Researchers report that apes and early humans evolved more flexible shoulders and elbows than monkeys to safely get out of trees. For early humans, these versatile appendages would have been essential for gathering food and deploying tools for hunting and defense. Credit: Luke Fannin, Dartmouth
Key points:
- Apes and early humans evolved free-moving shoulders and flexible elbows to descend from trees.
- “Downclimbing” was a key factor leading to the evolution of anatomical differences between monkeys and apes.
- The evolution of shoulder and elbow flexibility allowed early humans to gather food and use tools like spears for hunting and defense.
In a study published in Royal Society Open Science, researchers from Dartmouth reported that apes and early humans evolved free-moving shoulders and flexible elbows to slow their descent from trees.
The research team used sports analysis and statistical software to compare climbing videos and frames of chimpanzees with those of small monkeys called mangabeys. Chimps and mangabeys climbed up trees similarly, but chimpanzees’ behavior differed on the way down as they extended their arms above their heads to hold onto branches.
These findings point to the significance of “downclimbing” in the evolution of apes and early humans.
“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,” explained study author Luke Fannin.
Researchers also examined the anatomical structure of chimp and mangabey arms using skeletal collections at Harvard University and Ohio State University. Chimps have a shallow ball-and-socket shoulder, which allows for greater range of motion. Mangabeys and other monkeys have limited flexibility and movement as they have deep pear-shaped shoulder sockets.
Flexible shoulders and elbows passed on from ancestral apes allowed early humans to climb trees and come down safely. For example, the angle of a chimp’s shoulders was 14 degrees greater during descent than climbing. Their arm extended outward at the elbow 34 degrees more when coming down a tree than going up. Conversely, mangabeys’ shoulder and elbow angles were only slightly different—4 degrees or less—when they were ascending versus descending.
When early humans left the forest for the grassy savanna, the versatility of their shoulders and elbows allowed them to gather food and deploy tools including throwing spears for hunting and defense.
“Even once humans became upright, the ability to ascend, then descend a tree would’ve been incredibly useful for safety and nourishment,” said Fannin. “We’re modified, but the hallmarks of our ape ancestry remain in our modern skeletons.”