As cold cases go, they don’t come much older. Palaeontologists deciphered clues in the bones of the 3.18-million-year-old fossilised hominin Lucy’s to determine her cause of death – a 12-metre tumble out of a tree.
Scans taken by John Kappelman at the University of Texas at Austin in the US and published in the journal Nature show a fracture in the Australopithecus afarensis fossil’s right upper arm is consistent with a fall from a significant height, such as out of a tall tree.
Its part-time tree-dwelling nature means A. afarensis was likely preyed upon, and was far from an apex hunter.
Since the remains of Lucy’s skeleton were found in the Afar region of Ethiopia in 1974, it’s been clear her species walked on two legs. But their proclivity for hanging out in trees has been the subject of fierce debate over the years.
Some palaeontologists claim the species lived on the ground all the time, while others suggest they still spent some time in the treetops.
“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,” Kappelman says.
To reach this conclusion, Kappelman and his team took 35,000 high-resolution X-ray slices of the fossilised remains.
After noticing a series of sharp, clean breaks in Lucy’s humerus (upper arm), Kappelman consulted an orthopaedic surgeon at Austin Bone and Joint Clinic, Stephen Pearce, who said Lucy’s injury looked like she tried to soften a fall from a considerable height with an outstretched arm.
This seemed to be corroborated by the series of breaks in the left shoulder, right ankle, left knee, pelvis and first rib.
To try to piece together what happened, Kappelman compared Lucy’s case to how chimpanzees fall from trees and the kinds of injuries they sustain. From this he estimated that she landed feet first before bracing herself with her arms as she fell forward.
Kappelman calculated that the fall would have been roughly 12 metres. Lucy would have hit the ground at around 56 kilometres per hour. And given she was only a metre or so tall and weighing around 27 kilograms, it spelled the end for her.
“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 says.
“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.”
The researchers argue that the evolutionary adaptations her species developed to walk on two legs compromised their ability to climb as well, resulting in more falls like this one.
Varsha Pilbrow, a biological anthropologist from the University of Melbourne in Australia, says the paper had her convinced that Lucy died from a fall.
“It looks at evidence from a forensic point of view that is quite interesting. They go over what they’re trying to do very, very carefully and so it seems to make sense to me.”
Pilbrow noted that small hominin species such as Lucy’s were vulnerable to a number of predators, including leopards and eagles. They probably would have been forced up into the trees at night to avoid predation.
She also explains that Lucy’s species would have moved very quickly through the canopy, grabbing even the thinnest branches to keep moving. One of these branches likely broke under the hominin’s weight, sending her hurtling to the ground below.
Kappelman’s work, she adds, is part of an emerging trend of forensically investigating archaeological finds.
“It seems like now there are more and more papers coming out where we are looking at the same remains in a more detailed and nuanced way, so we are able to take a look at this from a more pathological point of view.”
She points in particular to a Scientific Reports paper published in October last year that involved similar investigation of fractures in the hominin species Australopithecus sediba.
But Kappelman’s work is notable because it continues a shift in the perception of early human culture.
“For a long time, particularly in the discipline of palaeo-anthropology, there was always a case of humans being adept at the environment, moving along, being the hunters. Now the field has flipped that idea,” Pilbrow says.
“When people [in the 1980s] started reconstructing skeletal remains based on how they were found, they said humans were not the ones that were hunting.
“They were more likely the ones who were hunted by other predators.”