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Our ultra-mobile arm joints may have evolved for climbing down trees

Explore the hypothesis of evolutionary arm joint adaptations for efficient tree descent

Human Arms Evolved
Image Source: Shutterstock @Denys.Kutsevalov

In a twist that’ll have you swinging from the trees of intrigue, researchers from Dartmouth are rewriting the playbook on how our primate ancestors came down from their leafy perches, and it’s a gravity-defying tale.

You see, those rotating shoulders and extendable elbows that we casually toss balls with or reach for high shelves with? They might not have started as a game, but rather as a life-saving act. Dartmouth’s own brainiacs are telling us that these limb marvels were once a natural emergency brake for our primate kinfolk, who had a tree-climbing reputation to uphold.

Published in the esteemed journal “Royal Society Open Science,” the Dartmouth gang’s paper, “Downclimbing and the evolution of ape forelimb morphologies,” has us pondering the past in new and exciting ways.

Now, when our early human pals decided to swap their treehouse digs for grassy savannas, these flexible limbs weren’t just for show. They became the key to survival, helping our ancestors hunt for food and defend themselves against the wild.

To figure this out, the Dartmouth dream team went all out with sports-analysis and statistical software. They had their cameras rolling as chimpanzees and the little monkeys known as mangabeys did their thing in the wild.

Turns out, when it comes to climbing up, these creatures have got a thing or two in common. Shoulders and elbows close to the body, it’s all about the ascent. But when it’s time to come down from the treetop mansion, chimps don’t mess around. they extend their arms overhead, like they’re descending a ladder, and gravity’s their relentless opponent.

Luke Fannin, the brains behind this operation, a graduate student in Dartmouth’s Ecology, Evolution, Environment and Society program, spilled the beans. He said that we’ve been focusing on apes going up trees for eons, but the grand exit from a tree? Not so much. The Dartmouth squad decided to shine a spotlight on this often-neglected half of the equation.

Why, you ask? Well, it turns out that getting out of a tree is like a gymnastic feat for apes and early humans. Falling? That’s a one-way ticket to the ER, my friends. So, evolution worked its magic, favoring those with flexible shoulders and elbows.

But wait, there’s more. These limbs weren’t just about tree gymnastics. They came in handy for night-time tree escapes. Australopithecus, one of our early ancestors, could climb trees at night for safety and gracefully descend in daylight. It’s like they were masters of the jungle nightlife.

Then came Homo erectus, the genius who discovered fire. Suddenly, those broad shoulders and limbs made sense. It wasn’t just about getting out of trees anymore. It was about hurling spears with deadly precision, something those apes couldn’t do to save their lives.

Jeremy DeSilva, the anthropology bigwig at Dartmouth, sums it up perfectly. He says that our early ape ancestors set the stage for us to throw footballs or spears. Evolution, as he puts it, is a world-class tinkerer.

Now, as clumsy as chimps may seem, they’ve got the art of descent down to a science. Their limbs? Remarkably similar to ours, just the blueprint we came from. So, even as we evolved into upright walkers, the ability to navigate trees like pros remained in our skeleton’s DNA.

The Dartmouth crew dove deep into the anatomical details, using skeletal collections from Harvard University and The Ohio State University. Chimps, like us, have shoulders with a shallow ball-and-socket setup, allowing for great movement. And they’ve got that elbow extension, thanks to the “olecranon process,” that makes our throws and their descents a breeze.

On the flip side, mangabeys and their quadrupedal buddies are built like cats and dogs. Their shoulder sockets are deep and pear-shaped, their elbows protrude like the letter ‘L.’ Steady but inflexible – that’s their motto.

Now, here’s where it gets wild. When chimps descend, their shoulders angle a whopping 14 degrees more than when they go up. Their elbows? They extend a mind-boggling 34 degrees further on the way down. Mangabeys? Barely any change in angles whether they’re climbing up or down.

So, what’s the deal with climbing down, you ask? It’s like a dance with gravity. You’re not just battling gravity; you’ve got to slow down too. That’s where chimps’ gravity-defying skills come into play. They’re all about that controlled fall – it’s like they’ve got a date with destiny.

Mary Joy, a key player in this study, watched chimp videos with a fresh perspective. She saw the erratic, controlled chaos of their descent and realized that weight might be the secret sauce. Momentum, it seems, is their ally, helping them reach the ground safe and sound.

And as a trail runner, Joy knows a thing or two about the art of descending. She’s felt the fatigue of inching down an incline when gravity’s whispering, “Just let go!” The slower you go, the quicker exhaustion creeps in. It’s all about efficient movement.

As Joy sums it up, our evolution has been one big masterpiece of compromises. That extended range of motion we inherited from our ape ancestors? It’s been a game-changer. So, why mess with success? Losing that range of motion? Not a great plan, evolution-wise.

In the end, we’ve learned that evolution doesn’t just tinker; it crafts masterpieces. And when an NFL quarterback throws that touchdown pass, remember, it’s thanks to our ape ancestors’ descent skills. The next time you reach for that high shelf, toss a ball, or even gracefully descend a staircase, tip your hat to your primate past – it’s in your bones.

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