Human ancestor "Lucy" spent significant time in trees: study

Lucy, perhaps the world's most famous early human ancestor whose bones were found 42 years ago this month, has long been at the center of a debate about whether the female hominin spent all of her time walking on the ground or instead combined walking with frequent tree climbing.

Now, high-resolution CT scans by scientists from the Johns Hopkins University and the University of Texas at Austin suggested that the 3.18-million-year-old human ancestor spent a significant amount of time in the trees and that evidence of this behavior is preserved in the internal structure of her bones, according to a study published Wednesday in the U.S. journal PLOS ONE.

Analysis of Lucy's partial fossilized skeleton, the investigators said, showed that all of her limb bones are very strong relative to her body size, indicating that she had exceptionally strong muscles, more like those of tree-climbing chimpanzees than humans.

Specially, the scans showed her upper limbs, similar to chimpanzees, were heavily built, supporting the idea that she spent time climbing and used her arms to move through trees, possibly to forage for food and escape predators.

In addition, analysis of Lucy's upper leg bones suggested that even when Lucy walked upright, she may have done so less efficiently than modern humans, limiting her ability to walk long distances on the ground.

Based on their findings, the researchers concluded that moving through trees may have remained important to some early human ancestors for millions of years.

"This is the most direct evidence to date that Lucy and her relatives actually spent a significant portion of their time in the trees," said Christopher Ruff, a professor of functional anatomy and evolution at the Johns Hopkins University School of Medicine, who led the study.

Exactly how much time Lucy spent in the trees is difficult to determine, the research team said, but another recent study suggests Lucy died from a fall out of a tall tree.

Therefore, "she may have nested in trees at night to avoid predators," the researchers said in a statement. "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 a 3.18 million-year-old specimen of Australopithecus afarensis -- or southern ape of Afar -- and 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 and previous studies suggested she weighed less than 65 pounds (29 kilograms) and was under four feet (1.2 meters) tall.

"It may seem unique from our perspective that early hominins like Lucy combined walking on the ground on two legs with a significant amount of tree climbing," said study coauthor John Kappelman, anthropology and geological sciences professor from the University of Texas at Austin.

"But Lucy didn't know she was 'unique' -- she moved on the ground and climbed in trees, nesting and foraging there, until her life was likely cut short by a fall -- probably out of a tree." Endit