Lucy’s beefy arms suggest she was a tree climber

(Credit: Getty Images)

The 3-million-year-old human ancestor known as “Lucy” appears to have spent much of her life climbing trees, CT scans show.

“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,” says anthropologist John Kappelman.

“But Lucy didn’t know she was ‘unique,'” says Kappelman, part of a team from the University of Texas at Austin and Johns Hopkins University that conducted a study published in the journal PLOS ONE. “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.”

Since the world-famous Lucy fossil was discovered in Ethiopia 42 years ago, paleontologists have debated whether she spent her life walking or combined walking with frequent tree climbing.

Lucy fossils
The fossils that make up the Lucy skeleton. (Credit: John Kappelman/UT-Austin)

The new analysis, the investigators say, shows that Lucy’s upper limbs were heavily built, similar to those of today’s tree-climbing chimpanzees, evidence that she used her arms to pull herself up into the tree canopy. The fact that her feet were better adapted for upright walking than grasping may mean that Lucy’s depended even more on her arms, resulting in more heavily built upper limb bones.

Spent her nights in trees

Exactly how much time Lucy spent in trees is difficult to determine, the researchers say, but another recent study suggests her fatal fall was from a tall tree.

This new study adds to evidence she may have nested in trees at night to avoid predators. An eight-hour slumber would mean she spent one-third of her time aloft. If she also occasionally foraged there, the total time above ground would be even greater.

Was hominid ‘Lucy’ about as big as the guys?

Lucy, housed in the National Museum of Ethiopia, is a 3.18 million-year-old specimen of Australopithecus afarensis, or southern ape of Afar. She is among the oldest, most complete fossil skeletons ever found of any adult erect-walking human ancestor.

Previous studies suggest she weighed less than 65 pounds and was under 4 feet tall.

“We were able to undertake this study thanks to the relative completeness of Lucy’s skeleton,” says Christopher Ruff, professor of functional anatomy and evolution at the Johns Hopkins School of Medicine. “Our analysis required well-preserved upper and lower limb bones from the same individual, something very rare in the fossil record.”

‘She is a rock’

During Lucy’s US museum tour in 2008, the fossil detoured briefly to the High-Resolution X-Ray Computed Tomography Facility at the University of Texas. For 11 days, Kappelman and Texas geological sciences professor Richard Ketcham carefully scanned her bones, creating a digital archive of more than 35,000 CT slices.

High-resolution scans were necessary because fossilized Lucy is so heavily mineralized that conventional CT is not powerful enough to image the internal structure of her bones.

“We all love Lucy,” Ketcham says, “but we had to face the fact that she is a rock. The time for standard medical CT scanning was 3.18 million years ago. This project required a scanner more suited to her current state.”

Bipedal ‘Millenium Man’ hung out in trees

The new study used the 2008 scans to quantify the internal structure of Lucy’s upper arm bones and left thigh bone.

“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,” Kappelman says. “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.”

‘The results are convincing’

A major issue in the debate over Lucy’s tree climbing has been how to interpret skeletal features that might be simply evolutionary “leftovers” from a more primitive ancestor with relatively long arms. The new study, Ruff says, focused on characteristics that reflect her actual behavior during life.

The team compared Lucy’s CT scans with scans from modern humans and from chimpanzees, a species that spends more time in the trees than we do.

“The upper limbs of chimpanzees are relatively more heavily built because they use their arms for climbing, with the reverse seen in humans, who spend more time walking and have more heavily built lower limbs,” Ruff says. “The results for Lucy are convincing and intuitive.”

The Paleoanthropology Lab Fund, the University of Texas at Austin College of Liberal Arts, the Houston Museum of Natural Science, and the National Science Foundation funded the research.

Source: Johns Hopkins University