A new study has introduced ‘downclimbing’ from trees as a driver in the early-human evolution of shoulders and elbows
The rotating shoulders and extending elbows that allow humans to reach for a high shelf or toss a ball with friends may have first evolved as a natural braking system for our primate ancestors who simply needed to get out of trees without dying.
Dartmouth College researchers report that apes and early humans likely evolved free-moving shoulders and flexible elbows to slow their descent from trees as gravity pulled on their heavier bodies.
When early humans left forests for the grassy savanna, the researchers say, their versatile appendages were essential for gathering food and deploying tools for hunting and defence.
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.
When climbing down, however, 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.
Luke Fannin, first author of the study and a graduate student in Dartmouth’s Ecology, Evolution, Environment and Society programme, said the findings are 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.
Manifest in humans
Existing research has observed chimps ascending and navigating trees – usually in experimental setups – but the researchers’ extensive video from the wild allowed them to examine how the animals’ bodies adapted to climbing down, Fannin said.
He said: “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.
“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.”
Study co-author Jeremy DeSilva, professor and chair of anthropology at Dartmouth, said: “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 behaviour.”
He added: “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 favoured those anatomies that allowed them to descend safely.”
The great tinkerer
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, DeSilva said.
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 – combined with free-moving shoulders and elbows – made our ancestors excellent shots with a spear (apes cannot throw accurately).
DeSilva said: “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.
“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.”
Despite chimps’ lack of grace, Fannin said, their arms have adapted to ensure the animals reach the ground safely—and their limbs are remarkably similar to those of modern humans.
Fannin said: “It’s the template that we came from – going down was probably far more of a challenge for our early ancestors, too.
“Even once humans became upright, the ability to ascend, then descend, a tree would’ve been incredibly useful for safety and nourishment, which is the name of the game when it comes to survival.
“We’re modified, but the hallmarks of our ape ancestry remain in our modern skeletons.”
The paper, ‘Downclimbing and the evolution of ape forelimb morphologies’, is published Royal Society Open Science.
Image 1: Dartmouth researchers report that apes and early humans evolved more flexible shoulders and elbows than monkeys (above) 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 defence.
Image 2: The researchers used sports-analysis software to compare the climbing movements of chimpanzees and mangabeys (pictured). They found that chimps support their greater weight when climbing down by fully extending their arms above their heads thanks to shallow, rounded shoulder joints and shortened elbow bones that are similar to those in humans. Mangabeys, which are built more like cats or dogs, have less flexibility and position their shoulders and elbows roughly the same when climbing up or down.
Credit both images: Luke Fannin, Dartmouth.