Lucy, the most famous fossil of a human ancestor, probably died after falling from a tree, according to a new study led by researchers at The University of Texas at Austin.
Lucy, a 3.18-million-year-old specimen of Australopithecus afarensis — or “southern ape of Afar” — is among the oldest, most complete skeletons of any adult, erect-walking human ancestor. Considered a “terrestrial biped,” Lucy has long been the subject of a vigorous debate about whether this ancient species also spent time in the trees.
“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,” said lead author John Kappelman, a UT Austin anthropology and geological sciences professor.
Kappelman worked with geological sciences professor Richard Ketcham to carefully scan all of Lucy’s 40-percent-complete skeleton to create a digital archive of more than 35,000 CT slices at the High-Resolution X-ray Computed Tomography Facility (UTCT) in the UT Jackson School of Geosciences.
“Lucy is precious. There’s only one Lucy, and you want to study her as much as possible,” Ketcham said. “CT is nondestructive. So you can see what is inside, the internal details and arrangement of the internal bones.”
While studying Lucy and the scans, Kappelman noticed something unusual: The end of the right humerus was fractured in a manner not normally seen in fossils, preserving a series of sharp, clean breaks with tiny bone fragments and slivers still in place.
“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,” said Kappelman, who consulted Dr. Stephen Pearce, an orthopedic surgeon at Austin Bone and Joint Clinic, using a modern human-scale, 3-D printed model of Lucy.
Pearce confirmed: The injury was consistent with a four-part proximal humerus fracture, caused by a fall from considerable height when the conscious victim stretched out an arm in an attempt to break the fall.
Kappelman noticed similar fractures at the left shoulder and compressive fractures throughout her skeleton, including a pilon fracture of the right ankle, a fractured left knee and pelvis. Even more subtle evidence such as a fractured first rib were seen — a hallmark of severe trauma and all consistent with fractures caused by a fall.
There was no evidence of healing, however, so Kappelman concluded the breaks happened perimortem, or near the time of death.
But how could Lucy have achieved the height necessary to produce such a high velocity fall and forceful impact? Kappelman argued that because of her small size — about 3 feet 6 inches and 60 pounds — Lucy probably foraged and sought nightly refuge in trees.
Using modern chimpanzees as a comparison, Kappelman suggests that Lucy probably fell from a height of more than 40 feet, hitting the ground at more than 35 miles per hour. Based on the pattern of breaks, Kappelman hypothesizes that she landed feet-first before bracing herself with her arms when falling forward, and “death followed swiftly.”
“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.”
Kappelman suggests that because Lucy was most likely both terrestrial and arboreal, her species was possibly subjected to frequent falls.
TOP IMAGE: UT Austin professor John Kappelman with 3D printouts of Lucy’s skeleton illustrating the compressive fractures in her right humerus that she suffered at the time of her death 3.18 million years ago Credit: Marsha Miller
JOURNAL CITATION: Perimortem fractures in Lucy suggest mortality from fall out of tall tree. John Kappelman, Richard A. Ketcham, Stephen Pearce, Lawrence Todd, Wiley Akins, Matthew W. Colbert, Mulugeta Feseha, Jessica A. Maisano & Adrienne Witzel; Nature (2016). doi:10.1038/nature19332