Aquatic Ape Human Ancestor Theory

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Descended larynx

A conspicuous and often-mentioned difference between human and non-human primates is that humans have a descended larynx. The larynx or voice box (Adam’s apple), housed in the thyroid cartilage, is an organ in the neck of mammals involved in protection of the trachea and in sound production. In most terrestrial mammals the larynx at rest is positioned high up in the throat, but in humans (except babies) the larynx is positioned relatively low in the neck [1, 2]. In chimpanzees and even more so in other primates, the larynx connects with the nasal passage at rest and its entrance is within the nasal cavity, so that the food passes on both sides of the laryngeal tube in the centre of the throat: although in mammals the food passage and the air passage cross each other in the pharynx (Fig. 1), the food passage (from mouth to oesophagus) and the air passage (from nose to trachea) are fully separated most of the time, so that most mammals can swallow fluids (and some species even semi-solid foods) and breathe
simultaneously [1, 2-4].

A lowered larynx is seen in a few aquatic mammals such as dugongs and manatees [5], but is probably less frequent in terrestrial mammals. Some mammals, like red deer, hammerhead bats, wolves and koalas, have a permanently low larynx, but have evolved (at least in red deer) a long and elastic velum (soft palate), which connects the nasal cavity with the larynx when at rest [2, 5-7]. This is lacking in humans. Thus, while most mammals,
including all apes and human babies, can swallow fluids and breathe simultaneously, it is remarkable that humans from about the age of six months cannot.

However, recent work by Nishimura [8-13] shows that laryngeal descent is not so unique to humans among primates as once thought, and that laryngeal descent evolved in at least two steps during hominoid evolution. In human newborns, the hyoid (tongue bone) and the larynx are positioned as high as in other mammals, but postnatally the laryngeal skeleton descends relative to the hyoid, and the hyoid descends relative to the cranial base. In chimpanzee infants the larynx also descends relative to the hyoid, but the hyoid does not descend relative to the mandible, possibly due to the strong growth of the muzzle (oral prognathism). In both apes and humans the larynx moves independently from the hyoid, whereas in monkeys the hyo-laryngeal complex is a functional unit [9]. Nishimura [9] hypothesizes that the ability of apes and humans to move the larynx independently of the hyoid might have helped to prevent aspiration. In other mammals, however, a low larynx able to be moved independently of the hyoid appears to be associated with loud and varied calls. Male hammerhead bats, for instance, which sing loudly to attract mates, have an extremely low (in fact, intrathoracal) larynx they can move freely, and male deer and wolves have larynges which lower considerably during sound production. According to Fitch [2], laryngeal descent lengthens the vocal tract and produces lower frequency formants in the calls, suggesting a large body size of the caller. It thus seems likely that the independently movable and descended larynx evolved in the early apes, before great apes and lesser apes split (~ 18 Ma), to allow the varied sound productions of the different ape species and especially the duets of the lesser apes (gibbons and siamang):

“Gibbons are monogamous and they define their territories with characteristic, far-reaching, very melodic hooting songs” [14], although songs follow a relatively rigid pattern, unlike that of our species.

Midsagittal sections through chimpanzee ad human heads

In human infants between four and six months, the larynx starts to descend [15]. The comparative data suggest that the descent of the larynx versus the hyoid bone, which is also seen in chimpanzees, and presumably exists in other apes as well, has to do with sound production that includes lower formants (suggestive of large body size) [6].

Also, the loose connection between the larynx and the hyoid in hominoids (but not monkeys [9]) has probably to do with the development of varied sound production (singing).

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Extract taken from: Fifty Years after Alister Hardy Waterside Hypotheses of Human Evolution, Chapter 12: Seafood, Diving, Song and Speech, Mario Vaneechoutte1, Stephen Munro2 and Marc Verhaegen3,*, p. 185-186

[1] Laitman JT. Evolution of the hominid upper respiratory tract: The fossil evidence. In: Tobias PV, Ed. Hominid evolution: Past, present and future. New York: Liss 1985; p. 281-6.
[2] Fitch WT. Production of vocalizations in mammals. In: Brown K, Ed. Encyclopedia of language and linguistics. Oxford (UK): Elsevier 2006; pp. 115-21.
[3] Davidson TM. The great leap forward: The anatomic basis for the acquisition of speech and obstructive sleep apnea. Sleep Med 2003; 4: 185-94.
[4] Crompton AW, German RZ, Thexton AJ. Mechanisms of swallowing and airway protection in infant mammals (Sus domesticus and Macaca fasicularis). J Zool 1997; 241: 89-102.
[5] Negus VE. The comparative anatomy and physiology of the larynx. New York: Hafler Publ Comp 1949.
[6] Fitch WT, Reby D. The descended larynx is not uniquely human. Proc R Soc Lond B 2001; 26: 1669-75.
[7] Rosevear DR. The bats of West Africa. London: British Museum 1965.
[8] Nishimura T. Comparative morphology of the hyo-laryngeal complex in anthropoids: Two steps in the evolution of the descent of the larynx. Primates 2003; 44: 41-9.
[9] Nishimura T, Mikami A, Suzuki J, Matsuzawa T. Descent of the larynx in chimpanzee infants. Proc Natl Acad Sci USA 2003; 100: 6930-3.
[10] Nishimura T. Descent of the larynx in chimpanzees: Mosaic and multiple-step evolution of the foundations for human speech. In: Matsuzawa T, Tomonaga M, Tanaka M, Eds. Cognitive development in chimpanzees. Tokyo: Springer 2006; pp. 75-95.
[11] Nishimura T, Mikami A, Suzuki, J, Matsuzawa T. Descent of the hyoid in chimpanzees: Evolution of face flattening and speech. J Human Evol 2006; 51: 244-54.
[12] Nishimura T. Understanding the dynamics of primate vocalization and its implications for the evolution of human speech. In: Masataka N, Ed. The origin of language. Tokyo: Springer 2008; pp. 111-30.
[13] Nishimura T, Oishi T, Suzuki J, Matsuda K, Takahash T. Development of the supralaryngeal vocal tract in Japanese macaques: Implications for the evolution of the descent of the larynx. Am J Phys Anthropol 2008; 135: 182-94.
[14] Ankel-Simons F. Primate anatomy. San Diego: Academic Press 2000.
[15] Doyle E. Developmental anatomy of the airway. Anaesth Intens Care Med 2009; 10: 183-5.


 
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