Lucy in the Sky: UT Study Says Ancient Human Ancestors Climbed Trees

UT Austin professor John Kappelman with 3D printouts of Lucy’s skeleton illustrating the compressive fractures in her right humerus that she suffered at the time of her death 3.18 million years ago. (Photo by Marsha Miller/UT Austin)

It’s 3.2 million years ago and you’re traveling through the forests of Ethiopia. There’s no familiar sign of civilization in sight, though the wet, warm weather might feel like any other day. Suddenly, there’s a rustle in the trees overhead. Maybe it’s the flap of a bird’s wings, or a fruit come loose from its branch. Or maybe it’s not a what but a who, peering down from her nest above—the most famous of our ancient human ancestors, Lucy.

In a recent study published in the journal PLOS ONE, UT-Austin professors John Kappelman and Richard Ketchum and Johns Hopkins University School of Medicine professor Christopher Ruff report evidence suggesting that Lucy was a “tree climber.” It’s estimated that Lucy and her species Australopithecus afarensis—or southern ape of Afar—spent around one-third of their time in trees, likely building nests and picking fruits and nuts to eat. 

“Our study shows the way Lucy’s skeleton is built and the way her upper limbs are very strongly built—more like a chimpanzee,” Kappelman says. “And on that basis, we argue that she very likely climbed in trees.”

Since Lucy was found in Ethiopia 42 years ago by paleoanthropologist Donald C. Johanson, there has been debate surrounding whether her kind spent the majority of time terrestrially on foot or also frequently climbed trees. With more than 40 percent of her skeleton recovered, it’s known that Lucy was built for bipedalism, a trait that allows humans to walk upright on two feet. Some experts maintain that she was solely a terrestrial animal, arguing that her strongly built upper limbs—characteristic of tree-climbing chimpanzees—may just be a “leftover” trait from her own ancestors. Kappelman, however, thinks differently.

“Our study helps to resolve the debate that’s been raging for well over 40 years about whether or not human evolution included a phase where these early human ancestors were both in the trees and on the ground,” he says. “Older ideas had ancestors in trees then boom—on the ground. But no in between. This argues the combined locomotion.”

Around eight years ago, Kappelman and Ketcham took CT scans (a series of X-ray images) of Lucy’s body, which is estimated to have been about 3 feet 6 inches tall and weighed 60 pounds, to create a digital archive of her bones that they could continue using to do research. In their latest study, the researchers focused on comparing Lucy’s upper arm bones and her thigh bone. Like the way the bone in a tennis player’s dominant arm builds up over time and use, Ruff said their research examined characteristics that showed Lucy’s actual behaviors while she was alive.

“Our study is grounded in mechanical engineering theory about how objects can facilitate or resist bending,” Ruff says. “Our results are intuitive because they depend on the sorts of things that we experience about objects, including body parts, in everyday life. If, for example, a tube or drinking straw has a thin wall, it bends easily, whereas a thick wall prevents bending. Bones are built similarly.”

The study also supports the researchers’ findings, which made national headlines in August, that suggested how Lucy most likely died: by falling from a tree. Kappelman found that the end of Lucy’s right humerus was severely fractured and had not healed before her death. He says a series of additional fractures on Lucy’s bones showed that she had experienced “severe trauma,” suggesting that Lucy must have fallen from a height of more than 40 feet.

Though some outside experts are impressed with the work that Kappelman and his colleagues have done, some critics argue that more still needs to be studied. Nathaniel Dominy, an evolutionary biologist at Dartmouth College, said in an interview with The New York Times that he considers it unlikely that Lucy’s kind lived in the trees. “For me, the much more likely scenario is that she was climbing for food,” he said.

Kappelman received permission to upload the bone data online. It can be accessed at, where scientists and schoolchildren are able to download three-dimensional renderings and inspect them on a computer. Though Kappelman says their research started long ago, he and his colleagues aren’t done with Lucy yet.

“We’ve got more stuff in the works on her as well,” Kappelman says. “I like to say she’s the fossil that just keeps on giving.”

UT-Austin professor John Kappelman with 3D printouts of Lucy’s skeleton illustrating the compressive fractures in her right humerus that she suffered at the time of her death 3.18 million years ago. Photo by Marsha Miller/UT-Austin.


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