Why do shoulders and elbows rotate? Apes on trees hold clue

The study unveils how these versatile limb structures continued to shape human evolution after our ancestors ventured from forests to grassy savannas. 

In the new environment, these limb adaptations became essential for tasks like gathering food and using tools for hunting and defense.

To support these findings, researchers employed sports analysis and statistical software. They analyzed videos and still frames capturing the climbing behaviors of chimpanzees and mangabeys in their natural habitats. 

Both chimpanzees and mangabeys exhibited similar climbing techniques, primarily keeping their shoulders and elbows bent close to their bodies during ascent. 

However, the pivotal difference emerged during descent, where chimpanzees extended their arms above their heads, akin to a person descending a ladder. 

This unique adaptation allowed them to maintain their balance, despite their greater weight pulling them downward rump-first.

Luke Fannin, the first author of the study and a graduate student in Dartmouth's Ecology, Evolution, Environment, and Society program, emphasized the findings are among the first to identify the significance of "downclimbing" in the evolutionary history of apes and early humans. 

Fannin elaborated on the challenges presented by downclimbing for early apes and humans, explaining that the risk of falls during this behavior would have exerted significant selective pressure on their morphology. 

However, until this study, scientific literature had predominantly focused on apes climbing up trees, neglecting the equally vital aspect of them getting out of trees.

The evolutionary timeline stretches back about 20 million years when the first apes emerged in dispersed forests. These early apes ascended trees for food and descended to move on to the next tree. 

The process of descending from a tree posed new challenges, particularly for larger apes who couldn't afford falls that might lead to severe injuries or death. Natural selection favored anatomical adaptations that allowed for safer descents.

Early human survival strategies

The flexible shoulders and elbows inherited from ancestral apes provided early humans, like Australopithecus, with the capability to climb trees at night for safety and descend during daylight without harm. 

With the advent of Homo erectus, who could use fire to protect against nocturnal predators, human anatomy underwent further changes. 

Broader shoulders capable of a 90-degree angle, combined with free-moving shoulders and elbows, endowed our ancestors with impressive spear-throwing abilities, distinguishing them from apes, which cannot throw accurately.

"It’s that same early-ape anatomy with a couple of tweaks. Now you have something that can throw a spear or rocks to protect itself from being eaten or to kill things to eat for itself. That's what evolution does—it's a great tinkerer," said study co-author Jeremy DeSilva, professor and chair of anthropology at Dartmouth.

"Climbing down out of a tree set the anatomical stage for something that evolved millions of years later," he said. "When an NFL quarterback throws a football, that movement is all thanks to our ape ancestors."

In essence, the study underscores how early adaptations to descending from trees set the anatomical stage for evolutionary developments millions of years later. 

The ability to ascend and descend trees efficiently remained invaluable for safety and nourishment even as humans transitioned to an upright posture. 

These adaptive limb structures became an enduring hallmark of our ape ancestry, influencing the way we interact with our world today, whether it's an NFL quarterback throwing a football or everyday actions that echo our evolutionary past.