Lucy, Australopithecus afarensis
Lucy, a fossil of the early human ancestor Australopithecus afarensis, had strong muscles and probably spent a lot of her time in trees. Dave Einsel / Getty Images

The 3.18-million-year-old fossil known as Lucy – an ancestor of modern humans – suggests that she spent a lot of time in trees. Her forearms were muscular and she was strong relative to her body size, a new study finds.

Bone scans show that Lucy, a member of the Australopithecus afarensis species, had arms that were strong relative to her leg bones, and that walking was probably not the most efficient method for getting around for her, according to a study published in the journal PLOS ONE.

The researchers scanned the bones of the 3.18-million-year-old fossil known as Lucy to see how much she used her arms and her legs. The relative strength of Lucy's arms and legs was in between chimps and humans, meaning she was probably not as good at climbing as chimps but better than humans.

Lucy's feet were better adapted to upright walking than to grasping trees, however, adding to her need for particularly strong arms while climbing.

These habits are recorded in Lucy's fossilised skeleton. "It is a well-established fact that the skeleton responds to loads during life, adding bone to resist high forces and subtracting bone when forces are reduced," says John Kappelman of the University of Texas and an author of the paper.

"Tennis players are a nice example: studies have shown that the cortical bone in the shaft of the racket arm is more heavily built up than that in the non-racket arm."

Lucy's bones
The fossils that make up the Lucy skeleton. John Kappelman/University of Texas at Austin

This is the most direct evidence yet that Lucy spent a lot of her time in trees, says Christopher Ruff from Johns Hopkins University School of Medicine in the US, an author of the paper.

"We were able to undertake this study thanks to the relative completeness of Lucy's skeleton," says Ruff. "Our analysis required well-preserved upper and lower limb bones from the same individual, something very rare in the fossil record."

Lucy is thought to have weighed less than 39 kilograms (65 pounds) and was under 4ft tall.

"It may seem unique from our perspective that early hominins like Lucy combined walking on the ground on two legs with a significant amount of tree climbing," says Kappelman. "But Lucy didn't know she was unique – she moved on the ground and climbed in trees, nesting and foraging there, until her life was likely cut short by a fall – probably out of a tree."