The Human descended larynx
From William T. Fitch’s speech on the Evolution of Language (2005), larynx can engage into the nasal cavity of animals, allowing the ease of breathing while swallowing simultaneously.
The larynx, also known as the “voice box” for humans are lower than that of apes, allowing more space for the the tongue to move during speech production.
The descended tongue root (i.e. the back of the tongue) allows additional degree of freedom for vocal tract acrobatics during speech. This contouring and mobility of the tongue allows it to have highly coordinated motions, therefore it is able to produce consonants and vowels.
Anatomy of chimpanzees’ vocal tract hence differs from humans’, preventing them from speaking well, even if they were trained to speak.
The comparative method on descent of the larynx
Such a descended larynx was believed until recently to be uniquely human, and thus was being reinforced in numerous accounts of language evolution
For example, animal vocalizations (i.e. production of sounds) indicates that the animals generally lower their larynx during vocalization (e.g. dog barking). Lowering of the larynx during vocalization may hence, be of a analogous trait (i.e. traits that are similar in function, but from a different evolutionary source). Additionally, several species of animals have similarly been observed to possess descended larynx like koalas, deers and even lions. Since the common mammalian ancestor of these species do not have a descended larynx, this convergence indicates that selective evolution may have taken place.
From this discovery, Fitch (2000) proposed that the motivation for the descent of the larynx was not only caused by the need for speech in humans, since this descended larynx in animals did not cause them to speak, unlike humans.
The evolution of the vocal tract thus requires consideration of other factors like selective evolution. Both homology (i.e. similar function, but different evolutionary source) and analogy traits must then be analysed for a comprehensive understanding of human’s capacity for speech.
This comparative approach presented by Fitch (2005) then suggests a probable hypothesis that the trait of a lowered larynx may have already been present even before the the humans and chimpanzees species split from a possible common ancestor.