Researchers who say they showed the pre-human "Lucy" died falling from a tree now say they've shown she probably lived up there, or at least spent a lot of time in trees.
Bone scans show "Lucy," who died 3.2 million years ago, had thick, strong upper arms compared to her thigh bones — the same pattern seen in chimpanzees as compared to modern humans or even later human ancestors.
While Lucy clearly walked upright on the ground, the evidence points to a creature who also climbed, hung and hoisted herself about in the trees, the team at Johns Hopkins University and The University of Texas at Austin reported Wednesday.
"The most likely explanation is that Lucy climbed trees with a greater reliance on her upper extremity much more frequently than modern humans or early Homo," they wrote in a report published in the Public Library of Science journal PLoS ONE.
How can they tell? They looked at CT scans — a kind of souped up x-ray — of Lucy's upper arm and thigh bones. When animals or humans use their muscles heavily, the pull on the attached bones makes the bone thicker and stronger.
By comparing the relative thickness and strength of the long arm bones to the long leg bones, scientists can tell which gets used more. For instance, gymnasts have stronger arm bones than most people, and babies who haven't yet learned to walk have weaker-looking leg bones.
"Tennis players are a nice example: Studies have shown that the cortical bone in the shaft of the racquet arm is more heavily built up than that in the non-racquet arm," said John Kappelman, a paleoanthropologist at UT who helped lead the study.
The scans of Lucy indicate that she spent far more time using her arms to lift her weight than modern humans or pre-humans such as Homo erectus, the researchers said.
"Possible reasons for using the trees more often include foraging for food and escape from predators," Kappelman's team wrote.
Lucy was an Australopithecus afarensis, a small pre-human that lived 3 million years ago in eastern Africa.
She's famous because her skeleton is so complete.
Her brain was slightly bigger than that of a modern-day chimpanzee, but her skeleton clearly shows she walked upright.
She was just over 3.5 feet tall had strong arms that could have swung her up easily into trees. And because she had flatter feet made for walking, she may have relied heavily on arm and shoulder strength to maneuver in the branches.
Although the area where Lucy was found is dry now, 3 million years ago it was a grassy woodland, the team said.
Their theories are controversial. Kappelman reported earlier this year that the broken bones in Lucy's skeleton show she tumbled from a high tree. Other paleontologists have said the breaks could easily have happened long after Lucy's death.