Researchers have shown that monkeys produce well differentiated proto-vowels, an advance that pushes back earlier estimates of when speech evolved in animals by about 2,00,000 years.
The review study, published in the journal Science Advances, mentioned the theory of the "descended larynx", according to which the larynx -- commonly called the voice box -- must be in a low position to produce differentiated vowels before speech can emerge.
According to the researchers, including those from CNRS in France, monkeys, which have a vocal tract anatomy similar to humans, had a higher larynx, and could not produce differentiated vocalisations.
But based on the current study, they said, speech may have emerged 200,000 years earlier than previous estimates.
The current study is a review of existing literature on the evolution of speech.
It notes the work of two pairs of researchers, in the 1930s-1950s, who had tested the possibility of teaching a home-raised chimpanzee to speak, at the same time and under the same conditions as their baby, but all their experiments ended in failure.
Explaining this result, in 1969 in a long series of articles a US researcher, Philip Lieberman, proposed the theory of the descended larynx (TDL).
Lieberman compared the human vocal tract to monkeys, and showed that these have a small pharynx -- related to the high position of their larynx—whereas in humans, this organ is lower.
He said this anatomic block reportedly prevented differentiated vowel production, which is present in all the world's languages, and necessary for spoken language, and this notion became accepted by most primatologists.
Considering the acoustic cavities formed by the tongue, jaw, and lips (identical in primates and humans), recent research showed the production of differentiated vocalizations is not a question of anatomy, but is related to the control of articulators.
But according to the current study, if the emergence of articulated speech is no longer dependent on the descent of the larynx—which took place about 200,000 years ago—scientists can now theorise much earlier speech emergence.
The researchers said this could be as far back as at least 20 million years, a time when our common ancestor with monkeys lived, and already presumably had the capacity to produce contrasted vocalizations.
