Researchers from Dartmouth University in the United States explored why the earliest human-like ancestors developed elbows and shoulders that could support life in the trees and a move toward standing and walking. The insights come from a study featured in Royal Society Open Science, illustrating how primate anatomy relates to movement through forest canopies.
The team analyzed motion videos of chimpanzees and smoky mangabeys as they navigate their arboreal habitats, using a specialized program to quantify joint angles and limb mechanics. The findings show that the structure of a chimpanzee’s hand bears striking similarities to human hands, particularly in how the wrist and finger bones collaborate during climbing. The shoulder joint, oriented as a shallow socket beneath the rounded head of the upper arm bone, permits a broader range of motion and can tolerate occasional dislocations more easily. This configuration also enables full arm extension when reaching for branches. In contrast, mangabeys demonstrate limb proportions and joint features more akin to quadrupedal primates, reflecting a different evolutionary path for locomotion.
Researchers observed that during tree climbing, both chimpanzees and mangabeys exhibit larger joint angles at the shoulder and elbow, illustrating a common architectural strategy among primates for climbing and gripping. As chimpanzees descended, they frequently raised and extended their arms overhead to secure hold on overhanging branches, a movement pattern that underscores their adaptation for transitioning between arboreal and ground navigation.
The results imply that early human and primate ancestors developed arm and shoulder configurations that helped slow falls from trees, a crucial advantage for a body of substantial size where descent could be dangerous. The movable elbows and shoulders acted as natural brakes, reducing the likelihood of a dangerous drop while enabling efficient control during descent and ascent.
From this perspective, the anatomy resembles that of the earliest monkeys, yet shows notable changes that enable new capabilities. The evolved limb design could support actions such as throwing projectiles, grasping essential objects, or aiding in climbs to reach food and shelter. The descent from the tree appears to have laid the anatomical groundwork for later developments in locomotion and tool use. In modern contexts, such shoulder and arm mechanics are linked to movements behind activities like throwing in sports, where the same principles trace back to our ape ancestors.
Continuing investigations by researchers who previously studied fecal samples and other fossils emphasize how these anatomical features contributed to social and survival strategies among primates. They point to a broad pattern in early primates where upper limb structure supported both locomotion and manipulation, shaping the course of human evolution. The current work adds a fresh layer of understanding about how trunk, shoulder, and arm articulation integrated to enable climbing efficiency, stability, and the gradual shift from tree life to ground-based activity.
Overall, the study reinforces how a suite of small, coordinated anatomical tweaks can drive major shifts in behavior and capability, influencing how early hominids interacted with their environments and prepared for new ways of living on two legs. The researchers caution that while the data illuminate a key phase of adaptation, they represent part of a broader narrative about evolution, movement, and the emergence of tools and complex social life in humans and their relatives.