But how do they flip around like that? How do they swim so fast? What's going on?
The Science of the Shoulder
Our shoulders allow for a near full-range rotation of our arms. Go ahead and swing your arms around for a while. Reach up over your head. Stretch out backwards behind you. Congratulations, you're an ape.
convergent evolution) can't do this. They can't reach straight up over their heads. They can't stick their arms out to their sides and behind their backs. They can't even do monkey bars. They aren't apes (and apes aren't monkeys, just to be clear).
But how do we, and our fellow apes (chimps, gorillas, gibbons, bonobos, and orangutans), do this? To answer that, we need to look at the bones.
See, when you want lots of flexibility, you need a large surface area at the joint for unhindered movements.
What you see below is a top-down view (or "proximal" view) of the humerus of a monkey (baboon) and an ape (human), via eskeletons.org. This is the end of the humerus that would come into contact with the scapula (aka the shoulder blade). It's known as a ball-in-socket joint.
Again, on the left we see the baboon humerus. It's easy to see from this side view the difference between the humerus head of the monkey versus the human (on the right). The head of the human humerus is much larger and rounder, a much better "ball" for the ball-in-socket joint.
Another thing to point out is how curved the monkey humerus is compared to the human's. The straightness of the human arm allows us to hang from things safely. To understand this, you just need to think about the tension. The straighter the bone, the less likely it is to break from tension stressing its curves.
But why do we need any of this? Why did we end up with super flexible shoulders and this ability to hang from stuff? Evolution wasn't exactly working to allow us to flip around like acrobats...or was it?
Actually, it was. Hanging from trees (or "suspension") is a great adaptation. It gives us the ability to move along a branch and take advantage of more area around us: everything above the branch (by sitting on the branch like a monkey) and everything below the branch (by hanging under it). That means more food!
Brachiating, or arm-swinging from branch to branch, was just the next step. Instead of having to climb back to the trunk of the tree, climb down, and climb back up another tree, it's easier to be able to move from tree to tree up in the canopy. Monkeys tend to do this by leaping between trees. Apes don't have tails and tend to be heavier, so leaping between trees is trickier for our balance. That makes it's safer to be able to reach an arm and pull oneself towards the next tree. Do that over and over, and you end up brachiating. The faster you can brachiate, the faster you get to the new food source (or get away from the scary thing chasing you!).
This acrobatic ability has made apes the top gymnasts in the world. And while humans tend to get all the attention for it (especially during the Olympics), sometimes it's good to remember that our cousins have this cool ability, too. I'll end this post with an awesome video of a gibbon showing off this super flexible shoulder joint to mess with a couple tigers. Someone get that ape a gold medal!