Lucy, our famous ancestor, was built for tree-dwelling

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A composite image of Lucy the Australopithecus (center) and two Malapa hominins (sides.)  Photo by Peter Schmid.

A composite image of Lucy the Australopithecus (center) and two Malapa hominins (sides.) Photo by Peter Schmid

Lucy, our ever-popular human ancestor, may have preferred a tree-dwelling lifestyle, based on bone scans published Wednesday in PLOS ONE. The research adds clarity to early human behavior and suggests our ancestors may have spent millions of years “monkeying around” the branches.

Since American paleoanthropologist Donald Johanson discovered Lucy’s remains in 1974, they have traveled the world to be carefully examined. Scientists have determined that she certainly walked upright like a human and had proportions somewhere between a human and a chimp. However, controversy and questions still surround how she behaved.

Lucy had long arms like a chimp, but did she move and live like one? Or did she merely inherit those leftover features from a tree-dwelling ancestor?

By analyzing high-resolution X-ray scans of Lucy’s upper arm bone, evolutionary anatomist Christopher Ruff demonstrated that early hominins developed arm strength through consistent use — likely by tree-climbing.

“She was still climbing trees on a regular basis,” Ruff said of his team’s new study. “You don’t develop this kind of strong upper-limb bones if you climb a tree once a week.”

Ruff’s team also examined Lucy’s femur and concluded that her walking gait would have been less efficient than humans.

Lucy’s inherited her long arms, so these features don’t expose much about her day-to-day behavior. However, the strength of your limb bones is a more “plastic” trait that changes based on how you use them as you grow. That’s why Lucy’s strong arms indicate that she was, in fact, supporting her weight in trees.

Scientists have speculated for a long time that Lucy and her family must have spent at least some time in trees, especially as recent analysis has demonstrated that she died falling out of one. That study found injuries at or around the time of Lucy’s death are consistent with wounds suffered by people who have fallen from a great height and then have put their arms in front of them to break the impact. Ruff noted that those results are further evidence of tree-dwelling.

But other experts disagree and believe Lucy lived a more terrestrial life. Evolutionary anatomy professor Carol Ward, who focuses on apes and early hominins, said that Lucy had many more adaptations for living on the ground.

For example, humans and Lucy have flat feet, which are suited for walking on the ground. Plus, tree-dwelling apes have grasping big toes, with feet that look like hands.

“We gave that up, Lucy gave that up, in favor of feet that were better at being on the ground.” Ward said. “So not only do we know that the most important thing was for these animals to be able to move effectively on the ground, we also know that being in the trees wasn’t very important to them.”

However, scientists agree that Lucy and her Australopithecus afarensis family moved in both land and tree environments.

“The question in some ways isn’t whether Lucy was able to climb trees,” Ward said. “My kids climb trees, people climb trees now.”

Likewise, tree-dwelling apes can walk on the ground when needed, but not as well or as upright as a human or Australopithecus.

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