Lucy -- the common name accorded to the very famous fossilized remains of a human ancestor -- probably died after falling from a tree, according to a new study.
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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 and was discovered in 1974.
"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 John Kappelman, Professor at University of Texas in the study published in the journal Nature.
Kappelman and his teammates carefully scanned all of her 40 per cent complete skeleton and noticed that 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," Kappelman added.
For the study, Kappelman consulted Stephen Pearce, an orthopedic surgeon at Austin Bone and Joint Clinic, using a modern human-scale, 3-D printed model of Lucy.
Pearce confirmed that 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 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 -- all consistent with fractures caused by a fall.
Without any evidence of healing, Kappelman concluded that the breaks occurred perimortem, or near the time of death.
Kappelman stated that because of her small size -- about three feet six inches and 60 pounds -- 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 35 miles per hour. Based on the pattern of breaks, Kappelman hypothesised that she landed feet-first before bracing herself with her arms when falling forward.
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.
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Using fracture patterns when present, future research may tell a more complete story of how ancient species lived and died.