MORE THAN THREE million years ago, ancient ancestors of humans were walking upright — but they could still climb trees like monkeys, a study showed Thursday in the US journal “Science.”

Based on careful analysis of a pair of shoulder blades — both exceptionally well-preserved from a skeleton of a three-year-old Australopithecus afarensis girl — scientists were able to settle a question intensely debated by anthropologists for more than three decades.

The girl, called Selam, lived some 3.3 million years ago, and is from the same species as Lucy, one of the earliest hominid skeletons ever uncovered.

Selam was discovered in 2000 in the Ethiopian region of Dikika. For 11 years after that, Zeresenay Alemseged of the California Academy of Sciences worked carefully with Kenyan lab technician Christopher Kiarie to extract her shoulder blades from the rest of the skeleton.

“Because shoulder blades are paper-thin, they rarely fossilize — and when they do, they are almost always fragmentary,” Alemseged said in a statement.

“So finding both shoulder blades completely intact and attached to a skeleton of a known and pivotal species was like hitting the jackpot.”

Alemseged said the study suggests that our ancestors stopped climbing behavior much later than suggested by previous research.

Once the shoulder blades were extracted, Alemseged and co-author David Green of Midwestern University compared them to other early human relatives and to adult skeletons of Australopithecus afarensis.

They also compared the bones to those of modern apes.

“When we compared Selam’s scapula with adult members of Australopithecus afarensis, it was clear that the pattern of growth was more consistent with that of apes than humans,” Green said.

“It’s really an interesting transition: they show evidence for bipedalism,” or walking on two feet, he told AFP. “But we think they maintained the adaptation for climbing as well. They climbed to escape predators, for food.”

Green said it remains unclear just when humans shifted away from climbing.

By the time homo erectus emerged around 1.9 million years ago, the skeleton had changed significantly: “it’s much longer, leaner, with proportions more like those of modern humans,” he said.

But “between Lucy, which is about 3.5 million years ago, to the homo erectus, there is a long gap we are trying to fill with fossils to understand,” Green explained.

Alemseged said the new information helps fill in the picture of the evolution towards homo sapiens.

“This new find confirms the pivotal place that Lucy and Selam’s species occupies in human evolution,” he said.

“Though not fully human, A. afarensis was clearly on its way.”