According to a new study published in the journal PLoS ONE, the relative strength of arms and legs of ‘Lucy’ — a 3.18-million-year-old specimen of Australopithecus afarensis — was in between that of modern chimpanzees and modern humans.
A group of Australopithecus afarensis. Image credit: Matheus Vieeira.
Early human ancestors are thought to have walked on the ground to some degree, but determining how much — and how well — has been difficult.
This is partly because their long arms could have been remnants of their forebears rather than adaptive in their own lives.
To assess how much Australopithecus afarensis used their arms in locomotion, a research team led by Johns Hopkins University School of Medicine scientist Dr. Christopher Ruff compared X-ray microtomography scans of Lucy’s upper arm bone (humerus) and upper leg bone (femur) with those of modern chimpanzees and modern humans.
The scans revealed that the relative strength of her humerus and femur was intermediate between those of today’s chimpanzees and humans.
This suggests that Australopithecus afarensis spent a significant amount of time using their arms to move through trees, possibly to forage for food and escape predators.
In addition, analysis of Lucy’s femur suggests that Australopithecus afarensis’ walking gait may have been less efficient than that of modern humans, limiting their mobility on the ground.
Based on their findings, Dr. Ruff and co-authors suggest that moving through trees may have remained important to some early human ancestors for millions of years.
“This is the most direct evidence to date that Lucy and her relatives actually spent a significant portion of their time in the trees,” Dr. Ruff said.
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C.B. Ruff et al. 2016. Limb Bone Structural Proportions and Locomotor Behavior in A.L. 288-1 (Lucy). PLoS ONE 11 (11): e0166095; doi: 10.1371/journal.pone.0166095
