Apes evolved shoulders as "brakes" to safely climb down trees

A team of scientists led by Dartmouth College has recently argued that the shoulder rotation and elbow extension we use to grab something from an elevated shelf or play catch may have originally evolved as a safety mechanism to help our primate ancestors descend from trees without facing injury. 

According to the experts, due to gravity acting on their heavier bodies, apes and early humans developed movable shoulders and bendable elbows to control their descent from trees. 

As early humans transitioned from forests to open grasslands, these flexible limbs became instrumental in food collection and tool usage for both hunting and protection.

Focus of the study 

To better understand various climbing techniques, the researchers used sports-analysis and statistical tools to study videos and images of wild chimpanzees and small monkeys known as mangabeys climbing in the wild. 

The investigation revealed that both species climbed trees in a similar manner, with their shoulders and elbows positioned close to their bodies. 

However, when descending, chimps stretched their arms upwards, gripping branches akin to how humans descend a ladder, with their weight causing them to descend rear-end first.

Downclimbing 

Luke Fannin, a graduate student in Ecology, Evolution, Environment and Society at Dartmouth and the study’s lead author, emphasized the significance of “downclimbing” in the evolution of apes and ancient humans, who are both genetically closer to each other than they are to monkeys. 

Through their extensive video recordings taken in natural settings, the researchers could analyze the physical adaptations these animals developed for downward climbing.

“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.”

Getting out of a tree 

“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,” added co-author Jeremy DeSilva, a professor 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. 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.”

Shoulder flexibility

This flexibility in shoulders and elbows, inherited from ancient apes, would have enabled early humans like the Australopithecus to safely ascend and descend trees. 

Once Home erectus began to use fire to protect itself from nocturnal predators, the human anatomy changed once more, with broader shoulders capable of a 90-degree angle allowing our ancestors to efficiently throw weapons such as spears.

Early-ape anatomy 

“It’s that same early-ape anatomy with a couple of tweaks. Now you have something that can throw a spear or rocks to protect itself from being eaten or to kill things to eat for itself. That’s what evolution does – it’s a great tinkerer,” DeSilva explained.

“Climbing down out of a tree set the anatomical stage for something that evolved millions of years later. When an NFL quarterback throws a football, that movement is all thanks to our ape ancestors.”

Despite chimpanzees’ lack of grace, their limbs – which are remarkably similar to those of modern humans – have efficiently adapted to make sure that the animals can reach the ground safely.

Shoulder movement 

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

The scientists also analyzed the anatomical structure of both chimp and mangabey arms using specimens from Harvard University and the Ohio State University. Chimps, like humans, possess a shallow ball-and-socket shoulder joint, allowing a broader range of movement. Moreover, they can fully extend their arms due to the reduced length of the bone behind the elbow (known as the “olecranon process”). 

By contrast, mangabeys and other monkeys are built more like quadrupedal animals such as cats and dogs, with deep pear-shaped shoulder sockets and elbows with a protruding olecranon process making the joint resembling the letter “L.” Although such joints are more stable, they have limited flexibility and range of movement.

Resisting the pull of gravity

The investigations also revealed that the angle of a chimp’s shoulders was 14 degrees larger during descent than when climbing up, while their arm extended outward at the elbow 34 degrees more when coming down from a tree then when going up. In the case of mangabeys, such differences were far less pronounced, not exceeding four degrees.

“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,” said co-author Nathaniel Dominy, a professor of Anthropology at Dartmouth.

“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.”

Controlled descent 

Mary Joy, another co-author and a Dartmouth 2021 graduate, observed distinct differences in the way chimps climb down trees compared to ascending them, while reviewing videos filmed by DeSilva. 

“It was very erratic, just crashing down, everything’s flying. It’s very much a controlled fall,” she explained. “In the end, we concluded that the way 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.”

Joy, also a trail runner, drew parallels from her personal experience, emphasizing that controlled descent, like chimps demonstrate, might be more energy-efficient. 

“When I’m moving downhill, the slower I’m going and restricting my movement, the more I’m fatiguing. It catches up to me very quickly. No one would think the speed and abandon with which chimps climb down from trees would be the preferred method for a heavier primate, but my experience tells me it’s more energy efficient.” 

“Movement in humans is a masterpiece of evolutionary compromises. This increased range of motion that began in apes ended up being pretty good for us. What would the advantage of losing that be? If evolution selected for people with less range of motion, what advantages would that confer? I can’t see any advantage to losing that,” she concluded.

The study is published in the journal Royal Society Open Science.

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