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DON GONYEA, host:

Scientists have long said: When it comes to evolution, it was making tools that really set humans apart. In our series, The Human Edge, NPR is examining the watershed moments that propelled human evolution.

As science correspondent Christopher Joyce reports, if tools define our species, then it's our hands we have to thank.

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CHRISTOPHER JOYCE: The flexing of the fingers at the card table.

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JOYCE: A young pianist's hands convert dots on a page into sound.

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Unidentified Woman: You have touched to be more profoundly than I...

JOYCE: With a quill pen on parchment, our hands can convey our hearts' desire.

Unidentified Woman: Hence forward I am yours to everything.

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JOYCE: It took millions of years for our hands to go from grasping tree limbs to writing poetry. And scientists believe that making stone tools helped propel that evolution.

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JOYCE: At George Washington University, anthropologist Erin Marie Williams is trying to find out how that evolution took place. How did making and using tools shape our hands and wrists?

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JOYCE: She studies flint-knapping, that's knapping with a silent K. Flint-knapping is the art of making stone tools the way our ancestors did.

Dr. ERIN MARIE WILLIAMS (Anthropologist): Everything that made us human arguably was given this big push by using stone tools. And so I'm trying to see what is it about our anatomy that allowed us to be good at it compared to other species who weren't good at it.

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JOYCE: Her flint-knapper today is Dennis Sandgathe, an archaeologist from Simon Fraser University in Canada.

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JOYCE: He sits on a chair in a windowless laboratory and cradles a large rock, or core, of jet-black flint in his left hand. In his right hand, he grasps a round, smooth hammerstone as if it were a baseball.

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JOYCE: He swings the hammerstone and whacks the core. A flake leaps off and falls to the ground. Each flake is razor sharp. These were the first knives.

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Dr. DENNIS SANDGATHE (Archaeologist): The angle at which I strike this will have a major effect on the result. There certainly is some math behind it, but, you know, typically when you're knapping like this, it's more kind of experience and intuition.

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Dr. SANDGATHE: It usually involves a little bit of blood.

JOYCE: Williams translates the intuition into science by filming hundreds of hours of knapping and analyzing the knapper's movements on a computer.

Dr. WILLIAMS: Okay. You ready? Recording.

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JOYCE: Today, she notices that Sandgathe snaps his right wrist forward when he strikes the flint. That's a clue. It could be that some quirk of human anatomy - say, the way the wrist bones are configured or how the thumb is shaped -allows him to do that especially well. If so, when did that quirk first appear in our ancestors?

If a physical change like that led to better tool making, then its owner would have survived better. Tools mean more meat and protection, so changes in the body that helped us make better tools stuck.

Dr. WILLIAMS: Some early humans were making stone tools and they were bad at it. Other early humans were good at it, and they had more kids and they out-competed their neighbors who weren't so good at it.

JOYCE: Those early humans were probably Homo habilis, which means handy man. Homo habilis appeared about two-and-a-half million years ago, and is thought to be the first human. Handy man made tools, but they were crude. That could be because his wrists and hands were still pretty ape-like.

Now, apes can make tools. Scientists have trained a bonobo, called Kanzi, to do that. But Kanzi's not much good at it.

Dr. WILLIAMS: He just can't get the motions down. And that's because he can't grip the stones, his thumbs aren't long enough and his fingers are too long and he's clumsy. He can't move his wrists. He can't extend it back and get this really important snap. He makes a mess.

JOYCE: An ape's brain is up to the task, but not his anatomy. And ape doesn't have the hands that Sandgathe has. It took millions of years of evolution to produce the hands of a skilled flint-knapper. You can see the before and after picture of that evolution at the Smithsonian Institution's Museum of Natural History.

Dr. CALEY ORR (Anthropologist): I grew up skateboarding, so probably used my feet more than my hand.

JOYCE: Anthropologist Caley Orr now prefers hands to feet.

Dr. ORR: My wife is constantly telling me to stop looking at her hands or mentally dissecting her forearm.

JOYCE: On an office table, Orr has laid out the skeletal hands of three apes and a human. The apes' hands are enormous. The orangutan's is like a catcher's mitt, but their thumbs are tiny and splayed out to the side; the fingers are long and curved. They look powerful, but Orr says the strength runs vertically, from the wrist up through the fingers. That's good for hanging on tree limbs, but not for much else.

The human hand is smaller and it works differently. Orr hands me a two-foot-long club to illustrate.

Dr. ORR: Here, try to hold this without using your little finger, and just using those other digits.

JOYCE: It's going to fly out of my hand. I think if I took a good swing at something, I would lose it.

The strength in my hand extends across the palm. My thumb is strong and so is my pinky. I can wrap that thumb over my other fingers and then secure the grip at the bottom with my pinkie. An ape can't do that very well. And my opposed thumb and wider fingertips also mean I can grip a round stone - like a hammerstone - with more control than an ape can. I have the hand of the ultimate toolmaker.

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JOYCE: Erin Marie Williams unzips a body bag at the George Washington University Medical Center. Inside is a cadaver donated for research. With a scalpel held delicately in her right hand, she reaches over with her left to show me what millions of years of evolution has produced.

Dr. WILLIAMS: When I flip the arm over so that the palm is up, you can see underneath these tendons we have just a ton of muscles that are just in our palms that help us finely move our fingers.

JOYCE: It's a spider's web of muscles and tendons underneath the skin, many of them unique to the human hand.

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JOYCE: And now we often define ourselves by what we can do with that hand -write, play music, even communicate. And all it took to get it was a couple of million years of whacking two rocks against each other.

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JOYCE: Christopher Joyce, NPR News.

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GONYEA: To learn more about how the human body has evolved, visit npr.org/science and click on The Human Edge.

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GONYEA: This is NPR News.

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