Our shoulders and elbows have first evolved as a natural braking system for our primate ancestors

Study introduces ‘downclimbing’ from trees as a driver in early-human evolution.

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When compared to monkeys, hominoid primates (apes) have forelimbs that are noticeably more flexible, particularly at the wrist, elbow, and shoulder joints. Attributing the increased joint mobility to the difficulties of below-branch suspension and vertical climbing is tempting.

However, field-based kinematic studies have found few differences between chimpanzees and monkeys when comparing forelimb excursion angles during upclimbing. There is, however, a strong theoretical argument for focusing instead of downclimbing. This motivated scientists to quantify the effects of climbing directionality on the forelimb kinematics of wild chimpanzees (Pan troglodytes) and sooty mangabeys (Cercocebus atys).

Their study revealed that the rotating shoulders and extending elbows may have first evolved as a natural braking system for our primate ancestors who needed to escape trees without dying. According to this study by Dartmouth University researchers, primates and early humans likely evolved flexible elbows and free-moving shoulders to help them descend from trees more slowly as gravity pushed on their heavier bodies. 

Scientists claim that early humans’ adaptable appendages were crucial for gathering food and using tools for hunting and defense as they moved from the forests to the grassy savanna.

Using sports analysis and statistical software, scientists compared videos and still-frames of chimpanzees and small monkeys called mangabeys climbing in the wild. They discovered that mangabeys and chimpanzees used close-to-the-body bending of the shoulders and elbows when climbing trees. Chimpanzees held onto branches while descending a ladder, extending their arms over their heads to support their weight.

Luke Fannin, first author of the study and a Ph.D. candidate in the Ecology, Evolution, Environment, and Society program in the Guarini School of Graduate and Advanced Studies, says, “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.”

“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.”

“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.”

Study co-author Jeremy DeSilva, professor and chair of the Department of Anthropology, said, “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.”

“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. Getting out of a tree presents all kinds of new challenges. Big apes can’t afford to fall because it could kill or badly injure them. Natural selection would have favored those anatomies that allowed them to descend safely.”

According to DeSilva, early humans like Australopithecus could have climbed trees at night for safety and descended safely during the day if they had inherited flexible shoulders and elbows from ancestors apes. He claims that once Homo erectus discovered how to utilize fire to defend itself from nocturnal predators, the human shape developed wider shoulders that could be bent at a 90-degree angle. This and free-moving shoulders and elbows allowed our predecessors to be outstanding spear throwers.

It still has the same early-ape anatomy, albeit with a few modifications. Now you have a creature that can throw a spear or boulders to fend off predators or to kill prey it can consume. That is what evolution does; it is a master experimenter. Climbing down from a tree set the anatomical stage for something that evolved millions of years later.

Fannin said, “Despite chimps’ lack of grace, their arms have adapted to ensure the animals reach the ground safely—and their limbs are remarkably similar to those of modern humans.”

“It’s the template that we came from—going down was probably far more of a challenge for our early ancestors, too. Even once humans became upright, the ability to ascend and descend a tree would’ve been incredibly useful for safety and nourishment, which is the name of the game for survival. We’re modified, but the hallmarks of our ape ancestry remain in our modern skeletons.”

Using skeleton collections from Harvard University and Ohio State University, scientists also looked at the anatomical makeup of chimpanzee and mangabey arms. According to Fannin, chimpanzees have shallow ball-and-socket shoulders similar to those of humans, which, despite being more prone to dislocation, have a wider range of motion. And like humans, chimps can fully extend their arms because of the shortened olecranon process, a bone directly behind the elbow.

With deep, pear-shaped shoulder sockets and elbows with prominent olecranon processes that give the joint the appearance of the letter L, mangabeys and other monkeys are constructed more like quadrupedal animals like cats and dogs. Although these joints are more stable, their flexibility and range of motion are significantly more constrained.

After careful investigation, the scientists discovered that a chimpanzee’s shoulders angled 14 degrees more downward than upward. And when they descended from a tree, their arm’s outward angle at the elbow was 34 degrees more than when they ascended. Only a small difference—4 degrees or less—was observed in the angles at which mangabeys positioned their shoulders and elbows when climbing a tree instead of when they were descending one.

Study co-author Nathaniel Dominy, the Charles Hansen Professor of Anthropology, said, “If cats could talk, they would tell you that climbing down is trickier than climbing up, and many human rock climbers would agree. But the question is, why is it so hard.”

“The reason is that you’re not only resisting the pull of gravity, but you also have to decelerate. Our study is important for tackling a theoretical problem with formal measurements of how wild primates climb up and down. We found important differences between monkeys and chimpanzees that may explain why the shoulders and elbows of apes evolved greater flexibility.”

Joy said“It was very erratic, just crashing down; everything’s flying. It’s very much a controlled fall. Ultimately, we concluded that how chimps descend a tree is likely related to weight. Greater momentum potentially expends less energy, and they’re much more likely to reach the ground safely than by making small, restricted movements.”

Journal Reference:

  1. Luke D. Fannin, Mary S. Joy, Nathaniel J. Dominy, W. Scott McGraw, and Jeremy M. DeSilva. Downclimbing and the evolution of ape forelimb morphologies. Royal Society Open Science. DOI: 10.1098/rsos.230145

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