicine, who was not part of the study, tells The Christian Science Monitor. "They really looked at the fossils and then brought to bear evidence to back up their suggestions."
Dr. Susman and his colleagues have long studied the morphology of extinct human relatives, including Lucy. But many other paleoanthropologists that have studied Lucy or casts of her famous skeleton are not convinced.
"My first impression is that it explains too much. Essentially they're attributing just about every little fracture, big breaks, little breaks, everything to this fall," David Begun, a palaeoanthropologist at the University of Toronto who was not part of the study, tells the Monitor in a phone interview.
"I wouldn't say they're wrong," Dr. Begun says. It's possible that Lucy did indeed die from falling from a tree, he says. But, "I would just say it's impossible to say."
Begun and others are skeptical because, they say, the cracks in the bones could have come from a myriad of other causes.
Donald Johanson, whose team discovered and excavated Lucy in 1974, writes in an email to the Monitor that "the suggestion that she fell out of a tree is largely a 'Just-So Story' that is neither verifiable or falsifiable and therefore unprovable."
Instead, he suggests, perhaps Lucy was trampled by a stampede of large animals.
But Kappelman says he too was skeptical of his own hypothesis, so he checked all the other scenarios he could think of against the evidence in Lucy's skeleton. Those other scenarios ran the gamut from a stampede or other altercation with a large animal, to being swept away by a flood, to the effects of being struck by lightning.
Still, "it looks like the fall is the most likely scenario," Kappelman says.
One of the fractures particularly pointed toward this kind of impact: compressive fractures in Lucy's joints. These fractures occur in humans today when using an outstretched arm to brace against a fall or an impact in a car crash.
Lucy likely had the same kind of instinctive response to falling, Kappelman says.
One challenge of determining the cause of the cracks in Lucy's bones is whether the bones broke before or after her death.
It was clear these weren't the result of injuries long before Lucy's demise because the bones would have shown some signs of healing, Kappelman says. But whether or not they happened before Lucy's bones dried out or later while her skeleton was buried underground for millions of years is a different question.
"In my opinion," Dr. Johanson says, "the breakage and plastic distortion seen on Lucy's bones, as well as many of the other hominin fossils at Hadar [the site where she was found] is a result of geological forces acting on the bones during the fossilization process."
Begun agrees, saying that he has seen some similar breaks in fossils of animals like pigs that would have never left the ground.
One way to tell the difference, Kappelman says, is whether or not the bones have a hinge fracture. Hinge fractures, he explains, are like when you try to break a stick of wood that is still green. The bones don't completely break apart cleanly. And many of Lucy's fractures appear to have occurred when the bone was still green, Kappelman says.
If Kappelman and his colleagues are right, then their research could add to the evidence that Lucy's species were arboreal as well as terrestrial.
"If you're going to fall out of a tree and die, you had to get up the tree," Susman says.
Scientists know that Lucy spent a lot of her life walking upright on the ground. But, until now, evidence that Lucy's species was arboreal, too, has largely only been from her morphology and the study of living primates. Still, most scientists agree that Lucy likely slept and foraged in trees.
This research highlights that human ancestors were not as terrestrial as humans, Begun says. Today, "there are virtually no primates that are as terrestrial as humans," he says, and researchers have been fascinated with what sets our species apart.
Kappelman says that this research has helped him see Lucy as an individual. "Understanding her death breathed life into her. It's this jump of empathy across time and space that really allowed me to identify with her in a way that I hadn't before."
Before this research, he says, "Lucy was just a box of bones to me."