X-rays Show Human Ancestor Lucy May Have Died From Falling Out of a Tree

A new study out of the University of Texas at Austin has proposed a cause of death for our most famous human ancestor, a hominid named Lucy. According to paleoanthropologist John Kappelman, Lucy died after falling from a tree.

In 1974, archaeologists in Ethiopia discovered fossilized bones belonging to one of humanity’s earliest known ancestors to walk upright on two feet. The skeleton reshaped our understanding of human evolution; however, until now, no scientist has been able to explain how she perished.

Lucy was a member of the ancient hominin species Australopithecus afarensis, which existed on the evolutionary timeline about halfway between apes and modern humans, approximately 3.9 million years ago. For the most part, Lucy looked like your average ape – long arms, protruding belly, low forehead. However, her ankles, feet, and pelvis show that she was capable of the uniquely human trait of walking upright. Lucy was also much smaller than the modern adult human, weighing in at about 60 pounds and standing only three and a half feet tall.

Scientists have never been sure if these hominids had spent any time in the trees. Their bodies were no longer built for agile climbing, but rather for walking on land. However, this new study suggests that tree-dwelling behavior (arborealism) was still a fixture among primates until recently.

The researchers hypothesized that Lucy had fallen from a tree after carefully scanning her bones. Using the High-Resolution X-ray Computed Tomography Facility (UTCT) in the UT Jackson School of Geosciences, they could see the inside of the fossils in order to visualize the internal features and arrangements of the bones. Different kinds of CT systems can accommodate very small to very large complex parts, ranging from .020 inches to 39 inches in length, so Lucy’s small appendages were the ideal specimens.

Researchers immediately noticed an oddly clean fracture on Lucy’s upper right 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,” said Kappelman.

The x-rays were consistent with injuries seen in modern humans after falling. The team did not find any evidence of healing, indicating that Lucy died shortly after sustaining the injuries.

“Those adaptations that allowed them to move more effectively on the ground are not so good for climbing. It may have predisposed her to more frequent falls,” Kappelman noted. “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.”


Photo attribution: By Andrew from Cleveland, Ohio, USA (Lucy) [CC BY-SA 2.0 (http://creativecommons.org/licenses/by-sa/2.0)], via Wikimedia Commons

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