Despite the ability to walk upright, early relatives of humanity represented by the famed "Lucy" fossil likely spent much of their time in trees, remaining very active climbers, scientists report in the Oct. 26, 2012, issue of the journal Science.
While scientists have known Lucy, an Australopithecus afarensis skeleton, and her kin were no knuckle-draggers, they have long debated how much time they spent in trees. The answer could reveal information about evolutionary forces that shaped the human lineage. (Shown here, a fragment of Lucy's lower arm bone.)
To help resolve this controversy, scientists looked at two complete shoulder blades from the fossil "Selam," an exceptionally well-preserved skeleton of a 3-year-old A. afarensis girl dating back 3.3 million years from Dikika, Ethiopia. The arms and shoulders can yield insights on how well they performed at climbing. (Shown here, Selam's cranium, face and mandible.)
Researchers spent 11 years carefully extracting Selam's two shoulder blades from the rest of the skeleton, which was encased in a sandstone block. Here, a dorsal view of Selam's cranium, with the occipital bone and vertebral column visible. Dorsal views of both the complete right and the fragmentary left scapulae.
"Because shoulder blades are paper-thin, they rarely fossilize, and when they do, they are almost always fragmentary," Zeresenay Alemseged, a paleoanthropologist at the California Academy of Sciences. "So finding both shoulder blades completely intact and attached to a skeleton of a known and pivotal species was like hitting the jackpot." Here, another view of Selam's cranium and mandible, with the vertebrae and complete right scapula visible.
The researchers found these bones had several details in common with those of modern apes, suggesting they lived part of the time in trees. For instance, the socket for the shoulder joint was pointed upward in both Selam and today's apes, a sign of an active climber. In humans, these sockets face out to the sides. Shown here, Selam's cranium as well as the complete right scapula and the right ribs.
The tiny right fossil shoulder blade of a 3-year-old Australopithecus afarensis female discovered in Dikika, Ethiopia who died 3.3 million years ago is held by lead author David J. Green of Midwestern University. The left blade (not pictured) is also preserved and both fossils display evidence that this early bipedal species maintained adaptations to climbing trees.
Here, a right lateral view of Selam's cranium, face and mandible, with the upper and lower dentition visible.