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Of all the things that make human beings unique, one that gets overlooked, literally, is the shoulder. And it did no less than alter the course of human evolution.

As NPR's science correspondent Christopher Joyce reports for our series The Human Edge, the shoulder gave us survival skills we never could have imagined without it.

CHRISTOPHER JOYCE: To understand our shoulder, look at a skeleton, as I did with anthropologist David Green at George Washington University's Medical Center. What you see is an intersection. The head of your arm bone meets your collar bone and part of the shoulder blade. They're held together with tendons and ligaments. The whole joint angles out horizontally from the neck, like a coat hanger.

Mr. DAVID GREEN (Anthropologist, George Washington University Medical Center): Because it's pointing straight out, our arms are allowed to just kind of hang freely, and then, you know, we can flex our arms at the elbow and we have our hands right in front, and that's useful for manipulation. Apes, the joint actually points almost toward the ceiling.

JOYCE: The ape shoulder is good for hanging from a tree. But when our ancestors started walking on two legs, our shoulders started to change. Early on, the joint descended lower on our chest. For a while, the shoulder blade was more on the side, over the rib cage. Then it moved onto our back.

Anatomist Susan Larson of Stony Brook University says even after early humans left the trees altogether - a little over two million years ago - the shoulder still wasn't settled.

Dr. SUSAN LARSON (Anatomist, Stony Brook University): The next stage was something not modern humanlike, nor apelike. It was something entirely different. 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.

JOYCE: These were the earliest humans, about two and a half million years ago. It took another two 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.

Dr. JOHN SHEA (Archaeologist, Stony Brook University): This is where all the weapons are kept.

JOYCE: John Shea is an archaeologist at Stony Brook.

Dr. SHEA: Spears, arrows, darts, that sort of thing. One of my faculty colleagues walked in here and said it looked like the Tower of London, but we call it the swamp.

(Soundbite of laughter)

JOYCE: Shea studies primitive technology, especially weapons. We're going out to a football field for a demonstration.

Dr. SHEA: Weapons are in these. These are the one of the spears, spear cases.

JOYCE: And what do you guys carry in the box?

Dr. SHEA: We use that as a target. It's less issue than carrying the deer head across the campus.

JOYCE: Shea says the secret of the modern shoulder was its ability to move the arm in almost any direction, even behind the back. That, combined with other early human traits, to enable us to throw with power and accuracy.

Dr. SHEA: We have a wrist that can move like a whip. You know, we can accelerate through throwing.

JOYCE: Even the lower body contributed to a good throw.

Dr. SHEA: Your gluteus muscles - you know, your rear end, your thighs, your calves - these are all things that make for good running, but they also make for good throwing.

JOYCE: Shea hands me what might have been the first shoulder-fired human projectile.

Dr. SHEA: It's a limestone spheroid.

JOYCE: In other words, a rock. This one is about a million and a half years old. I throw it; it goes about 20 yards. I didn't bring down an antelope.

Shea thinks our ancestors threw rocks pretty well. The problem was rocks weren't good weapons, even against slow-moving prey.

Dr. SHEA: Best-case scenario, you've annoyed it. Worst-case scenario, this is one of these animals that deals with annoying primates by trying to stomp them into paste.

JOYCE: So humans invented something sharper: wooden spears.

The oldest were discovered in Germany, about 9 feet long and date back 400,000 years, probably made by Neanderthals. Shea has made copies for himself. He throws one.

He's accurate, but the spear is heavy and travels only about 30 yards. I soon discover that it's harder than it looks.

Dr. SHEA: Yeah, just like that. Thumb and four finger in opposition like that.

JOYCE: Yeah. I broke it again. Right motion, lousy weapon. Humans eventually discovered physics. For example, make a throwing motion with your arm. Okay? You'll see your hand moves a lot farther than your elbow does. It also moves faster. So if you could make your arm longer, your throwing speed would be much faster. It's the same principle as a catapult.

And the atlatl - that's A-T-L-A-T-L - does just that. It's a slender, 2-foot length of wood. You hold one end in your palm, straps to your fingers. At the other end, you attach an arrow or a dart with the point directed out toward that herd of wildebeest that you've been tracking.

Dr. SHEA: And then you throw just by stepping forward like that.

JOYCE: Wow.

Dr. SHEA: If that hit a person, it would've knocked them off their feet onto the ground. A little bit closer than that would've pinned them to the ground.

JOYCE: Really?

Dr. SHEA: These things are serious weapons.

JOYCE: A hundred and fifty feet is easy. One of Shea's students threw a dart the length of a football field. 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.

Dr. SHEA: With a weapon system like this, body size doesn't matter. It's all in length and speed of rotation.

JOYCE: Length of arm and rotation of the shoulder. Yes, it's a quirky joint -that shoulder - complicated and prone to injury, but it helped turn a puny primate into the planet's most efficient predator.

Christopher Joyce, NPR News.

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