Monkeys’ elbows and shoulders evolved differently from monkeys, allowing us to shoot with precision

Mary Joy is an undergraduate student in the Department of Anthropology at Dartmouth University – as well as a climber and runner. In fact, Joy’s athletic interests fueled her scientific pursuits in at least one important way: they helped her develop a hypothesis about the evolution of the shoulders, elbows, and wrists in humans. And since humans seem to use fast movements with a large range of motion when running downhill, could the same increased range of motion be found in the shoulders and elbows of primates to reduce fatigue and muscle contraction when they’re going downhill?

Alan Joy is co-author of a paper, recently published in the peer-reviewed journal Royal Society Open Science, that provides real evidence behind this hypothesis (which later became her thesis). To provide this evidence, the Dartmouth researchers studied the “up” and “down” movements of wild chimpanzees (Pan Caves) and soot mangabee (circus Attis, little monkey type), using mathematical analysis and statistical software.

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

Thanks to this discovery, scientists can now begin to answer one of the great mysteries of primate evolution: Why do apes have a much greater range of motion in the shoulder, elbow and wrist joints than do apes?

Dr. Nathaniel J. Dominy, co-author of the study and professor of anthropology at Dartmouth University, told Salon: “Researchers have long puzzled over this question, with most arguments focused on (a) climbing up or (b) hanging down a branch.” by email. The new study focuses on the importance of climbing, as “a controlled descent is more challenging than an ascent even though gravity is identical in both directions, so it follows that monkeys and monkeys must use their shoulders and elbows differently while climbing, and they do!”

Mangabe close-up (Photo by Luke Fannin)

This is especially true of chimpanzees, Dominy said, which are heavier than monkeys and therefore use their shoulder and elbow joints more widely than monkeys “to maximize the use of their feet as brakes, a position known as ‘reclining’ among human rock climbers.” This study is the first to show these differences in body motion, or kinematics, during vertical climbing.

“But if you ask any rock climber, they’ll tell you that it’s harder to climb down than to climb up, even with all the advantages of our super-flexible shoulders and elbows,” Dominy said.

Joy explained the importance of continuing to study climbing, as this seemingly simple act contains movements that could help unlock some of the mysteries of human evolution.

“In my view, this suggests that climbing is a potential evolutionary force with implications for skeletal shape of primates – and ultimately humans – rather than just residual motion,” Joy told Salon by email. “In other words, it is a powerful form of movement in its own right and should be studied further.”

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To explain why she feels this way, Joy notes that our bodies likely evolved to adapt to changes in our environments and lifestyles. Viewed from this perspective, it makes sense that creatures that constantly need to move their bulky bodies through trees would benefit from anatomical features acting as brakes. It just so happens that these same features make it possible to play organized sports, craft sophisticated gadgets, and do a host of other activities specifically related to humans.

“For me, it made me think a lot about how other forms of human locomotion arose as safe (or other) adaptations of locomotion that our common ancestors might have had to make as they transitioned between living patterns and environments.” to explain. “Based on our study, it seems plausible that our common ancestors evolved this greater range of motion in their upper arms in response to the need for safer climbing, but even in the absence of this pressure we use the same adaptation to stretch, throw balls (or weapons) and climb rocks more or less. entertaining.”

It is easy to envision how human climbing led to these evolutionary quirks. As Dominy puts it, these delicate climbing behaviors “favored the stiff spines and thicker lumbar vertebrae needed for upright walking, along with shoulders and elbows equipped for throwing objects and making tools.” The end result was that “climbing gave us the anatomical structure needed to thrive as hominins on the savannas of Africa.”

It also provided the Dartmouth students behind this project with experiences they will never forget. Dominy himself commented, “This study was supervised by two students, and I will never tire of seeing the joy of discovery when the students see the first results of their hard work.” In this case, the hard work “improves our understanding of how primate bodies evolved to solve basic problems safely and efficiently. And our bodies are a legacy of that process.”

It also had the spin-off effect of allowing the students to form bonds with the monkeys they observed. As Joy recalled, “I really enjoyed observing and measuring the climbing behavior of the mangabey in the videos. They are amazing animals, and despite the fact that they were such different individuals, I called them all Geronimo.”