The 3.18 million-year-old human ancestor Lucy may have frequently climbed trees and nested in them at night to avoid predators, a new study of the hominim's fossilised skeleton suggests.
Scientists from the Johns Hopkins University and the University of Texas at Austin in the US said that the scan shows that Lucy's upper limbs were heavily built, similar to champion tree-climbing chimpanzees, supporting the idea that she spent time climbing and used her arms to pull herself up.
The fact that her foot was better adapted for upright walking than grasping may mean that climbing placed additional emphasis on Lucy's ability to pull up with her arms and resulted in more heavily built upper limb bones, researchers said.
Exactly how much time Lucy spent in the trees is difficult to determine, they said.
A recent study suggested that Lucy died from a fall out of a tall tree.
This new study adds to evidence that she may have nested in trees at night to avoid predators, they added.
An eight-hour slumber would mean she spent one-third of her time up in the trees, and if she also occasionally foraged there, the total percentage of time spent above ground would be even greater.
Lucy, housed in the National Museum of Ethiopia is among the oldest, most complete fossil skeletons ever found of any adult, erect-walking human ancestor.
She was discovered in the Afar region of Ethiopia in 1974 by Arizona State University anthropologist Donald Johanson and graduate student Tom Gray.
The new study analysed CT scan images of her bones for clues to how she used her body during her lifetime. Previous studies suggest she weighed less than 65 pounds and was under four feet tall.
"We were able to undertake this study thanks to the relative completeness of Lucy's skeleton," said Christopher Ruff, professor at Johns Hopkins.
"Our analysis required well-preserved upper and lower limb bones from the same individual, something very rare in the fossil record," said Ruff.
The research team first had a look at Lucy's bone structure during her US museum tour in 2008, when the fossil was detoured briefly to the High-Resolution X-Ray Computed Tomography Facility in the University of Texas at Austin Jackson School of Geosciences.
For 11 days, researchers carefully scanned all of her bones to create a digital archive of more than 35,000 CT slices.
High-resolution CT scans were necessary because Lucy is so heavily mineralised that conventional CT is not powerful enough to image the internal structure of her bones.
The study appears in the journal PLOS ONE.