How Early Humans Evolved Flexible Shoulders and Elbows to Safely Get Out of Trees, Dartmouth Researchers Find

Dartmouth researchers have discovered that apes and early humans developed more flexible shoulders and elbows compared to monkeys, which allowed them to safely descend from trees. The study, published in Royal Society Open Science, suggests that the versatile appendages were essential for early humans to gather food and use tools for hunting and defense. The researchers used sports-analysis and statistical software to compare the climbing movements of chimpanzees and mangabeys in the wild. They found that while both species climbed trees in a similar manner, chimpanzees extended their arms above their heads when descending, using their shoulders and elbows like a ladder. This adaptation allowed them to safely scale down trees and avoid falls. The researchers believe that descending from trees presented new challenges for early humans, as their larger bodies made them more prone to injury or death from falls. Therefore, the free-moving shoulders and flexible elbows that evolved in apes and early humans were likely a response to the risk of descending safely. The study also suggests that these flexible shoulders and elbows were later used by early humans to climb trees at night for safety and gather food. Once Homo erectus could use fire for protection, their broader shoulders and free-moving joints made them adept at using tools for hunting and defense. The researchers compared the skeletal structure of chimpanzee and mangabey arms and found that chimpanzees have shallow shoulder sockets and shorter elbow bones that allow for greater flexibility and range of movement, similar to humans. In contrast, mangabeys have deeper shoulder sockets and longer elbow bones, which provide stability but limit their flexibility. The study highlights the importance of “downclimbing” in the evolution of apes and early humans and emphasizes the role of environment and behavior in shaping anatomical adaptations.