A 3.2 million-year-old mystery solved: Lucy may have died in a long fall from a tree

In 1974, the paleoanthropologist Donald C. Johanson led an expedition to Ethiopia to look for fossils of ancient human relatives.

In an expanse of arid badlands, he spotted an arm bone. Then, in the area surrounding it, Johanson and his colleagues found hundreds of other skeletal fragments.

The fossils turned out to have come from a single three-foot-tall female who lived 3.2 million years ago. The scientists named her species Australopithecus afarensis, and the skeleton was dubbed Lucy.

Four decades later, Lucy remains one of the most famous discoveries in paleontology. Finding a single bone of that age would have been reason to celebrate; finding so much from a skeleton revealed a tremendous amount about Lucy - and about human evolution in general.

A 3D model of the early human ancestor, Australopithecus afarensis, known as Lucy, on display at the Houston Museum of Natural Science. Credit: AP

Her death, on the other hand, has been a mystery. Now, after poring over the celebrated bones, a team of scientists has concluded that Lucy died most unceremoniously: from a long fall out of a tree.

If they are right, the discovery could yield an important clue to how our ancestors evolved from tree-dwelling apes into bipeds that walked the African savanna. But the new study has experts deeply divided. Some researchers are praising the research, while others, including Johanson, think the authors have failed to adequately make their case.

The study, published Monday in the journal Nature, came about because Lucy's skeleton - normally housed at the National Museum of Ethiopia - was taken on a tour of the United States in 2007. After a show at the Houston Museum of Natural Science, Lucy spent 10 days at the University of Texas at Austin, where scientists put her bones through a CT scanner.

"We just decided, by golly, we were going to scan every little bit of Lucy because it may never be done again," said John Kappelman, a paleoanthropologist at the university.

This undated photo provided by the University of Texas at Austin shows the distal radius - a wrist bone - of Lucy, a fossil specimen of an early human ancestor, Australopithecus afarensis, undergoing computed tomographic scanning at the university in Austin, Texas.Credit: AP

Since then, Kappelman and his colleagues have painstakingly turned the scans into 3D models, piecing together the virtual fragments to get a more accurate idea of their original shapes. Last December, he noticed a puzzling break in Lucy's upper right arm.

A look through surgical journals suggested that the break might be a so-called compressive fracture, in which a force pushes down on a bone, sometimes even driving one bone into another.

A close-up of the 3D model of the early human ancestor, Australopithecus afarensis, known as Lucy.Credit: AP

What intrigued Kappelman was what could have caused such a compressive fracture: a fall from a great height. "It's not something that would happen if you just tripped and fell," he said.

Kappelman printed out a human-sized, 3D model of Lucy's shoulder and took it to Dr Stephen Pearce, an orthopedic surgeon at the Austin Bone and Joint Clinic. Pearce agreed that the break was a compressive fracture. Other orthopedic surgeons consulted by Kappelman made the same diagnosis.

This undated image provided by the University of Texas at Austin shows the skeleton of Lucy.Credit: AP

Kappelman and his colleagues decided to inspect all of Lucy's bones for fractures that might have been caused by a fall. In addition to studying the virtual models, Kappelman examined the original fossils in Ethiopia.

They found a number of breaks that looked as if they had occurred after Lucy died. But he also observed more compressive fractures, as well as so-called greenstick fractures, in which a bone only cracks on one side, much like what happens when a living tree branch breaks. Both kinds of fractures can happen during falls.

Lucy suffered many such fractures, the scientists concluded, from her ankles to her jaw. The fractures suggest that she came down feet-first and then tumbled forward, holding out her hands in a futile hope of protecting herself.

"It tells us she was conscious when she reached the ground," Kappelman said.

If so, Lucy did not stay conscious for long. The fractures to her rib cage suggest crushing injuries to her internal organs that would have killed her.

Kappelman and his colleagues believe Lucy must have fallen from a tree. They base that conclusion on what geologists have determined about the environment where she lived: At the time, it was a low-lying wooded area around a stream, with no cliffs nearby.

William L. Jungers, a paleoanthropologist at Stony Brook University who was not involved in the research, called it "a provocative but plausible scenario."

Laura Martin-Frances, a postdoctoral researcher at the National Research Centre on Human Evolution in Burgos, Spain, said she was impressed with the level of detail in the new study.

"For me, it's quite accurate what they have done," Martin-Frances said.

But other experts said Kappelman and his colleagues had not done enough to rule out other explanations for the fractures.

Ericka N. L'Abbé, a professor of anthropology at the University of Pretoria in South Africa, said that when living bones break, some parts bend. A close inspection of Lucy's bones might have revealed traces of that bending.

"The major drawback is that they didn't look under a microscope," L'Abbé said.

Johanson said it was far more likely that the fractures Koppelman attributes to a fall had occurred long after her death, as her skeleton was buried under sand.

"Elephant bones and hippo ribs appear to have the same kind of breakage," Johanson said. "It's unlikely they fell out of a tree."

Monkeys and apes spend a lot of time in trees and have impressive adaptations for that sort of life. One of the most striking features of Lucy's skeleton is the shape of her leg and knee bones, which look suited for walking on the ground instead.

Since the discovery of Lucy, paleoanthropologists have found more fossils from Australopithecus afarensis. They suggest Lucy had flat feet and other traits needed for walking.

Some researchers have argued that by Lucy's time, our forerunners were no longer good tree-climbers, having evolved to find food on the ground. "Australopithecus afarensis was essentially a terrestrial animal," Johanson said.

But other experts believe Lucy's hook-like hands and her flexible shoulders suggest she was a decent tree-climber. Perhaps she split her time between the ground and the canopy. Some researchers have even argued that our distant ancestors originally evolved bipedalism in the trees for walking on branches.

"This is a big debate," said William Harcourt-Smith, an anthropologist at the American Museum of Natural History.

Kappelman and his colleagues considered the possibility that Lucy fell out of a nest in which she was sleeping. Chimpanzees build their nests an average of 40 feet above the ground. A fall from that height could have killed Lucy, the scientists calculated.

But Nathaniel Dominy, an evolutionary biologist at Dartmouth College, considers it unlikely. "For me, the much more likely scenario is that she was climbing for food," he said.

Chimpanzees sometimes gather honey from hives that are far above their nests. They have to use one hand to hold on to a branch while jabbing a stick into a hive with the other.

"Lucy was just enduring the stings as a chimpanzee would. It would be intense," Dominy said.

Kappelman has received permission from the Ethiopian government to post the bone data online, and he hopes his critics will look closely for themselves. At eLucy.org, scientists and schoolchildren alike will be able to download 3D renderings and inspect them on a computer - or even print out casts of Lucy's bones on 3D printers.

"I'm happy that the 3D files are out there," Kappelman said. "People can much more fully evaluate our hypothesis by looking at them, and it will be fun to see where it goes."

New York Times