If you could change the way a monkey or an ape's brain is wired, that animal would be capable of producing perfectly intelligible speech.
Researchers then used that information to create a computer model of what it would sound like if the monkey were able to say phrases such as "happy holidays."
The finding calls into question long-held assumptions about how humans developed their unique ability to use spoken language.
"What you'll find in the textbooks is that monkeys can't talk because they don't have the appropriate vocal tract to do so," says Tecumseh Fitch, a cognitive biologist at the University of Vienna. "That, I think, is a myth. My colleagues and I all get very tired of seeing this. But you see it in all the textbooks. Lots of popular books, and also scholarly books about the evolution of language, assume that in order to evolve speech we had to have massive changes in our vocal tract. "
In the past, scientists looked at dead animals to judge what their vocal tracts could do. But Fitch says that made people vastly underestimate the flexibility of nonhuman mammals.
He and his colleagues monitored a long-tailed macaque named Emiliano as he made a wide range of different gestures and sounds, including lip-smacks, yawns, chewing, coos and grunts. Their special equipment took a rapid series of X-rays that allowed them to capture the full range of movement in the monkey's vocal tract. Then they used computer models to explore its potential for generating speech.
Friday, in the journal Science Advances, his team reports that monkeys would be physically capable of producing five distinguishable vowels — the most common number of vowels found in the world's languages.
And human listeners could clearly understand phrases they created with their synthesized monkey speech, including a marriage proposal.
The bottom line, says Fitch, is that a monkey's speech limitations stem from the way its brain is organized.
"As soon as you had a brain that was ready to control the vocal tract," Fitch says, "the vocal tract of a monkey or nonhuman primate would be perfectly fine for producing lots and lots of words."
The real issue is that monkeys' brains do not have direct connections down to the neurons that control the larynx and the tongue, he says. What's more, monkeys don't have critical connections within the brain itself, between the auditory cortex and motor cortex, which makes them incapable of imitating what they hear in the way that humans do.
Rise of the Planet of the Apes, a science fiction movie from 2011, actually has the right idea, notes Fitch. In that film, after a lab chimp named Caesar undergoes brain changes, he eventually is able to speak words such as "No."
"The new Planet of the Apes is a pretty accurate representation of what we think is going on," says Fitch.
SCOTT SIMON, HOST:
Humans can talk. I'm not always the best ad for it. But we can. Monkeys and apes can't talk. But what if they could? NPR's Nell Greenfieldboyce reports on a new study that explored what monkeys' voices would sound like if they had humanlike brains to help them speak.
NELL GREENFIELDBOYCE, BYLINE: Did you ever see a movie called "Rise Of The Planet Of The Apes?"
(SOUNDBITE OF "RISE OF THE PLANET OF THE APES" FILM)
GREENFIELDBOYCE: It came out a few years ago. And in it, a chimp named Caesar gets exposed to a brain-enhancing drug. Later, he escapes from a cage and is grappling with a guard, who tells him to get away.
(SOUNDBITE OF FILM, "RISE OF THE PLANET OF THE APES")
UNIDENTIFIED ACTOR: (As security guard) Take your stinking paw off me, you damn dirty ape.
ANDY SERKIS: (As Caesar) No.
GREENFIELDBOYCE: It's a shocker. Caesar can speak. He can say no. And all it took was changes in the brain. Tecumseh Fitch thinks that could really be true. He's a cognitive biologist at the University of Vienna. And he says there's long been this assumption that the evolution of speech required massive changes in the vocal tract. But he doesn't buy it.
TECUMSEH FITCH: What you'll find in the textbooks is that monkeys can't talk because they don't have the appropriate vocal tract to do so. That, I think, is a myth.
GREENFIELDBOYCE: He says monkeys normally make all kinds of sounds and lip smacks.
FITCH: So they make grunts, cues. They have these threat vocalizations that are kind of like, heh, where they open their mouth really wide. They would scream if they were in pain.
GREENFIELDBOYCE: He and some colleagues recently used special X-ray equipment to observe a long-tailed macaque named Emiliano. As the machine beeps, you can hear the monkey coo.
(SOUNDBITE OF MONKEY COOING)
GREENFIELDBOYCE: The researchers closely tracked the movements of his lips, tongue and larynx.
FITCH: So what we were interested in is, what are the possible shapes that a monkey vocal tract can take?
GREENFIELDBOYCE: With this information plus computer models, they could figure out what aspects of speech would be physically possible for a monkey. And it turns out monkeys could do a lot. They could produce five vowels. And five vowels is pretty standard for human languages.
FITCH: So what we found is that they can make a very, very clearly - eh, ehh, ah, uh, oh - all of those vowels are within range of a monkey.
GREENFIELDBOYCE: Then the researchers used the computer to simulate what it would sound like if a monkey talked. Let's say a monkey got in to the spirit of the season and said, happy holidays.
COMPUTER GENERATED VOICE #1: Happy holidays.
GREENFIELDBOYCE: Or what if a monkey asked to marry you?
COMPUTER GENERATED VOICE #2: Will you marry me?
GREENFIELDBOYCE: They picked that phrase because it had a lot of vowels. But Fitch says monkeys would be able to make plenty of consonants too. All it would take to talk is the right kind of brain.
FITCH: As soon as you had a brain that was ready to control the vocal tract, the vocal tract of a monkey or any other non-human primate would be perfectly fine for producing lots and lots of words.
GREENFIELDBOYCE: The work appears in the journal Science Advances. And Fitch hopes it kills off the idea that monkeys' vocal anatomy is incapable of speech once and for all. Nell Greenfieldboyce, NPR News.
(SOUNDBITE OF PATRICK DOYLE COMPOSITION, "LOFTY SWING") Transcript provided by NPR, Copyright NPR.