Lucy climbed trees, walked with a wobbly gait and had stronger muscles than humans, according to a new analysis of our ancient ancestor's fossilized remains.
The work, published Wednesday in PLOS One, is the first to mine the internal bone structure of the world's most famous Australopithecus afarensis for information on how the petite hominid lived her life 3.18 million years ago.
"Most people have agreed for a while that she did some tree climbing, or had done tree climbing in the recent past, but there were a lot of questions about whether it was a major part of her lifestyle," said Christopher Ruff, a professor of functional anatomy and evolution at Johns Hopkins University School of Medicine and the lead author on the study. "We're saying she probably used trees on a daily basis."
To come to this conclusion, the authors turned to a micro-CT scan of Lucy's skeleton that was taken at the University of Texas at Austin in 2008. Lucy's fossilized bones had been scanned not long after their discovery in the early 1970s, but the instruments back then were not powerful enough to show the internal structure of her bones.
"Lucy is a fully mineralized fossil, so she's like a rock, and the problem with lower energy CT is that they can't see through rock," said John Kappelman, a UT professor of anthropology and geological sciences who did the recent scans. "Up until 2008, we had had no data at all on the internal structure of her bones. She was radiographically opaque."
Being able to peer into the interior of Lucy's bones has shed a whole new light on how Lucy lived her life, Ruff said.
Like our bones, Lucy's bones respond to pressure. The more force exerted on them, the stronger they become.
For example, the bones in a professional tennis player's playing arm can be 50% stronger than in her non-playing arm. The same is true for pitchers and their pitching arm.
"Bones adapt," Ruff said. "With overuse you add more bone, with underuse you lose bone."
Ruff and his team concentrated on cross-sectional scans of Lucy's one remaining thigh bone and her two remaining upper arm bones. Particularly, they were looking for how tissue was distributed along the bone shaft as an indication of strength.
"It's the same type of analysis that an engineer would use in designing a bridge," Ruff said.
Next, the group compared the relative strength of Lucy's bones to those from a database of more than 1,000 pre-20th century humans and 100 chimpanzees.
Previous work on chimpanzees and gorillas revealed that measurements like these matched up with locomotion behavior. For example, animals that climbed trees had relatively stronger upper limbs compared with those that did not climb trees.
The authors found that Lucy's upper limb strength was intermediate between humans and chimps, but a bit closer to the chimp side. This suggests that she used her upper limbs significantly more than we do, although not as much as chimpanzees, which frequently climb trees.
"You don't retain bone strength genetically, you have to use them or they won't get strong," Ruff said.
He added that it is hard to imagine what other factors besides tree climbing would have created the bone tissue distribution he observed in Lucy's upper arm bones.
"There is really no other explanation for that kind of overloading," he said.
The shape of Lucy's feet and her pelvis clearly indicate that when she was on the ground she walked on two legs, Ruff said. Therefore, he does not imagine that Lucy spent most of her life swinging from branches. However, he said she probably climbed trees daily to gather food and perhaps to sleep.
"She was only 3.5 feet tall and while she had access to very primitive tools, there was nothing that would send off a large animal," Ruff said. "It seems logical that she would be using the trees for sleeping at least."
He also notes that a reexamination of Lucy's pelvis suggests that she would have swayed left and right as she walked, requiring more energy than we do to move across land. This would limit her ability to walk great distances and might also explain why she was more likely to spend time in trees than we might.
Ian Tattersall, a paleoanthropologist at the American Museum of Natural History in New York, said the study provides additional evidence that the long arms of A. Aferensi were not "evolutionary baggage" as some had suggested.
"The authors show quite convincingly that bone architecture in A. afarensis supports the notion of significant arboreal activity," said Tattersall, who was not involved in the work. "We need to view the australopiths not as some kind of 'transitional' form, but as stably adapted to a lifeway that was neither that of great apes nor of Homo."
Ruff said Lucy's overall bone strength was much higher than modern humans', suggesting she also had much stronger muscles than we do.
This could provide insight into the types of trade-offs that were made when humans developed their large brains, he said.
About 2 million years ago an interesting shift happened in the evolution of our genus — bones became less strong (and by association, muscle strength decreased), and brain size increased.
"Brains are metabolically expensive," he said. "It's possible that when you are feeding the brain, you take resources away from the muscles."
Therefore, he suggests that another difference between Lucy and us is that she used her brawn to gather food and protect herself from predators, while modern humans use their brains.
MORE IN SCIENCE
5 p.m., Dec. 1: This article was updated with comments from Ian Tattersall of the American Museum of Natural History in New York.