The Unpredictable Species by Lieberman Philip

The Unpredictable Species by Lieberman Philip

Author:Lieberman, Philip
Language: eng
Format: epub
Publisher: Princeton University Press
Published: 2013-03-24T16:00:00+00:00


Figure 5.4. (A) Infant vocal tract. A newborn human’s head in many ways resembles a hairless infant chimpanzee’s. That’s also the case for the infant’s tongue and the position of its larynx (the voice box) and vocal cords. The newborn human tongue is positioned almost entirely within the mouth, and its larynx, which connects to the root of the tongue, is close to the opening to the nose. (B) Adult human tongue and airway. The human tongue after age six to eight years is different from that of any other living species. Half of the human tongue is positioned in the mouth, half is in the pharynx, down in the throat. The airway above the larynx formed by the tongue positioned in this manner has two equal-length segments that meet at a right angle. This configuration permits us to produce vowels that enhance the intelligibility of human speech, but there is a biological cost. The human larynx has been carried down into the throat because it is connected to the root of the tongue. The low position of the adult human larynx increases the risk of choking on food and causes swallowing disorders.

Figure 5.4b shows an adult human tongue, airway, and the position of the larynx and entrance to the esophagus. The esophagus leads to the stomach. When it comes to swallowing solid food, apes and most other animals also have an advantage. Their tongues first propel food along the roof of the mouth, past the larynx, and into the esophageal pathway leading to the stomach. As Daniel Lieberman points out in his 2011 book The Evolution of the Human Head (pp. 295–302), swallowing and breathing patterns differ profoundly between humans and nonhuman primates. In nonhuman primates, the pharynx forms a sort of “tube within a tube” in which air flows directly from the lungs through the nose. Food can’t interfere with the air flow through this inner pathway. The larynx, the entry to the inner air pathway, is positioned high, within the opening to the nose, sealed off from the outer tube through which liquids and soft solids are swallowed. Two soft tissue”flaps,” the epiglottis and velum, overlap each other (think of the rubber gaskets on a refrigerator’s door) to seal off the raised larynx from the food pathway when drinking. When swallowing solid food, the epiglottis flips down, and breathing stops momentarily. The increased risk of choking faced by older humans derives from the human tongue having gradually changed its shape, migrating down into the pharynx in the first six to eight years of life, carrying the larynx down with it.

When swallowing, we have to perform a set of coordinated maneuvers to avoid choking. We have to pull the larynx and the hyoid bone (which supports the larynx) forward and upward to get it out of the way of solid bits of food that are being forcefully propelled down our pharyngeal air-food pathway. At the same time, we have to flip our epiglottis down to cover the larynx.



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