Her partial skeleton was discovered embedded in the sandstone rocks of Ethiopia’s Afar desert a dozen years ago by Zereseney Alemseged, the noted anthropologist at the California Academy of Sciences. He named her Selam and still works to reconstruct her life.

After 11 years of painstaking laboratory analysis, the anthropologist and his colleagues reported the findings Friday in the journal Science. Although little Selam and all her families were clearly bipedal, the intricate shape of her two fossilized shoulder blades shows that their lifestyles must have involved “a substantial amount of climbing,” the scientists wrote. They may well have nested in trees at night and sought refuge there by day, Alemseged said.

Their shoulder bones also resembled those of tree-climbing gorillas, he said.

The fossil child was the same species as Lucy, the famed adult woman in the line known as Australopithecus afarensis whose skeleton was discovered in the Afar in 1974. Lucy’s bones were dated at 3.2 million years ago, but Selam was alive many generations earlier, about 3.3 million years ago.

Discussing his report this week, Alemseged said that over the long course of human evolution, Selam’s shoulder blades – her scapulae – show that she and all of Lucy’s species “had not fully abandoned the arboreal lifestyle, which would have been useful for nesting in trees, evading predators and provisioning themselves.”

“This was a significant adaptation, that enabled this short-statured hominin, with no sophisticated tools, to survive in a dangerous landscape filled with large felines and other carnivores,” Alemseged said.

Shoulder blades

The specialized structure of those shoulder blades in Selam’s and Lucy’s species wasn’t just a useless leftover from evolutionary ancestors but a crucial key to their ability to survive during their lifetimes, he concluded.

Alemseged co-authored his study with David J. Green, a specialist in ancient fossil bone structures at Midwestern University College of Dental Medicine in Downers Grove, Ill.

Their conclusions are supported in a “Perspectives” commentary published in the same issue of Science by Susan Larson, an anatomy professor and expert in fossil and human bone structures at Stony Brook University in New York.

“Overhead use of their upper limbs to climb and balance in trees remained part of their overall survival strategy,” she wrote of the species.

In an e-mail, she added: “While a modern human 3-year-old is still considered a toddler, a 3-year-old chimp or gorilla are quite capable of walking with the troop and of climbing trees. In fact, they are very adept at moving in trees. It seems likely, therefore, that Selam was similarly able to climb as well as move independently.”

But two other noted anthropologists familiar with Alemseged’s report argued this week that he’s wrong.

‘Not adapted’

“No doubt it could climb trees,” said Kent State University anthropologistOwen C. Lovejoy, a veteran leader in studies of Lucy and her species, in an e-mail about Selam. “But it was certainly not adapted to living in them after 3 million years of adapting to walking upright and living on the ground.”

Yohannes Haileselassie of the Cleveland Museum of Natural History, another leading anthropologist working in the Afar region, said other shoulder blade fossils of Selam’s species would place her close to the average of those bones in humans and “weaken the argument” that she was a possible tree climber – or a tree dweller in emergencies.

David Perlman is The San Francisco Chronicle‘s science editor. E-mail:dperlman@sfchronicle.com