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Oct. 25, 2012

Lucy in the Trees?

by Charles Bergquist

Click to enlarge images
Our hominid ancestors may have spent time in the trees more recently than previously thought, according to a paper published this week in the journal Science. Researchers examined the shoulder blades of an Australopithecus afarensis child nicknamed "Selam" and found that, much like a ape's shoulder blades, the fossil bones were adapted for spending significant amounts of time off the ground. Selam's shoulder joint socket is pointed upward, rather than facing out to the side as in modern humans. That upward-pointing arrangement, which still exists in apes, is the mark of an active climber, the researchers say.
 
Australopithecus afarensis is the hominid species known largely to the public through "Lucy," a set of fossil remains discovered by paleoanthropologist Donald Johansen and colleagues in Ethiopia in 1974. "Selam," who is sometimes referred to as "Baby Lucy," was an approximately 3-year-old Australopithecus afarensis girl found in 2000 in Dikika, Ethiopia, just a few miles away from the "Lucy" site.  The Australopithecus afarensis fossils have been dated to around 3 million years ago.
 
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The "Selam" fossils are the most complete Australopithecus afarensis remains yet found. The researchers on the new study say they needed 11 years to extract the fragile shoulder blades from the sandstone matrix surrounding them. After making digital models of the shoulder blades, they compared them to those of modern humans, chimps, gorillas, orangutans, and with the shoulder blades of other early human relatives such as "Turkana Boy" and Homo floresiensis ("The Hobbit").
 
The researchers were quick to point out that they were not claiming that the Australopithecus afarensis species primarily lived in the trees -- the hip bones, feet, and legs of Australopithecus afarensis appear adapted for upright walking as in modern humans. Rather, they say, the work "strengthens the hypothesis that these hominins participated in a behavioral strategy that incorporated a considerable amount of arboreal behaviors in addition to bipedal locomotion."
 
"These remarkable fossils provide strong evidence that these individuals were still climbing at this stage in human evolution," said David Green of Midwestern University, who co-authored the report with Zeresenay Alemseged of the California Academy of Sciences. "This study moves us a step closer toward answering the question 'When did our ancestors abandon climbing behavior?,'"  Alemseged added. "It appears that this happened much later than many researchers have previously suggested."
 
{"input":{"width":490,"photo":"selamback","row":"4477","table":"DOCUMENT"}}
"Their lower limb features leave little doubt that these early members of our lineage walked on two legs," explained Susan Larson of Stony Brook University, commenting on the findings in Science.  However, she said, the shoulder bone shape in Selam was noticeably different from that seen in humans. "Humans do various things with their upper limbs, but without a major focus on overhead postures their scapulae follow a very different growth trajectory," she said.  
 
The question of when exactly human ancestors came down from the trees remains open. The tree-optimized shoulder arrangement seen in Selam is not present in Turkana Boy (an approximately 1.8 million year old find classified as either Homo erectus or Homo ergaster). "This reconfiguration was likely part of the emergence of our own genus Homo and a growing dependence on tools and culture for survival," Larson said.
About Charles Bergquist

Charles reminds Ira when it's time to stop talking, helps wrangle this Web site, and produces segments for the radio program. His favorite stories involve chemistry, inventions, nanotechnology, and shiny things with blinking lights.

The views expressed are those of the author and are not necessarily those of Science Friday.

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