Paying the Price for the Ability to Speak

by Robert Sack, M.D.

Here's a Trivial Pursuits question: What do humans have in common with English bulldogs but with no other animal? The answer: Bulldogs and humans are the only two species that suffer from obstructive sleep apnea. Most diseases that occur in humans have their counterpart in animals -- a principle that makes basic medical research possible. Occasionally (as in the case of HIV infection) the only other species to contract a disease is our closest relative, the chimpanzee; but even chimps do not get obstructive sleep apnea. Yet 4% of men and 2% of women of our species (about 18 million people in the U.S. ) have this disorder (most of them, undiagnosed).

According to a theory proposed by Terence Davidson, M.D. (Department of Otolaryngology, U.C. San Diego), humans suffer from sleep apnea for somewhat the same reason that English bulldogs do: we are bred for it. In the case of English bulldogs, intentional selective breeding has created a dog with a distinctive appearance that resembles Winston Churchill, with hanging jowls and a flattened snout, but that is unfortunately prone to snoring and sleep apnea. In the case of humans, evolution (which involves a kind of selective breeding) has also produced a creature with a precarious windpipe that is vulnerable to obstruction. How did this apparently defective design in the human airway happen?

Davidson thinks that a vulnerability to sleep apnea, as well as the risk of choking, is the price our species paid (in evolutionary terms) for the rich repertoire of sounds we can make with our voice. In other words, sleep apnea is the price we paid for our ability to speak. The modification of the human airway has generated such enormous advantages for survival through improved communication that the overall benefits have been well worth the risks. But evolutionary adaptation can involve significant tradeoffs.

So how did evolution reconfigure our head and neck? If one compares our anatomy (modern humans) to that of the chimpanzee, or for that matter, the Neanderthal, there are some major differences. To summarize (using non-technical terms) our face has been considerably flattened (we must look awfully unattractive to chimps). As a result, our vocal tract has been pushed back and down, forming an acute right angle between the mouth and the vocal cords that are housed in the larynx (Adam's apple or voice box). To accomplish this remodeling, 1) our jaws were shortened, 2) the base of our bulky tongue was pushed deeper into the throat (not confined to the mouth cavity as in other primates), and 3) our larynx descended down towards the chest.

Each of these anatomical differences contributed to the success of our marvelously versatile sound generator, but each alteration has had its associated drawback. The shortened human jaw means that the upper airway is narrower and more likely to obstruct. (Furthermore, the shortened jaw increased the tendency for impacted wisdom teeth; Neanderthal dentists presumably didn't have to deal with this problem.) Although obesity is the most frequent contributing cause of sleep apnea, in some people, a shorter than normal lower jaw (manifesting as a large overbite) can be the main problem. In such cases, you could think that evolution went too far in the direction in flattening the face. One of the more aggressive treatments for sleep apnea is to surgically rebuild the jaws, elongating them outward -- in effect, reversing the course of evolution.

The base of the tongue is the usual site for airway obstruction from sleep apnea because the retro-displaced tongue tends to fall into the back of the throat, especially when we lie on our backs. If the tongue were confined to the mouth, as it is in most other mammals, it wouldn't be so likely to get in the way and block the flow of air.

What's more, the descended larynx in humans leaves a gap between the larynx and the soft palate that increases the risk of choking. Newborns are relatively protected because, at birth, the larynx has not yet descended to its adult position. Infants can suck and breathe at the same time because they are able to achieve what is called an "epiglottic-soft palate lock up." As children grow, the epiglottis -- the flap valve that covers the trachea (wind pipe) when we swallow -- descends with the rest of the larynx and a risky chasm develops. To protect us from choking, the epiglottis has to actively seal off the airway every time we swallow; otherwise food would drop into the trachea. (In boys, the larynx descends even further at puberty, deepening the voice.)

Although the capacity for speech was the eventual outcome of these alterations in the head and neck, it is unlikely that these anatomical changes were initially engineered for that purpose. As an evolutionary process, it's probably too big a jump. So there must have been some intermediate reasons for the rearrangement our anatomy. Some experts think that the changes came about in the first place in order to accommodate the upright posture adopted by our ancient ancestors. Although upright posture has many advantages, it too has its drawbacks and has left us a legacy of medical problems including difficult childbirth, lower back problems, and inguinal hernias.

The moral of the story, so far, is that evolutionary design does not start from scratch but works with the existing structures at hand and inevitably involves compromises between useful adaptations and unfortunate liabilities. If the evident liabilities are not too great, or if there are no good alternative solutions, the liabilities are conserved, even if they appear to be design flaws.

It takes more than a violin to make music and likewise, it takes more than a well-shaped vocal track to make speech; it also takes a good big brain to control the fine movements of the lips, tongue and breathing muscles that produce the subtle variations in sound that we recognize as speech. These movements constantly reconfigure the shape of the upper airway in order to filter or stop the vibrations in our breath that are generated down in the vocal cords. Thus, the words beet, boot, bought, and bat may be uttered with the same pitch, but we can distinguish them by the subtle difference in the timber (technically called formant) of the sound that results from different ways of puckering of the lips and curling the of the tongue. Although speech takes a little practice (starting with babbling), we are born with the connections ready to go and there is no stopping it. By the age of two, babies have learned to produce an amazing array of sounds by adjusting the shape of their tongue and lips.

It is notable that the Neanderthals had a brain about as big as our lineage, but left no artifacts to indicate they were capable of complex thoughts. It is unknown if they were able to speak (in the sense that we think of it), or to think abstractly. Perhaps their ability to develop language may have been hampered by the shape of their jaw. It is apparent that the sophisticated mental processes that emerged in homo sapiens (us) only about 50 to 100 thousand years ago, could not have occurred without language. And language in humans is dependent on our virtuoso ability to generate and detect subtle variations in sound of the voice. Language and speech must have evolved in parallel. To be able to speak requires both a big brain and a highly refined sound generator -- albeit one with a risky tendency to obstruct when we fall asleep.

So (it follows), if we weren't prone to sleep apnea, we might not be able to speak. If we weren't able to speak, we might not have language. If we didn't have language, we wouldn't have art, religion and science. In other words, if we weren't prone to sleep apnea, we wouldn't be human.