How apes’ natural brakes led to human shoulders and elbows

The human ability to reach, throw, and manipulate objects with our arms may have originated from an ancient adaptation that helped our ancestors climb down from trees safely, a new study suggests.

Apes and early humans faced the challenge of downclimbing

According to the researchers from Dartmouth College in the US, apes and early humans evolved more flexible shoulders and elbows than monkeys to cope with the challenge of downclimbing, or descending from trees head-first. This behavior required them to extend their arms above their heads and rotate their shoulders to slow down their descent and avoid falling.

How apes’ natural brakes led to human shoulders and elbows
How apes’ natural brakes led to human shoulders and elbows

The study, published in Royal Society Open Science, is among the first to identify the significance of downclimbing in the evolution of apes and early humans, which are more genetically related to each other than to monkeys. The researchers used sports-analysis and statistical software to compare videos and still-frames they took of chimpanzees and small monkeys called mangabeys climbing in the wild.

They found that chimps and mangabeys scaled trees similarly, with shoulders and elbows mostly bent close to the body. However, when climbing down, chimpanzees extended their arms above their heads to hold onto branches like a person going down a ladder as their greater weight pulled them downward rump-first.

Downclimbing shaped the anatomy of apes and early humans

Luke Fannin, first author of the study and a graduate student in Dartmouth’s Ecology, Evolution, Environment and Society program, said that 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.

“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,” Fannin said.

The researchers suggest that flexible shoulders and elbows passed on from ancestral apes would have allowed early humans such as Australopithecus to climb trees at night for safety and come down in the daylight unscathed. Once Homo erectus could use fire to protect itself from nocturnal predators, the human form took on broader shoulders capable of a 90-degree angle that made them excellent shots with a spear.

Downclimbing has been ignored by previous research

Co-author Jeremy DeSilva, professor and chair of anthropology at Dartmouth, said that previous research has focused on how apes climbed up trees but neglected how they got out of them.

“Our field has thought about apes climbing up trees for a long time—what was essentially absent from the literature was any focus on them getting out of a tree. We’ve been ignoring the second half of this behavior,” DeSilva said.

He added that the first apes evolved 20 million years ago in the kind of dispersed forests where they would go up a tree to get their food, then come back down to move on to the next tree.

“Getting out of a tree presents all kinds of new challenges. Big apes can’t afford to fall because it could kill or badly injure them. Natural selection would have favored those anatomies that allowed them to descend safely,” DeSilva said.

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