Skeletal Analysis Suggests Lucy Died After Falling From a Tree

By George Dvorsky on at

The world’s most famous human ancestor, an extinct hominid named Lucy, died after falling from a tall tree, according to scientists. It’s a revelation that points to tree-dwelling behaviour in recent evolutionary history, but some scientists aren’t convinced.

In a new study published in Nature, palaeoanthropologist John Kappelman and colleagues from the University of Texas at Austin posit the hypothesis that Lucy met her fate after falling feet first from a tree.

It is ironic that the fossil at the center of a debate about the role of arborealism in human evolution likely died from injuries suffered from a fall out of a tree.

The researchers arrived at this surprising conclusion after studying the many fractures on parts of Lucy’s 13.2-million-year-old skeleton, some of which are quite severe. The new study suggests that the extinct hominin Australopithecus afarensis engaged in tree dwelling behaviour, or arborealism, and that this behaviour still existed among bipedal hominids just as humans were first emerging.

Her skeleton may only be 40 per cent complete, but Lucy represents one of the oldest and most complete fossil hominids ever discovered. The species to which she belonged, A. afarensis, lived in Africa between 3.9 and 2.9 million years ago. Like humans, these hominids walked on two feet, but scientists aren’t entirely sure if they spent time up in the trees. This new study suggests they probably did, and that tree dwelling behaviour remained a fixture among primates until relatively recently.

Skeletal Analysis Suggests Lucy Died After Falling From a Tree

The Lucy fossil. (Image: Ethiopian National Museum)

“It is ironic that the fossil at the centre of a debate about the role of arborealism in human evolution likely died from injuries suffered from a fall out of a tree,” noted Kappelman in a release.

The researchers arrived at this conclusion after carefully scanning Lucy’s skeleton and creating a digital archive of more than 35,000 slices. Using the High-Resolution X-ray Computed Tomography Facility (UTCT) in the UT Jackson School of Geosciences, the researchers were able to peer inside the fossil and visualise the internal details and arrangements of the bones.

Skeletal Analysis Suggests Lucy Died After Falling From a Tree

A bone undergoing scanning. (Image: Marsha Miller/UT Austin)

One of the first things the researchers noticed was a rare and oddly clean fracture at the end of Lucy’s right upper arm. “This compressive fracture results when the hand hits the ground during a fall, impacting the elements of the shoulder against one another to create a unique signature on the humerus,” noted Kappelman.

Lucy was no longer simply a box of bones but in death became a real individual: a small, broken body lying helpless at the bottom of a tree.

With the help of an orthopaedic surgeon, the researchers concluded that the injury was likely caused by a plunge from a considerable height (about 12 metres) at the moment when Lucy stretched out her arm in an effort to break the fall. Similar fractures were observed at the left shoulder and other parts of Lucy’s skeleton, as well as modern examples. Based on the pattern of breaks, the researchers say she landed feet-first before bracing herself with her arms as she fell forward. She likely died soon thereafter from the severe injuries.

“When the extent of Lucy’s multiple injuries first came into focus, her image popped into my mind’s eye, and I felt a jump of empathy across time and space,” Kappelman said. “Lucy was no longer simply a box of bones but in death became a real individual: a small, broken body lying helpless at the bottom of a tree.”

So what was Lucy — a bipedal hominid weighing about 30 kilos and measuring about three and a metre tall doing so high up in a tree? The researchers hypothesise that she was probably foraging, and/or seeking nightly refuge in trees.

Skeletal Analysis Suggests Lucy Died After Falling From a Tree

John Kappelman with 3D printouts of Lucy’s bones. (Image: Marsha Miller/UT Austin)

But not everyone is buying the “Lucy fell from a tree” hypothesis. As noted by Carl Zimmer in the New York Times, living bones don’t always break. Sometimes they bend. He quotes Ericka N. L’Abbé, a professor of anthropology at the University of Pretoria in South Africa, who said, “The major drawback is that they didn’t look under a microscope.” It’s also possible that the fractures formed long after Lucy’s death as her skeleton was buried under sand.

Indeed, the details of the new work are compelling, but it may be a stretch to draw such a specific conclusion from such meagre and circumstantial evidence. Moreover, this narrative has to be reconciled with other pre-existing evidence, such as the fact that A. afarensis was flat footed and very well adapted to walking. Lucy, unlike her distant ancestors, were no longer well adapted for climbing (which interestingly enough may actually explain why she fell from the tree, assuming that’s what happened). Kappelman, on the other hand, believes that Lucy was both terrestrial and arboreal, exhibiting features that helped her to move on the ground and in the trees.

For those who are sceptical, the researchers have posted their bone data online at eLucy.org. With the permission of the Ethiopian National Museum, the public can now download and print Lucy’s skeleton so that they can scrutinise this hypothesis for themselves. [Nature]