Christopher Joyce at NPR:
Of all the things that make human beings unique, one that gets overlooked — literally — is the shoulder. It turns out that the shoulder altered the course of human evolution by giving us survival skills we never could have imagined without it.
To understand the shoulder, look at a human skeleton. What you see is an intersection. The head of your arm bone (the humerus) meets your collar-bone (the clavicle) and part of the shoulder-blade (scapula). They're held together with tendons and ligaments. The whole joint angles out horizontally from the neck, like a coat hanger.
“Because it's pointing straight out,” says David Green, an anthropologist at George Washington University who studies the evolution of the shoulder, “our arms are allowed to just kind of hang freely, and then we can flex our arms at the elbow and have our hands out front, and that's useful for manipulation. In apes, the joint actually points almost toward the ceiling.”
The ape shoulder is good for hanging from a tree, but when our ancestors started walking on two legs, the shoulder started to change. Early on, the joint descended lower on the chest. For a while, the shoulder-blade was more on the side, over the rib cage. Then it moved onto the back.