A group of scientists says flexible shoulders and movable elbows may have evolved as a safety mechanism in ape ancestors and early humans. These anatomical features emerged as a response to gravity, acting as “brakes” to ensure a safe descent from trees. As early humans moved from forests to open grasslands, those flexible limbs also became essential for foraging and for using weapons in hunting and defense.
The Importance of Descent in Ape and Early Human Evolution
To better understand different climbing techniques, researchers used sports-analysis tools and statistical methods to study videos and images of wild chimpanzees and small mangabey monkeys climbing in their natural habitats. They found that both species climbed trees in a similar way, keeping their shoulders and elbows close to their bodies. But during descent, chimpanzees extended their arms above them to grasp branches, much like a person descending a ladder, leaning back as they lowered themselves.
Lead author Luke Fanning emphasized the importance of “descent” in the evolution of apes and early humans, groups that are genetically closer than once believed. Thanks to numerous videos captured in natural settings, researchers were able to analyze the physical adaptations that help monkeys descend from trees.
Mr. Fanning says this research challenges the idea that climbing is an underrated but crucial factor in the anatomical differences between mangabeys and great apes, differences that ultimately show up in humans. Descending posed a serious challenge given the size of these animals; their morphology adapted through natural selection to reduce the risk of falling.
Co-author Professor Jeremy De Silva adds that scientists have long focused on how great apes climb, while the literature has largely overlooked how they descend. The first great apes evolved around 20 million years ago in fragmented forests, where they climbed trees to find food and then descended to move to the next tree. Falling was not an option for these animals because a fall could cause death or severe injury. Natural selection favored anatomical structures that let monkeys descend safely.
Flexibility of Shoulders and Elbows in Monkeys and Humans
The flexible shoulders and elbows inherited from ancient apes enabled early humans, such as Australopithecines, to climb trees and descend safely. After early humans began using fire to protect themselves from nocturnal predators, their anatomy changed further: broader shoulders and more flexible elbows let them throw weapons like spears. As Mr. De Silva explains, it’s basically the same ape anatomy with a few tweaks that allowed humans to throw spears or stones for hunting and defense. Descending from trees helped lay the anatomical foundation for skills that would evolve millions of years later — even the simple act of an athlete throwing a ball traces back to those adaptations.
Despite differences from humans, chimpanzee limbs, which closely resemble ours, are well adapted for safe descent. According to Professor Fanning, while humans have changed, traces of ape ancestry remain in modern human skeletons. Scientists also analyzed the shoulder and arm anatomy of chimpanzees and mangabeys. Chimpanzees, like humans, have a shallow ball-and-socket shoulder joint that allows a wide range of motion. They can fully extend their arms because of a shorter bone length beyond the elbow. By contrast, mangabeys resemble quadrupedal animals such as cats and dogs, with deep, pear-shaped shoulder sockets and elbows that bend like the letter L. That structure is stable but limits flexibility and range of motion.
Counteracting Gravity During Descent
The study also found that chimpanzees held their shoulder angle 14 degrees higher during descent than during ascent. Their elbows were about 34 degrees more extended when descending than when climbing. In mangabeys, those differences were much smaller. Professor Nathaniel Dominy says that, if cats could talk, they would probably tell you that descending is harder than climbing. Many climbers would agree. The reason is that descent requires not only fighting gravity but also controlling and slowing your movement.
Researcher Mary Joy, reviewing Mr. De Silva’s videos, observed clear differences between how chimpanzees descend and how they climb. She noted that chimpanzee descent was quick and controlled, which may be more energy-efficient and safer than the limited movements of heavier primates. From her own experience, Ms. Joy says the slower she moves downward and the more she restricts her movements, the faster she tires. She adds that human movement is an evolutionary compromise that began with our ape ancestors.
