Innovations to the human vocal apparatus

Aug 30, 2010 by

To make speech sounds we use the same basic apparatus that apes have: lungs, throat, voice box, tongue and lips. But we are the ones who can speak and sing. Why?

Although speech organs have evolved primarily for other survival functions — lungs to exchange oxygen and carbon dioxide, vocal cords to prevent food and liquids from entering the lungs and lips to seal the oral cavity — they have undergone important design modifications that allowed them to be used for speech. Thus, even though we are very similar to apes in our anatomy, it is the small anatomical differences that allow us to speak. Chimps and other apes, on the other hand, cannot speak the same way we do.

These important innovations include the changes in the tongue muscles. The tongue originally evolved to move food within the mouth. But a much more complex and precise motor control is necessary to move the tongue in all the positions necessary to create the different human speech sounds: the tongue can move forward as in “th”-sounds or back as in /k/, up for /i/ and down for /a/; it can curl at the sides for /l/ and curl backwards for the English /r/ — and much much more. The same can be said about the lips: the round for /u/ and stretch for /i/, move sharply for /w/ and so on. Putting all these motions together is a complex dance that requires an enormous amount of fine motor control. Unsurprisingly, it takes children up to 10 years to get it right.

Or take, for example, our voice box (also known as the larynx). It’s a small cartilage box, about the size of a walnut. It is visible in men as the Adam’s apple. Stretched across the top of the larynx are the vocal chords, which are two folds of mucous membrane. When we exhale and push the air through the larynx, the vocal chords vibrate, making the sound. The frequency of this buzzing is what gives sound the pitch. We change the pitch by tightening the vocal chords to make our voice higher and loosening them to make a lower sound.

Two important anatomical modifications allow us — but not the apes — to speak. First, the vocal chords in humans are more muscular and less fatty than in our relatives such as chimps and gorillas. This allows us greater control over the chords’ precise configuration. Another important difference between us and other primates is in the lower position of the larynx in humans, illustrated in the picture below (the position of the larynx is shown in yellow). Strikingly, this design modification, which permits a greater variety of articulations with the tongue, has the consequence of making it much easier for humans to choke. Which means that the evolutional disadvantage of being more vulnerable to choking is outweighed by the ability to speak!

Another one of such amazing design innovations is the muscle and nerve system that allows us to precisely control our breath — breathlessly! Apes cannot control how they inhale and exhale in the same way we can; they can make only short sounds a few seconds long before they have to take another breath. But we humans can control our breath to an astonishing degree, by evening air pressure in our lungs. Our lungs resemble air balloons; however, a regular balloon gets very low pressure when it is nearly deflated. In contrast, we can control how quickly or how slowly our lungs release air. So before we say something, we estimate how long of a sentence/phrase/utterance we are going to produce and pace ourselves accordingly by holding back on the lungs with the muscles. Not only that, but also we have this complicated mechanism that allows us to hold back less and less and less as the lungs deflate. As a result the air pressure is more or less even. If we did not have this amazing mechanism, the pitch would rapidly descend as we got to the end of the lung balloon, and we’d blow our vocal chords apart with high pressure. So we barely ever “run out of breath” while speaking.

Of course, this necessity to control one’s breath while speaking means that speaking is physically tiring. Unsurprisingly, one feels drawn out after giving a two-hour lecture. It’s like running a marathon only you also have to think — literally on your feet — at the same time. Hope it makes you appreciate your teachers even more.

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