Articulate Human Speech Starts With Fine Control

To this day, young mammals are born with fully developed hyoid bones, even when other bones are still partly grown. Adult mammals, too, benefit, masticating and manipulating food with their mouths in ways impossible for reptiles. Although the primary function of the hyoid is to support feeding, evolution has also put it to use in the shaping of sound. The larynx imparts sound to air flowing up the windpipe from the lungs. These sonic vibrations then stream into the upper part of the windpipe, the mouth, and the nasal cavities, before flying free to find listeners. The mammalian hyoid and its muscles allow animals to change the shape and resonance of throat and mouth, giving sound its timbre and nuance, squelching some frequencies and lifting others. The hyoid both supports the mouth and tongue and anchors the larynx. When we call the knobby larynx in our throats the voice box, we do a disservice to the complex architecture within our upper throats and heads, places where voice finds its shape and character. The mammalian vocal system, then, acts like many musical instruments. The larynx is the reed in an oboe. The upper vocal tract is the oboe’s body and finger keys. Evolution has crafted many variations of the mammalian vocal tract, each suited to the ecological or social context of the species.

Can't Keep This From You

In echolocating bats, part of the hyoid connects the larynx to a bony plate at the base of the middle ear. This connection allows the nervous system to compare the outgoing pulse of sound from the larynx with the returning echo in the ear. Toothed whales use their giant vocal folds to make whistles, but their echolocating pulses come from nasal air sacs below the blowhole. These whales feed not only by biting and grasping but by sucking large prey like squid out of the water then swallowing them whole. To support this predatory sucking, their hyoid bones are massive and have flattened surfaces for the attachment of muscles. This descent happens seasonally, dropping during the breeding season, and during the roar itself when the larynx falls then springs back up. Because low sounds come from big bodies, the larynx’s movement presumably serves to make an impression on listeners, the equivalent perhaps of human motorcyclists modifying their exhaust pipes to give the sonic impression of large, powerful engines. The vocal tracts of primates seem especially amenable to evolution’s creative powers. Compared with those of carnivores, for example, the larynges of primates are larger, have evolved faster, and are more variable in relation to body size. Many primates have large air sacs connected to the larynx that act as bellows and resonators. Strangely, we humans have no extraordinary elaborations of our vocal equipment. Our larynx and hyoid are about the size we would expect for an animal of our weight.

Run Of The Mill

Somehow, we’ve achieved the great complexity and nuance of spoken language with tweaks to basic mammalian gear. Losing the laryngeal sacs was likely a key early step. The bulbous sacs of our close cousins, the other great apes, are fabulous for making screams and moans that carry through the forest, but not so good for subtlety. We do not know why our ancestors lost these throat balloons. Perhaps early hominins benefited from quieter, more nuanced vocalizations or the sacs may have impeded them when they became bipedal runners and stalkers on the savannah. Whatever the reason, the loss of these encumbrances likely cleared the way for the neck and mouth to take on their modern human form. Gently press your fingertip in the soft space under your chin, behind your lower jawbone. Now extend your chin a little and run your finger backward. At the junction of neck and jaw underside, your fingertip will find the front of the hyoid bone that wraps back into your neck. This is the only bone in the body not attached to any other bone. Instead, it is suspended from the skull and jaw by strong straps of tissue. Keep moving your fingertip back and down.

Better Days

The next hard lump is the larynx, a thickened part of the windpipe. Inside, inaccessible to probing fingers, are the vocal folds. The larynx is suspended from the hyoid. When we are born, the hyoid and larynx are pressed up against the back of the palate, as they are in many other mammals. As we grow, they both drop down. In adulthood, the hyoid sits just below the level of the lower jaw with the larynx suspended below, in the neck. Sound waves from vocal folds in the human larynx flow upward into a vertical stretch of windpipe leading to the back of the mouth. From there, sound moves forward, from the back of the throat to the lips. Say aah into the mirror and you’ll see the horizontal space of the mouth take an abrupt downward turn behind your tonsils. No sound passes from one to the other without its involvement. Articulate human speech starts with fine control of breath from the lungs. In the larynx, the vocal folds are drawn into the flow of breath and start vibrating, just as the mouth of a balloon vibrates when air rushes out. In most mammals, these folds are entrained in the flow of air, and their elasticity causes them to move back and forth, creating sound waves in the air. In the purr of a cat, these vibrations are boosted by rapidly pulsing muscles, but other mammals lack this enhancement. Sounds from the larynx then pass to the upper part of the throat and into the mouth. There the shape of airway and mouth enhances some frequencies and suppresses others. The tongue further filters the sounds as they flow into the mouth, where tongue, cheeks, jaw, and teeth also sculpt the sound. After departing the oral cavity, the lips impart plosive emphasis or hiss and, finally, the sound wings free into the air. Every part of this web of interacting muscles, bones, and soft tissues plays an essential role.