Every time you go for a swim, throw a ball or head off to do a bit of climbing, you use your shoulders and elbows. A recent study concludes that you can thank apes for the ability to extend your elbows and rotate your shoulders.
Essential for survival
Flexible shoulders and elbows initially evolved as a survival mechanism. Apes needed the flexibility to control their descents from trees. Many early humans dwelled in forests, and the ability to climb and get down safely was essential. Flexible arms acted as a braking system.
“Downclimbing represented such a significant physical challenge given the size of apes and early humans that their morphology would have responded through natural selection because of the risk of falls,” Luke Fanning, lead author of a recent study explained.
A chimpanzee climbs down a tree trunk. Photo: Shutterstock
While watching chimpanzees, researcher Mary Joy was captivated by the downclimb. Joy is a climber and watching the chimps descend reminded her how taxing downward movement is on the body, both while climbing and mountain running.
“I started to think a lot more about how tiring downward movement over steep slopes could be and how much I needed to adapt my movement specifically to deal with the stresses it would place on my quads, hips, and knees,” Joy told UK Climbing. “When downclimbing, not only am I putting muscles under contraction that are probably used less, but unless I am fully lowering myself to a hanging position, that contraction is also placing increased stress on the joints of my upper arm.”
Comparing downclimbing styles
It became clear to researchers that downclimbing was a “highly important factor in the diverging anatomical differences between monkeys and apes that would eventually manifest in humans.” Before the discovery of fire, early humans climbed into trees at night for safety, only coming down at dawn.
By looking at the climbing habits of chimpanzees and sooty mangabeys, the researchers compared climbing styles and descent mechanisms. They filmed the apes and monkeys in their natural habitat and then used sports analysis and statistical software to analyze their movements.
A young sooty mangabey. Photo: Shutterstock
The monkeys and apes made their way up trees in an almost identical way. They kept their shoulders and elbows tucked in close to their bodies. But descending was a different story.
On the way down, it became clear that chimpanzees had much more flexibility in their shoulder, elbow, and wrist joints. They extended their elbows and shoulders more (by 34% and 14% respectively) when downclimbing compared to climbing.
Meanwhile, sooty mangabeys only extended their joints by 4% during a descent. Chimpanzees resemble a person descending a ladder when climbing out of trees. Their arms extend above their head and they descend backside first.
An athlete throws a javelin. Photo: Shutterstock
Leaving the forest spurred further evolution
Most research on the development of our arm morphology has focused on how apes climb rather than how they descend. The arms of apes and modern humans are incredibly similar. As humans moved out of forests and into the open savanna, our arms evolved further. Our shoulders became much broader and could eventually move over a 90° angle. This allowed our ancestors to throw spears, hunt, and make tools.
“Climbing down out of a tree set the anatomical stage for something that evolved millions of years later. When an NFL quarterback throws a football, that movement is all thanks to our ape ancestors,” co-author Jeremy DeSilva explained.
