The study indicates that human shoulders and elbows first evolved as brakes for climbing monkeys

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

Dartmouth researchers report that apes and early humans may have evolved free-moving shoulders and flexible elbows to slow their descent from trees as gravity was pulling on their heavier bodies. The paper, “Downward Climbing and Evolution of Monkey Forelimb Morphologies,” was published in Royal Society for Open Science.

Researchers say that when early humans left the forests for the grassy savannah, their various appendages were essential for gathering food and using hunting and defense tools.

The researchers used mathematical analysis and statistical software to compare videos and still frames they captured of chimpanzees and baby monkeys called mangabey climbing in the wild.

They found that chimpanzees and mangabees climb trees similarly, with shoulders bent and elbows mostly close to the body. However, when going downhill, the chimpanzees would extend their arms over their heads to grab onto the branches like someone descending a ladder as their greater weight pulled them down rump-first.

Luke Fannin, first author of the study and a graduate student in the Program in Ecology, Evolution, Environment and Society at Dartmouth, said the findings are among the first to identify the importance of “regression” in the evolution of apes and early humans, which are even more important. They are genetically related to each other from monkeys.

Existing research has observed chimpanzees climbing and moving between trees — usually in experimental settings — but the researchers’ extensive video from the wild allowed them to examine how the animals’ bodies adapt to climbing, Fannin said.

“Our study brings up the idea of ​​climbing as an underappreciated, yet incredibly important, factor in the divergent anatomical differences between monkeys and great apes that will eventually emerge in humans,” Fannin said. “Climbing is very physically challenging given the size of apes and early humans, as their shape would have responded through natural selection due to the risk of falling.”

The study co-author said: “Our field has considered tree-climbing great apes for a long time, but what was essentially absent from the literature was any focus on their exit from the tree. We ignored the second half of this behavior.” The author is Jeremy DeSilva, professor and chair of the Department of Anthropology at Dartmouth.

“The first monkeys evolved 20 million years ago in a kind of sparse forest, where they would climb up a tree to get their food, and then come back down to move to the next tree,” DeSilva said.

“Getting out of the tree presents all kinds of new challenges. The great apes couldn’t stand falling because that would kill or seriously injure them. Natural selection would have favored those anatomy that allowed them to descend safely.”

De Silva said the flexible shoulders and elbows passed down from the ancestors of apes would have allowed early humans such as Australopithecus to climb trees at night in search of safety and to descend safely in daylight.

Once Homo erectus could use fire to protect itself from nocturnal predators, the human form took on wider shoulders capable of 90-degree angles—along with freely moving shoulders and elbows—which made our ancestors deliver excellent spear shots (apes couldn’t throw). accurately).

“It’s the same early ape anatomy with a few modifications. Now you have something that can throw spears or rocks to protect itself from being eaten or kill things to eat itself. That’s what evolution does — it’s a great fix,” DeSilva said.

“The descent from the tree paved the anatomical way for something that evolved millions of years later,” he said. “When an NFL quarterback throws a football, that motion is all thanks to our ape ancestors.”

Although chimps lack agility, Fannin said, their arms are adapted to ensure the animals reach the ground safely — and their limbs are remarkably similar to those of modern humans.

“That’s the model we came from,” Fannin said. “The descent to the bottom was probably a much bigger challenge for our early ancestors as well.” “Even when humans are upright, being able to climb and then descend, the tree will be incredibly useful for safety and nutrition, which is the name of the game when it comes to survival. We have been modified, but the defining features of humans are our ape ancestors still in our skeletons.” modern osteopathy.”

The researchers also studied the anatomy of the arms of chimpanzees and mangabeans using skeletal collections at Harvard University and Ohio State University, respectively. Fannin said that, like humans, chimpanzees have a shallow ball-and-socket scapula that allows for greater range of motion, although it is easier to dislocate. Like humans, chimpanzees can fully extend their arms thanks to a reduction in the length of the bone just behind the elbow, which is known as an olecranon process.

The build of the mangabey and other primates is very much like that of four-legged animals such as cats and dogs, with deep, pear-shaped shoulder sockets and elbows with a prominent operculum that makes the joint L-like. While these joints are more stable, they have a sturdier build. More flexibility and limited range of motion.

The researchers’ analysis showed that the angle of the chimpanzee’s shoulders was 14 degrees greater when descending than when climbing. Their arms extended outward at the elbow 34 degrees more when descending from the tree than ascending. The angles at which the mangabey placed their shoulders and elbows were only slightly different—4 degrees or less—when they were ascending a tree versus descending.

“If cats could talk, they would tell you that climbing is harder than climbing, and many human rock climbers would agree,” said Nathaniel Dominy, study co-author and Charles Hansen Professor at Charles Hansen University. “But the question is why is it so difficult?” Anthropologist and advisor to Fannin.

“The reason is that not only are you fighting the pull of gravity, you also have to slow down,” Dominy said. “Our study is important for addressing 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 monkeys’ shoulders and elbows evolved to be more flexible.”

Co-author Mary Joy, who led the study with Fannin on her undergraduate thesis and graduated from Dartmouth in 2021, was reviewing videos of chimpanzees DeSilva had taken when she noticed the difference in how the animals descended trees from how they climbed into them.

“It was very erratic, everything collapsed, everything was flying,” Joy said. “It’s a very controlled fall.” “Ultimately, we concluded that the way chimps descend a tree is likely to be related to weight. Greater momentum likely takes less energy, and they are more likely to reach the ground safely than making small, constrained movements.”

But as a runner, Joy knew the agonizing feeling of slowly descending a slope in short segments rather than simply sprinting down the track with the pull of gravity, her legs stretched forward to catch her at the end of each stride.

“When I’m moving down a slope, the slower and more constrained I am, the more tired I get. It’s catching up to me so fast. No one would think that the speed and abandon with which chimpanzees descend from trees would do that.” “It’s the preferred method for heavier primates, but my experience tells me it’s more energy efficient,” she said.

“Movement in humans is a masterpiece of evolutionary compromise,” Joy said. “This increased range of motion that started in monkeys has ended up being very beneficial to us. What is the advantage of losing that? If evolution were to pick up on people with less range of motion, what advantages would that give? I can’t see any of them as an advantage to losing that that.”