Lucy’s bone scans point to life in the trees

Our hominin ancestor Australopithecus afarensis spent a significant amount of time living in the trees – not just on the ground – a bone study has found.

Christopher Ruff at Johns Hopkins University in the US and colleagues scanned arm and leg bones from Lucy, an A. afarensis skeleton found in Ethiopia in 1974, and found she was well suited to life in the trees. They published their work in PLOS One.

Since Lucy’s discovery, there has been much debate about whether the 3.18-million-year-old hominin was ground-dwelling or preferred the arboreal life.

Lucy’s long arms give clues to a penchant for the high life, along with theories that she may have died falling from a tree.

Now, new research found A. afarensis arm bones fall somewhere between humans and chimpanzees and were likely very strong compared to the species’ leg bones.

Ruff and his crew used CT scans of Lucy’s femur and humerus to create 3-D models, which then let them study the bone structures in detail. Chimpanzee-like proportions coupled with particularly strong arms suggest “a significant arboreal locomotor component”, they write.

“This is the most direct evidence to date that Lucy and her relatives actually spent a significant portion of their time in the trees,” Ruff says.

Interestingly, the study also found Lucy’s gait was probably not as efficient as our stride and would have proved more energy-sapping.

Together, these strongly suggest that A. afarensis may have spent quite a lot of time among the branches.

“Although bipedal when on the ground, the limb bone structural proportions of [Lucy] provide evidence for substantially more arboreal, i.e. climbing behaviour than either modern humans or Homo erectus,” the researchers write.

“Possible reasons for using the trees more often include foraging for food and escape from predators.”

Along the human evolution timeline, it seems tree-dwelling remained a pastime of early human ancestors for a huge chunk of pre-history.