How do apes and monkeys use their mouth and throat to produce the vowel sounds in speech, and how did its development become significant in the evolution of human language? This is the question which Louis-Jean Boe of the University of Grenoble and his colleagues pondered on as they formulate their research to answer this question.
In an article written by Louis-Jean Boe and Thomas Sawalis in The Conversation, Boe notes that if a language is a foundational distinction between humans and other species of Great Apes, how can linguists study the emergence of language in early humans?
Louis-Jean Boe and his colleagues spent decades figuring out how do apes and monkeys produce vowels since vowels are the key to speech emergence. He notes, "spoken language in humans is an intricately woven string of syllables with consonants appended to the syllables' core vowels. We believe that our multidisciplinary findings push back the date for that crucial step in language evolution by as much as 27 million years."
ARE VOWELS REALLY THAT IMPORTANT?
Louis-Jean Boe explains that all words begin and end the same and that the difference among the vowel sounds is what makes them distinct. For instance, in the words "but", "bet", "bat", "bought", and "boot", if the consonants are dropped, the vowels have their distinct sound qualities and may require a different position of the jaw, tongue, and lips to be able to produce that sound.
"The configuration of the vocal tract-the resonating tube of the throat and mouth, from the vocal folds to the lips -- determines the sound." Louis-Jean Boe explains. "That, in turn, means that the sound carries information about the vocal tract configuration that made it. This relationship is the core understanding of speech science."
According to Boe, speech scientists are able to create models of vocal tracts and can calculate what sound it will make, and analyzing a sound to calculate what vocal tract shape made it for over half a century with the help of technology but comparing human vocal tract to that of the other great apes is not easy.
For instance, a baboon's vocal tract is way different than that of humans. A baboon's larynx and vocal folds (which are located close to its chin) are shortly connected to the pharynx then connect to the long, horizontal oral cavity. In addition, a baboon's tongue is long and flat compared to a human's, which is short in the mouth area then curves in the throat. This means that the human larynx has evolved to be situated lower in the throat, which enables the human pharynx to be larger.
HOW DID SPEECH SCIENTISTS END UP TO THIS CONCLUSION?
In his article, Louis-Jean Boe narrates how speech scientists formulated the laryngeal descent theory 50 years ago where they manipulated a macaque's vocal tract and used plaster cast to create a model of the said vocal tract to calculate the vowel sound produced by particular configurations. The laryngeal descent theory became the basis for further studies in speech evolution.
The laryngeal descent theory tried to explain two phenomena in speech science. One, if laryngeal descent is necessary for human vowels, then primates would never talk. This was done by having chimpanzees raised in human homes to encourage human-like behavior, especially language and speech. The second phenomenon states that humans' ability to create jewelry, burial goods, and cave painting is caused by increased cooperation due to the development of language.
Another finding of the laryngeal descent theory is that to be able to produce the full human vowel inventory, the vocal tract should have long oral and pharyngeal cavities, meaning developed to be situated further into the respiratory tract, both of which are not present in other Great Apes but also only observed with modern adult humans that arrive 200,00 years ago.
Speech scientists have, throughout the years, developed models after models that rethink the theory with the emergence of new pieces of evidence that help them understand the importance of vowels. The article by Boe discusses the methodology which speech scientists took to forge the recent explanation for the laryngeal descent theory. The paper by Louis-Jean Boe and his colleagues is published on Science.