The iconic 3.18-million-year-old human ancestor, Lucy, may have died of injuries sustained after falling from a tree, according to a new study.
Lucy, an ancient specimen of Australopithecus afarensis, is among the oldest, most complete skeletons of any adult, erect-walking human ancestor. Since her discovery in the Afar region of Ethiopia in 1974, Lucy has been at the centre of a vigorous debate about whether this ancient species also spent time in the trees.
“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,” said lead author John Kappelman, professor at the University of Texas at Austin in the US. Studying Lucy and her 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. “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,” said Stephen Pearce, an orthopedic surgeon at Austin Bone and Joint Clinic.
Kappelman observed similar but less severe fractures at the left shoulder and other compressive fractures throughout Lucy’s skeleton including a pilon fracture of the right ankle, a fractured left knee and pelvis, and even more subtle evidence such as a fractured first rib – “a hallmark of severe trauma” – all consistent with fractures caused by a fall.
Without any evidence of healing, Kappelman concluded the breaks occurred perimortem, or near the time of death. Kappelman argued that because of her small size – about 3 feet 6 inches and 27kg – Lucy probably foraged and sought nightly refuge in trees. In comparing her with chimpanzees, Kappelman suggested Lucy probably fell from a height of more than 40 feet, hitting the ground at more than 56km per hour. Based on the pattern of breaks, Kappelman hypothesised that she landed feet-first before bracing herself with her
arms when falling forward, and “death followed swiftly.”
Kappelman conjectured that because Lucy was both terrestrial and arboreal, features that permitted her to move efficiently on the ground may have compromised her ability to climb trees, predisposing her species to more frequent falls.
The study appears in the journal Nature.