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.
Anatomist Susan Larson of Stony Brook University in New York says even after early humans left the trees altogether -- a little over 2 million years ago -- the shoulder wasn't settled.
"The next stage was something not modern humanlike, nor apelike -- it was something entirely different," she says. "The range of motion would have been mostly sort of in front of your body -- so, great for making tools, but not so good for throwing."
The Shoulder's Secret
These were the earliest humans, living about 2.5 million years ago. It took another 2 million years or so for the collar bone to lengthen and the joint to lie horizontal as it does now. When it did, though, the shoulder gave us something novel -- the ability to throw.
And throwing changed everything. It turned us into dangerous hunters.
John Shea is an archaeologist at Stony Brook who specializes in the history of weaponry and how our ancestors shaped weapons to capitalize on our physical strengths and weakness. He shows off his "primitive technology" lab, locked up in the university anthropology department.
"Spears, arrows, darts -- one of my faculty colleagues walked in here and said it looked like the Tower of London," Shea says. "We call it the swamp."
Shea takes me to a football field for a demonstration. We haul spears and other weapons and a cardboard box for a target.
Shea explains that the secret of the modern shoulder is its ability to move the arm in almost any direction, even behind the back. That, combined with other early human traits, enabled us to throw with power and accuracy.
"We have a wrist that can move like a whip, that can accelerate through throwing," he explains. "And your gluteus muscles -- you know, your rear end, your thighs, your calves -- these are things that make for good running, but they also make for good throwing."
Advances In Weaponry
Shea hands me what might have been the first shoulder-fired human weapon. He calls it a "limestone spheroid." I call it a rock. It's about a million-and-a-half years old. I throw it. It goes about twenty yards -- hardly a threat to a running antelope.
Shea thinks our ancestors threw rocks pretty well. The problem was that rocks weren't good weapons, even against slow-moving prey. "Best-case scenario, you've annoyed it," he says. "Worst-case scenario, this is one of these animals that deals with annoying primates by trying to stomp them into paste."
So humans invented something sharper: wooden spears.
The oldest were discovered in Germany. They are about 9 feet long, date back 400,000 years and were probably made by Neanderthals. Shea has made copies and throws one. It buries its point into the ground with a satisfying "thunk."
Shea is pretty accurate with it, but the spear is heavy and travels only about 30 yards. He thinks the earliest spears were not much good for throwing, and instead were used more as lances.
The Atlatl -- A Catapult For The Arm
But humans eventually discovered physics. Try this simple experiment: Make a throwing motion with your arm. You'll see that your hand moves a lot farther than your elbow does. It also moves faster. So if you could make your arm even longer, your throwing speed would be much faster. It's the same principle as a catapult.
The atlatl does just that. It's a slender, 2-foot length of wood, and the name comes from the Aztecs, though Shea believes it long predates that culture. It's a spear-thrower. You hold one end in your palm, strapped to your fingers. At the other end, you attach an arrow or dart, with the point directed out toward that herd of wildebeest you've been tracking. You throw by stepping forward and basically flinging the dart forward with the arm and a flick of the wrist.
Shea fires one off with very little effort, and the dart travels a good 50 yards. "If that had hit a person, it would've knocked them off their feet onto the ground," he says. "A little bit closer than that, it would've pinned them to the ground. These things are serious weapons."
Even 150 feet is easy. One of Shea's students threw a dart the length of a football field, and Shea suspects that the hungrier you are, the better you throw.
Primitive as these weapons may have been, they gave early humans a huge advantage in hunting for calorie-rich meat and for defending themselves from predators or other humans.
"With weapons systems like this, body size doesn't matter -- it's all in length and speed of rotation," Shea says. The length refers to the length of the arm; because the throwing motion is rotational from the shoulder out, the atlatl simply lengthens the arm to increase the throwing speed and force.
The shoulder is a quirky joint and one that's complicated and prone to injury. But it helped turn a puny primate into the planet's most efficient predator.