The famous human ancestor Lucy spent at least a third of her time in the trees, new research shows.

Image credit: H. Lorren Au Jr., Orange County

The 3.2 million-year-old ancestor, whose fossilized skeleton was found in Ethiopia in 1974, probably lived a life similar to what the chimpanzee does today, according to US researchers who have published their findings in PLOS One.

By scanning Lucy’s skeleton with a High-resolution computed X-ray tomography (CT), powerful enough to get through the mineral layers that cover her remains, the scientists have produced 35,000 high-resolution images.

The images reveal internal structures suggesting that her upper limbs were built for heavy load bearing — much like chimpanzees’ arms, which they use to pull themselves up tree trunks and to swing between branches.

Model of Lucy at the Bowers Museum perpetuates the anatomically unsupported notion that the extinct ape known as “Lucy” could walk with an ordinary upright bipedal gait. Image credit: H. Lorren Au Jr., Orange County

“It is a well-established fact that the skeleton responds to loads during life, adding bone to resist high forces and subtracting bone when forces are reduced,” co-author John Kappelman, a paleoanthropologist at the University of Texas at Austin.

The study is grounded in mechanical engineering principles, lead author Christopher Ruff, a professor of functional anatomy and evolution at the Johns Hopkins University School of Medicine, said in a statement.

Lucy’s legs appear to have been relatively weak, and not well adapted for climbing or walking. The researchers believe that Lucy, like chimpanzees, likely spent a good portion of time in trees, perhaps to escape from predators or to find food.

Christopher B. Ruff , M. Loring Burgess, Richard A. Ketcham, John Kappelman, Limb Bone Structural Proportions and Locomotor Behavior in A.L. 288-1 (“Lucy”), Published: November 30, 2016