Our ancestor "Lucy" spent at least a third of her life in trees 3.18 million years ago, according to new research that could help explain how our species evolved brain power over brawn.
Lucy (Australopithecus afarensis) turns out to have been built like a muscular champion tree-climbing chimpanzee in her upper body, but she walked on two legs, concludes the new study, published in PLOS ONE.
"Lucy probably walked on the ground in a way similar to us, but with more side-to-side sway with each step," lead author Christopher Ruff, a professor of functional anatomy and evolution at the John Hopkins University School of Medicine, told Seeker. "This would have taken more energy, and may have limited her ability to travel long distances on the ground."
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Using CT scans, Ruff and his team analyzed the internal structure of Lucy's upper arm bones and thigh bones. Lucy's remains, found in the Afar region of Ethiopia 42 years ago this month, represent one of the most complete fossil skeletons ever found of any adult, erect-walking human ancestor.
The researchers next compared Lucy's scans to those of Paleolithic and modern humans, who spend the majority of their time walking on two legs on the ground, and with chimpanzees, which spend more of their time in trees. When on terra firma, chimps usually walk on all four limbs.
Lucy's upper limbs were heavily built because they were often used for tree climbing, Ruff and his colleagues concluded. Our species, on the other hand, tends to have more heavily built lower limbs, although certain athletes also have impressive upper body strength.
"Lucy would not have looked more like a modern gymnast, necessarily, although her ratio of arm to leg bone strength would have been more similar to a gymnast than to other people," Ruff said.
A day in the life of Lucy might have gone like this:
"Like modern African apes, she likely spent a good deal of time during the day on the ground, foraging, resting and traveling-at least short distances," Ruff said. "Some food items, including many fruits, were likely obtained by climbing trees. Trees would likely also have been used for sleeping, again as in modern African apes."
To this day, diets including plenty of tree-sourced items, such as nuts and fruits, are often recommended. And if you feel comforted by trees, that attraction could go way back to Lucy, who probably slept for around eight hours a night in them.
Lucy's story doesn't have a happy ending, though. She died before reaching old age, from injuries sustained after falling out of a tree. So it's possible that our ancestor's transition from tree-swinging apes to terrestrial beings went through some awkward stages.
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Ruff, however, was quick to point out that even chimps, who are well adapted to tree-swinging also fall out of trees, sometimes fatally. Then again, he added, "if she spent more time in the trees overall, there would be a higher chance of this happening than if she were essentially only ground-living, obviously."
He and other anthropologists suspect that a drying of the climate around 2.5 million years ago in Africa could have led to increased patchiness of forest areas, increasing the advantage of more efficient terrestrial locomotion between those regions. Changes in diet, ecology and even social organization might have also compelled Lucy's later relatives to leave the trees.
There were probably trade-offs between brain expansion and muscle size during human evolution.
Ruff explained, "The brain is a metabolically very expensive organ. Muscle tissue also requires energy input to grow and maintain. Lucy was quite muscular, judging from her relative bone strength, but she had a small brain. Reductions in average muscularity later in human evolution could have freed up metabolic resources for a larger brain."
William Jungers is a distinguished teaching professor emeritus in anatomical sciences from Stony Brook University Medical Center told Seeker that the new paper "independently corroborates other evidence" that Lucy and her kind were capable of both two-legged walking on the ground and movement in trees. Jungers believes "this combination was a long-lived and very successful locomotor adaptation in the human career."
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