Monkey elbows and shoulders are turned differently than monkeys, which allows us to throw more accurately

Mary Joy is a graduate student at Dartmouth University in the Department of Anthropology – and, in addition, she is a conservationist and an athlete. Indeed, Joy’s interest in sports has inspired her scientific endeavors in at least one important way: They have helped her develop a hypothesis about the evolution of the human shoulder, elbow and wrist. Since humans appear to use rapid movements with a greater range of motion when running downhill, does the same increased range of motion exist in primate shoulders and elbows to reduce fatigue and muscle contraction when running downhill?

Joy is now the co-author of a scientific paper, recently published in the peer-reviewed journal Royal Society Open Science, that provides the real evidence behind that theory (which later became her thesis). To provide this evidence, Dartmouth researchers studied the “up” and “down” movements of both wild chimpanzees.Pan troglodytes) and sooty mangabeys (Cercocebus ays, kind of little monkey), using sports-analysis and statistical software.

While comparing videos and photos, they noticed that chimpanzees did not descend in the same way as mangabeys: while mangabeys grow trees with their shoulders and elbows bent close to their bodies, chimpanzees support their weight by extending their arms far above their heads. .

Thanks to this research, scientists can now begin to answer one of the great mysteries of primate evolution: Why do monkeys have more mobility in their shoulders, elbows and wrists than monkeys?

“Researchers have been puzzled by this question for a long time, most of the discussions focused on (a) the promotion or (b) the suspension of the department,” Dr. Nathaniel J. Dominy, co-author and professor of anthropology at Dartmouth University told Salon. by email. The new study focuses on the importance of descending, as “a controlled descent is more challenging than ascending even though gravity is the same on both sides, so it follows that monkeys and apes must use their shoulders and elbows differently during descent, and they do!”

Mangabey Close (Photo by Luke Fannin)

This works especially well for chimpanzees, who are heavier than apes and therefore use their shoulders and elbows more than apes “to increase the use of their feet as brakes, a position known as ‘laybacking’ among rock climbers,” Dominy said. The study is the first to show these differences in body movement, or kinematics, during vertical climbing.

“But if you ask any rock climber,” said Dominy, “they’ll tell you that going down is harder than going up, even with all the advantages of our very flexible monkey shoulders and elbows.

Joy explained the importance of continuing to learn to descend, as this seemingly simple act includes movements that can help unlock some of the secrets of human evolution.

“In my view, it shows that descent is an evolutionary force that may have influenced primate — and ultimately human — skeletal morphology, as opposed to residual movement,” Joy told Salon via email. “In other words, it’s a powerful mechanism of movement in itself, and it should be studied more.”

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To explain why he felt this way, Joy noted that our bodies may have evolved to adapt to changes in our environment and lifestyles. When viewed from this vantage point, it makes sense that creatures that often need to transport their large bodies through trees would benefit from anatomical features that act as brakes. It just so happens that these same features make it easier to play structured games, create complex tools and perform other tasks that are directly related to people.

“For me, it made me think a lot about how some forms of human movement may have started as safety (or other) adaptations to the movements that our ancestors would have made as they moved between paths and habitats,” Joy. he explained. “Based on our study, it seems that our common ancestors developed this large range of motion in their upper arms in response to the need to descend safely, but even without stress, we use this same coordination to stretch, throw balls (or tools) and climb rocks for fun.”

It is easy to imagine how the decline of humans led to these evolutionary problems. As Dominy puts it, these careful climbing behaviors “would have favored the rigidity of the spine and the heavy lumbar vertebra needed to walk upright, as well as the shoulders and elbows equipped for throwing things and making tools.” The end result was that “climbing gave us the anatomical necessary to succeed as hominins in the savanna-woodlands of Africa.”

It also gave the Dartmouth students behind the project an experience they will never forget. Dominy himself said that “this study is led by two students, and I will never tire of seeing the joy of discovery when students see the first results of their hard work.” In this case that hard work “improves our understanding of how primate bodies evolved to solve important problems safely and efficiently. Our bodies are the legacy of that process.”

It also had the unintended effect of allowing students to develop relationships with the monkeys they saw. As Joy recalled, “I actually really enjoyed watching and mimicking the behavior of the mangabeys climbing in the videos. They are lovely animals, and although they were different people, I named them all Geronimo.”