Monkey’s can’t talk because they lack the brain power to do so confirming a theory first proposed by Charles Darwin 150 years ago, a new study found.
Scientists have been divided over why monkeys can’t speak with those such as Darwin claiming it was to do with the brain’s ability to control vocal chords.
But since the 1960s scientists have believed the lack of speech was down to the differences in vocal anatomy between humans and primates.
Now a joint Princeton-University of Vienna study found Darwin was right all along and human speech stems mainly from the unique evolution and construction of our brains.
Professor Tecumseh Fitch at the University of Vienna said: “Despite repeated attempts, no non-human primates have ever been trained to produce speech sounds, not even chimpanzees raised from birth in human homes.

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Monkeys lack of speech could be down to their vocal chords (Photo: Getty)

“Humans appear to be the only primates with a capacity to flexibly control their vocalisations and to integrate respiration, phonation, and vocal tract movements in an intricate manner as required for speech.

“Since Darwin’s time, two hypotheses have been considered to be the likely explanations for this fact.

“The first ‘neural’ hypothesis is that other primates lack the brain mechanisms required to control and coordinate their otherwise adequate vocal production system.
“Darwin favoured this hypothesis, and it was widely accepted until the 1960s
“The second ‘peripheral’ hypothesis, in contrast, identifies the basis of primate vocal limitations as the anatomy and configuration of the nonhuman primate vocal tract.”
He added this later work largely accepted the vocal apparatus of monkeys was “ inherently incapable of producing the range of human speech.”

Scientists are now agreeing with Charles Darwin about their lack of speech (Photo: Getty)

But this conclusion was based on postmortem plaster casts of vocal tracts.
The new study used x-ray videos to capture and then trace the movements of the different parts of a macaque’s vocal anatomy such as the tongue, lips and larynx in living macaques during vocalisation, facial displays, and feeding.
Human speech stems from a source sound produced by the larynx that is changed by the positions of the vocal anatomy such as the lips and tongue
For example, the same source sound lies behind the words “bat” and “bot” with the facial anatomy generating the different sound we hear.
It found the macaque vocal tract could easily produce an adequate range of speech sounds to support spoken language.
In other words Professor added: “Macaques have a speech-ready vocal tract but lack a speech-ready brain to control it.”
The source sound of a macaque’s grunt call was put into a computer model of the primate’s vocal anatomy and found a macaque could produce comprehensible vowel sounds, and even full sentences, with its vocal tract if it had the neural ability to speak.

Some monkeys can produce vowel sounds (Photo: Getty)

However, while a macaque would be understandable to the human ear, it would not sound precisely like a human.
He said: “The key conclusion from our study is that the basic primate vocal production apparatus is easily capable of producing five clearly distinguishable vowels (for example, those in the English words “bit,” “bet,” “bat,” “but,” and “bought”).
“Five vowels are the worldwide norm for human languages, and many of the world’s languages make do with only three vowels.”
He added it was more challenging to estimate the range of consonants that would be produced by a monkey.
He said: “We do not of course argue that a talking macaque would sound precisely the same as a human or that a macaque could create every possible vowel.
“We conclude that if a macaque monkey had a brain capable of vocal learning and combinatoric operations over speech sounds, its vocal tract would be able to produce clearly intelligible speech.”
He noted the importance of human vocal anatomy for speech had been overestimated and was not as exceptional as first thought.

Turns out Darwin was right all along (Photo: Getty)

He added: “These findings refute the widespread opinion that nonhuman primate vocal tracts are ‘unsuited to speaking.’
“We conclude that the inability of macaques and other primates to speak is a reflection not of peripheral vocal tract limitations but of their lack of neural circuitry enabling sophisticated vocal control.
“In short, primates have a speech-ready vocal tract but lack a speech ready brain to take advantage of its latent operating range.”
Commenting on the study published in the journal Science Advances, Professor Laurie Santos at Yale University said: “This new result tells us that there’s still a big mystery concerning where human speech came from.
“If a species as old as a macaque has a vocal tract capable of speech, then we really need to find the reason that this didn’t translate for later primates into the kind of speech sounds that humans produce.
“I think that means we’re in for some exciting new answers soon.”
Princeton University’s researcher, Asif Ghazanfar, a professor of psychology in the Princeton Neuroscience Institute, was the co-lead along with the University of Vienna’s Prof. Fitch.

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