Shoulders and elbows likely first evolved as brakes for climbing apes

[Kamaji]

Human shoulders and elbows first evolved as brakes for climbing apes

Study introduces 'downclimbing' from trees as a driver in early-human evolution

Date:  September 5, 2023
Source:  Dartmouth College
Summary:  Researchers report that the flexible shoulders and elbows that allow us to throw a football or reach a high shelf may have evolved as a natural braking system that let our primate ancestors get out of trees without dying. The researchers used sports-analysis software to compare the climbing movements of chimpanzees and small monkeys called mangabeys. While the animals climb up trees similarly, the researchers found that the shallow, rounded shoulder joints and shortened elbow bones that chimps have -- similar to humans -- allow them to fully extend their arms above their heads when climbing down, holding onto branches like a person going down a ladder to support their greater weight. When early humans left forests for the grassy savanna, these versatile appendages would have been essential for gathering food and using tools for hunting and defense. The findings are among the first to identify the significance of 'downclimbing' in the evolution of apes and early humans.

The rotating shoulders and extending elbows that allow humans to reach for a high shelf or toss a ball with friends may have first evolved as a natural braking system for our primate ancestors who simply needed to get out of trees without dying.

Dartmouth researchers report in the journal Royal Society Open Science that apes and early humans likely evolved free-moving shoulders and flexible elbows to slow their descent from trees as gravity pulled on their heavier bodies. When early humans left forests for the grassy savanna, the researchers say, their versatile appendages were essential for gathering food and deploying tools for hunting and defense.

The researchers used sports-analysis and statistical software to compare videos and still-frames they took of chimpanzees and small monkeys called mangabeys climbing in the wild. They found that chimps and mangabeys scaled trees similarly, with shoulders and elbows mostly bent close to the body. When climbing down, however, chimpanzees extended their arms above their heads to hold onto branches like a person going down a ladder as their greater weight pulled them downward rump-first.

Luke Fannin, first author of the study and a graduate student in Dartmouth's Ecology, Evolution, Environment and Society program, said the findings are among the first to identify the significance of "downclimbing" in the evolution of apes and early humans, which are more genetically related to each other than to monkeys. Existing research has observed chimps ascending and navigating trees -- usually in experimental setups -- but the researchers' extensive video from the wild allowed them to examine how the animals' bodies adapted to climbing down, Fannin said.

"Our study broaches the idea of downclimbing as an undervalued, yet incredibly important factor in the diverging anatomical differences between monkeys and apes that would eventually manifest in humans," Fannin said. "Downclimbing represented such a significant physical challenge given the size of apes and early humans that their morphology would have responded through natural selection because of the risk of falls."

"Our field has thought about apes climbing up trees for a long time -- what was essentially absent from the literature was any focus on them getting out of a tree. We've been ignoring the second half of this behavior," said study co-author Jeremy DeSilva, professor and chair of anthropology at Dartmouth.

"The first apes evolved 20 million years ago in the kind of dispersed forests where they would go up a tree to get their food, then come back down to move on to the next tree," DeSilva said.

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Source:  https://www.sciencedaily.com/releases/2023/09/230905202500.htm