A group of scientists claims that flexible shoulders and movable elbows may have evolved as a safety mechanism during the development of our ape ancestors and early humans. These anatomical features emerged as a response to gravity, acting as “brakes” to ensure a safe descent from trees. According to experts, when early humans transitioned from forests to open grasslands, these flexible limbs became essential tools for foraging and using weapons for both hunting and protection.
The Importance of Descent in Ape and Early Human Evolution
To better understand various climbing techniques, researchers employed 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 manner, keeping their shoulders and elbows close to their bodies. However, during descent, chimpanzees extended their arms upward, grasping branches much like humans descend a ladder, with their weight causing them to lower themselves backward first.
Lead author Luke Fanning emphasized the significance of “descent” in the evolution of apes and early humans, who are genetically closer to each other than previously thought. Thanks to numerous videos captured in natural settings, researchers were able to analyze the physical adaptations of monkeys for descending from trees.
Mr. Fanning notes that this research challenges the notion of climbing as an underrated yet crucial factor in the anatomical differences between mangabeys and great apes, which ultimately manifest in humans. The act of descending posed a serious challenge, considering the size of both monkeys and early humans, prompting their morphology to adapt through natural selection to mitigate the risks of falling.
Co-author Professor Jeremy De Silva adds that scientists have long pondered how great apes climb trees, as literature has largely overlooked how these animals 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, as it could lead to death or severe injury. Natural selection favored anatomical structures that allowed monkeys to descend safely.
Flexibility of Shoulders and Elbows in Monkeys and Humans
The flexibility of shoulders and elbows inherited from ancient apes enabled early humans, such as Australopithecines, to climb trees and descend safely. After the first humans began using fire to protect themselves from nocturnal predators, their anatomy underwent further changes. They developed broader shoulders and more flexible elbows, allowing our ancestors to effectively throw weapons, such as spears. As Mr. De Silva explains, this is the same anatomy of early apes with a few adjustments. Thanks to this evolutionary adaptation, humans could throw spears or stones for defense or hunting. In essence, evolution paved the way for remarkable adaptations in human anatomy. The act of descending from trees laid the anatomical foundation for something greater that would develop millions of years later. For instance, when an athlete throws a ball, it is made possible by our ape ancestors.
Despite the lack of sophistication in chimpanzees, their limbs, which closely resemble human ones, have effectively adapted for a safe descent to the ground. According to Professor Fanning, while humans have changed, traces of their ape ancestry remain in their modern skeletons. Scientists also analyzed the anatomical structure of the hands of chimpanzees and mangabeys. The former, like humans, possess a shallow ball-and-socket shoulder joint that allows for a wider range of motion. Additionally, chimpanzees can fully extend their arms due to a reduced bone length beyond the elbow. In contrast, mangabeys resemble quadrupedal animals, such as cats and dogs, with deep, pear-shaped shoulder sockets and elbows shaped like the letter L. While this anatomical structure is more stable, it offers limited flexibility and range of motion.
Counteracting Gravity During Descent
The study also revealed that the shoulder angle of chimpanzees was 14 degrees greater during descent than during ascent. Their arms were extended outward at the elbow by 34 degrees more when descending from trees compared to climbing. In the case of mangabeys, these differences were much less pronounced. Professor Nathaniel Dominy states that if cats could talk, they would undoubtedly say that descending is more challenging than climbing. Many climbers would agree. The reason is that it requires not only countering gravity but also slowing down.
Researcher Mary Joy observed clear differences in how chimpanzees descend from trees compared to how they climb, reviewing videos taken by Mr. De Silva. She noted that it was a quick, controlled descent, which may be more energy-efficient and safer than the limited movements of heavier primates. Ms. Joy points out from her own experience that the slower she moves downward and the more she restricts her movements, the more quickly she tires. She adds that human movement is a masterpiece of evolutionary compromise that began with our ape ancestors.