Larynx and Speech as Determinants in the Evolution of Man

LARYNX AND SPEECH AS DETERMINANTS IN THE EVOLUTION OF MAN B. RAYMOND FINK, M.D.* Two ofthe leading factors in man's behavioral evolution from apelike ancestors were elaboration of the brain and perfecting of the upright posture fi]. I propose here that these characteristics were parts ofa complex ofstructural modifications related to the emergence ofspeech as a behavioral dominant. I think this complex also included the chin, a shrinking mandible, a flattened face, finely subdivided facial muscles, and fully muscular vocal folds. The acquisition of rudimentary speech itself may have been associated with the other dominant behavioral trait of hominoids , tool making. Although speech behavior has been widely regarded as mainly a cerebral endowment [2, 3], reflection suggests that the difference in speech capacity between ape and man must also involve many integrated modifications ofthe skeletal and muscular effector organs. DuBrul has pointed out that many human modifications of pharyngo-oral anatomy are, on the one hand, correlated with speech and, on the other, "in direct collaboration with the change to upright posture" [4]. The absence ofsuch adaptations may partly explain the linguistic poverty ofthe chimpanzee, whose versatile finger movements indicate a highly developed motor control, possibly suited for quite elaborate vocalization. That he can learn many digital coordinations but very few phonatory coordinations [5] argues that an important bar to speech behavior in the chimpanzee may He in the sound-making and articulatory machinery. In hominids, the brain attracts emphasis, perhaps because the evolutionary increase in size is obvious and clearly testifies to increased complexity ofcontrol. Compa- * Department ofAnesthesiology, College ofPhysicians and Surgeons, Columbia University, and the Anesthesiology Service, The Presbyterian Hospital in the City ofNew York. This work has been supported by Grant No. RG-9069 ofthe United States Public Health Service. 85 rabie development of the peripheral speech apparatus may well go unnoticed , since the functionally significant result—multiplication of the number of graduated speech postures—is achieved by increased mobility ofthe phonatory skeleton and finer motor units in the controlling muscles rather than by increase in bulk. Modifications toward greater and more delicately controlled maneuverability are distinguishable throughout the speech mechanism of man: in the mandible, hyoid bone, and laryngeal cartilages (see below), and in muscles of the face [6], pharynx [4], and larynx. The architecture of muscular control reaches its ultimate refinement in the human vocal folds, where motor units comprising a single muscle fiber have been reported [7]. I next lay emphasis on the larynx, because recent studies have brought to light the presence in the human organ ofseveral structural modifications apparently specifically adapted to speech behavior [8-10]. Since the larynx leaves no fossilized relics, the lineage ofthese modifications has to be inferred from the study ofliving forms. It is quite striking how many animals exhibit laryngeal adaptations related to their mode of life. To review some ofthese briefly, the simplest, the lungfish Lepidosiren larynx, presents groups ofmuscle fibers on either side ofthe orifice between the food passage and the lung [11]. These fibers have been compared to a sphincter and presumablyserve to keep the duct to the lungs closed under water. The amphibia need more air, and in them a laryngeal skeleton develops, comprising cricoid and arytenoid cartilages, the latter giving attachment to occluding muscles said to be analogous with those of the lungfish. Additional muscles open the passage, their existence evidenced in the frog by periodic separation ofthe arytenoids coinciding with "inspiration ." Vocal Anura have folds ofmucosa, sometimes reinforced by cartilage, projecting medially from the arytenoids [12]. These are not thyroarytenoid vocal folds, since the amphibia lack a thyroid cartilage. Monotreme and marsupial mammals do have a thyroid cartilage [12], but the arytenoids are relatively very large and the thyroarytenoid folds quite rudimentary. With such diminutive "vocal" folds, it is hardly surprising that the marsupials are in general silent or squeaky creatures. Their thyroid cartilages seem to function primarily as a scaffold, enlarging and stabilizing the entrance to the trachea in association with the large air intake required by warm-blooded creatures. The placental mammals, on the other hand, vocalize freely, and in them the arytenoid cartilages are 86 B. Raymond Fink · Larynx and Speech as Determinants Perspectives in Biology...