Although she was a biped who regularly walked on the ground, the ancient human predecessor known as Lucy was more muscular than modern people and had strong arm bones that suggest she spent a lot of time climbing trees, a newly-published PLOS One study has found.
Led by Christopher Ruff, a professor of functional anatomy and evolution at the Johns Hopkins University School of Medicine, the new study is the first to examine the internal bone structures of “the world’s most famous Australopithecus afarensis,” according to the Los Angeles Times.
The analysis of the approximately 3.2 million-year-old specimen revealed that she had hips, legs, and feet that were clearly adapted for bipdealism, but also had extremely strong arm bones which would have allowed her to easily hoist herself up tree branches, the Washington Post added.
Paleoanthropologist John Kappelman seen with 3D printouts of Lucy’s skeleton (Credit: Marsha Miller/The University of Texas at Austin)
Upper limb strength indicative of daily tree climbing
“Most people have agreed for a while that she did some tree climbing, or had done tree climbing in the recent past,” Ruff told the Times, “but there were a lot of questions about whether it was a major part of her lifestyle. We’re saying she probably used trees on a daily basis.”
Ruff and his colleagues reached their conclusion after analyzing micro-CT scans performed on Lucy’s skeleton at the University of Texas at Austin in 2008. In particular, they found that cross-sectional scans on one remaining thigh bone and two upper arms bones revealed that her upper limb strength was slightly closer to that of a chimpanzee than a modern human.
This suggests that Lucy used her upper limbs significantly more than we normally do, but not quite as often as chimps, which regularly climb trees, the Times explained. Climbing, Ruff said, is the only logical explanation for the bone tissue distribution he witnessed in Lucy’s upper arm bones – “there is really no other explanation for that kind of overloading.”
Lucy was bipedal, except when acquiring food or sleeping
Furthermore, the micro-CT scans indicated that the 3.5 foot tall Australopithecus afarensis did walk on two legs when she was on the ground, based on the shape of her feet and pelvis, Ruff’s team reported in their study. Their analysis paints a picture of a pre-human which likely walked on the ground most of the time, but climbed trees daily to obtain food or to sleep.
By examining Lucy’s pelvis, the researchers also found that she likely would have swayed from side-to-side while walking, forcing her to expend more energy than modern-day humans to move across the land, the Times said. Furthermore, they discovered that her overall bone strength was much greater than ours, indicating that she had far stronger muscles than her progeny.
“Hominins had slowly developed adaptations for walking on the ground, but for millions of years we were still using the trees in a significant way. Really, it was only with evolution of Homo the genus that we became fully committed to the ground,” Ruff told the Post. “This is what makes Lucy so fascinating. She had crossed a lot of thresholds on the path to becoming human, but not all of them.”
Image credit: John Kappelman/University of Texas at Austin
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