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Most of us do it every day without even thinking about it, yet talking is a uniquely human ability. Not only have we evolved brains that produce language, but we can make a huge range of sounds to form hundreds of thousands of words.

In our series "The Human Edge," we're exploring the features that make humans so human, and our distinctly shaped vocal tract is one of them.

NPR's Kathleen Masterson has the story.

(Soundbite of singing)

KATHLEEN MASTERSON: Baritone Jonathan Najmi is a college senior training in singing at the University of Maryland, College Park. He's practicing for his final recital, with the help of an accompanist and his instructor.

Mr. GRAN WILSON (Teacher, University of Maryland, College Park): Good, but see if you can let more huh. Make enough space that you could go...

MASTERSON: Najmi's teacher, Gran Wilson, has toured the world as an opera singer.

Mr. WILSON: Yes. Now you've got a better sound. Now, enjoy these Ds. And deep...

MASTERSON: To make sounds like these, humans use the same basic apparatus as chimps: lungs, throat, voice box, tongue and lips. But we are the only ones singing opera and talking on the phone. This is because humans have evolved a longer throat and smaller mouth better suited for shaping sound. And we have a flexibility in our mouth, tongue and lips that lets us form a wide range of precise sounds that chimps simply can't produce.

(Soundbite of singing)

MASTERSON: It works like this: When we talk or sing, we release controlled puffs of air from our lungs through our larynx, or voice box. The larynx is about the size of a walnut. In men, you can see it; it's the Adam's apple. It's mostly made up of cartilage and muscle. Stretched across the top are vocal cords, which are two folds of mucous membrane. When we expel air from the lungs and push it through the larynx, the vocal cords vibrate, making the sound.

Mr. WILSON: The surface area of the cords that's actually vibrating is probably half of your smallest fingernail against each other - I mean, a very small amount of flesh buzzing.

MASTERSON: The frequency of this buzzing is what gives the sound the pitch. We change the pitch by tightening the vocal cords to make our voice higher, and loosening them to make a lower sound.

Mr. WILSON: It's like if you take a balloon and blow it up, and you can manipulate the pitch by pulling the neck.

MASTERSON: The vibrating air gets made into a specific sound - like an ee or an ah or a tuh or a puh - by how we shape our throat, mouth, tongue and lips. Blending these sounds together to form words and sentences is a complex dance. It requires an enormous amount of fine-motor control.

Dr. PHILIP LIEBERMAN (Professor of Cognitive and Linguistic Science, Brown University): Speech, by the way, is the most complex motor activity that any person acquires - except maybe, you know, violinists or acrobats. It takes about 10 years for children to get to adult levels.

MASTERSON: That's Dr. Philip Lieberman, a professor of cognitive and linguistic science at Brown University. Hes studied the evolution of speech for over five decades.

He says that when you look back in human evolution, you can see that after we diverged from an early ape ancestor, the shape of our vocal tract changed. Hundreds of thousands of years ago, our mouths started getting smaller and less protruding. We developed more flexible tongues we could control more precisely, and our necks started getting longer.

Dr. LIEBERMAN: The reason why the neck is getting longer: The tongue is going down, pulling the larynx down, and there has to be room for all this stuff in the neck.

The first time we see human skulls, fossils, that have everything in place is about 50,000 years ago, where the neck is long enough and the mouth is short enough that they couldve had a vocal tract like us.

MASTERSON: But all these changes came at a price.

Dr. LIEBERMAN: Now, the downside of this was that because you're pulling the larynx all the way down, when you eat, all the food has to go past the larynx and miss it, and get into the esophagus. And that's why people choke to death.

MASTERSON: So we evolved this crazy airway that allows us to choke to death more efficiently - all to further our ability to make more sound, and speak.

Dr. LIEBERMAN: We have all sorts of a number of vowels, numbers of consonants. A monkey will just say uh, uh, uh.

MASTERSON: Not only can we make more sounds, we can control how we string them together. And that is because of our amazing and precise control of our breath. Monkeys can't do this.

(Soundbite of monkeys)

MASTERSON: Monkeys can only make short sounds a few seconds long before they have to take another breath. But we humans can control our breath to an astonishing degree.

Dr. LIEBERMAN: One of the interesting things about speech and indeed, singing is that we go through a very complicated process to have an even air pressure in our lungs.

MASTERSON: Lieberman says the lungs are like a set of balloons, except unlike a balloon, which gets very low pressure when it's nearly deflated, we can control how quickly or slowly our lungs release air.

Dr. LIEBERMAN: When we talk, we first guess the length of the sentence we are going to produce - this is quite amazing - and we hold back on the lungs with the muscles. And they have this complicated function where as the lungs deflate, they hold back less and less and less and less. So you end up with a more or less even air pressure.

MASTERSON: If we didn't do this, Lieberman says, we'd blow our vocal cords apart with high pressure, and the pitch would rapidly descend as we got to the end of the lung balloon. Instead, we can do this:

(Soundbite of singing)

MASTERSON: Well, maybe not all of us. Kathleen Masterson, NPR News.

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