Shoulder fossil may put Lucy’s kind up a tree

Contested analysis portrays ancient hominid as frequent climber

The ancient hominid known as Lucy is getting shouldered into the trees by a recently uncovered fossil child. But scientific onlookers disagree about whether Lucy’s long-extinct species mixed tree climbing with walking.

The right shoulder blade from a 3.3-million-year-old fossil child provides evidence that members of an ancient hominid species regularly scaled trees as well as walked across the landscape. Courtesy of Zeresenay Alemseged/Dikika Research Project

Apelike shoulder blades from the ancient skeleton of a roughly 3-year-old girl that belonged to Australopithecus afarensis — the same species as Lucy, a famous 3.2-million-year-old partial female skeleton found in 1974 — suggest that these early members of the human evolutionary family split time between scrambling up trees and walking on the ground, say paleobiologist David Green of Midwestern University in Downers Grove, Ill., and anthropologist Zeresenay Alemseged of the California Academy of Sciences in San Francisco.

Scientists have argued for more than 30 years about whether A. afarensis was built mainly for walking or possessed physical attributes suitable for ascending trees as well. Shoulder blades of a fossil child discovered in 2000 in Dikika, Ethiopia, indicate that Lucy’s crew could indeed scale trees beginning early in life, Green and Alemseged report in the Oct. 26 Science.

A. afarensis inhabited East Africa 3 million to 4 million years ago.

Lucy and her cohorts spent plenty of time on foot but climbed trees to forage for fruits and to escape predators, Green proposes. Based on the new analysis of the Dikika fossils, he says, “juvenile members of A. afarensis may have been more active climbers than adults.”

A previous analysis of the Dikika child, dubbed Selam by its discoverers, suggested that the youngster’s shoulder blades — partly encased in rock at the time — resembled those of gorillas (SN: 9/23/06, p. 195). Green and Alemseged have since freed the fossils from surrounding rock. Comparisons to other hominid fossils, modern apes and humans suggest that Selam’s shoulder blades are generally apelike enough to have enabled regular tree-climbing.

As in living apes, Selam and Lucy had upward-pointing shoulder sockets, Green and Alemseged say. People are born with slightly downward-pointing shoulder sockets that eventually shift to face laterally.

A bony ridge on the back of Selam’s shoulder blades runs diagonally, as in living apes, the researchers add. The same ridge runs horizontally across the top of people’s shoulder blades.

Selam’s apelike shoulders represent an evolved accommodation in A. afarensis for mixing walking with climbing, writes anthropologist Susan Larson of Stony Brook University in New York in the same issue of Science.

But Green and Alemseged need to conduct a more detailed analysis of various bony landmarks on Selam’s shoulder blades to support a scenario of frequent tree climbing, says anthropologist Yohannes Haile-Selassie of the Cleveland Museum of Natural History.

Haile-Selassie led a team that discovered a 3.6-million-year-old partial male A. afarensis skeleton known as Big Man (SN: 7/17/10, p. 5). Everything about that find, including its legs, chest and a surviving shoulder blade, point to a nearly humanlike gait for Lucy’s species, he says.

If Green and Alemseged show in future work that the Dikika child’s leg and foot worked in concert with her shoulder to promote climbing, Haile-Selassie says, “then they may very well have a species different from A. afarensis.”

Skeletal traits, such as the shoulder socket’s orientation, can pass from an ancestral species to a descendant species because they’re crucial for survival or as functionally irrelevant hangers-on, says anthropologist Carol Ward of the University of Missouri in Columbia. “This makes the significance of the Dikika shoulder blades difficult to interpret.”

A. afarensis probably retained some shoulder characteristics of earlier tree-climbing hominids that had no impact on what had become a ground-based lifestyle, says anthropologist Owen Lovejoy of Kent State University in Ohio. Lovejoy participated in the investigation of Big Man’s remains.

Bruce Bower has written about the behavioral sciences for Science News since 1984. He writes about psychology, anthropology, archaeology and mental health issues.

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