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Encore: Communication among monkeys could help explain how humans evolved to talk

Transcript

Some monkeys make complex vocalizations, others don't. The reason why could help explain how the human brain evolved to produce speech. (Story first aired on All Things Considered on June 21, 2022.)

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Some monkeys have astounding vocal abilities; others not so much. NPR's Jon Hamilton explains why that could provide scientific clues about the origins of human speech.

(SOUNDBITE OF ARCHIVED NPR BROADCAST)

JON HAMILTON, BYLINE: This is a tale of two kinds of monkeys. The first is the macaque. It's larger and lives in Asia and North Africa. Christina Cerkevich at the University of Pittsburgh says when it comes to vocalizing, macaques are kind of limited.

(SOUNDBITE OF MACAQUES VOCALIZING)

CHRISTINA CERKEVICH: They don't seem to have a lot of control over their vocalizations. They don't change the pitch. They don't change the internal timing. A lot of times, their calls don't have syllables.

HAMILTON: The second monkey is the marmoset. It's smaller and lives primarily in South America. Cerkevich says the vocal skills of marmosets are remarkable.

(SOUNDBITE OF MARMOSETS VOCALIZING)

CERKEVICH: These guys have wonderful control over their vocalizations. They'll change the pitch, get louder because somebody's far away. They'll change the timing so that if you keep artificially cutting them off, they'll wait for the noise to go away so they can get their message heard.

HAMILTON: Both species have the same basic vocal tract, so the team figured the difference must be in how their brains control the vocal muscles. Cerkevich says the team decided to focus on one particular muscle in the larynx.

CERKEVICH: It's this tiny little muscle, especially in the marmosets because they're very small. This muscle, when it contracts, it increases tension on the vocal cords so that the pitch goes up.

HAMILTON: The team designed an experiment to identify the brain areas that control this tiny muscle. Peter Strick, a neurobiologist, says the experiment used a substance that follows the nerve pathways from muscle tissue to the brain.

PETER STRICK: And so we said, look; if we inject the same muscle, we might be able to see what's changed in the marmoset that allows it to vocalize.

HAMILTON: The experiment worked, and the results are published in the journal PNAS. The team found differences in two areas of the brain. One area seems to help shape a particular sound. The other appears to control the timing and sequence of sounds. Strick says, in both of these areas, the South American marmosets had many more brain cells sending signals to the tiny muscle in the larynx.

STRICK: We believe that these two areas are really key in enabling marmoset complex vocalization.

HAMILTON: These areas are separate from what's known as the primary motor cortex, which is involved in planning and executing all kinds of muscle movement. But Strick says the areas work together with the primary motor cortex to help a monkey vocalize.

STRICK: In a sense, it has multiple separate computers running at the same time to deal with that complex task.

HAMILTON: Strick says that's also true in people who devote a lot of brainpower to speech.

STRICK: Speech is remarkably complex. You have to control breathing appropriately. You have to control your lips and your tongue and produce sound.

HAMILTON: And you have to have very fine control of muscles in the larynx. Dr. Eddie Chang of the University of California, San Francisco, has spent years mapping the brain areas involved in human speech.

EDDIE CHANG: What's been a missing piece of the puzzle in all of this is whether or not the part of the brain that controls the larynx is similar in other species, including some of our closest relatives - monkeys.

HAMILTON: Chang says now it looks like that puzzle piece has been found. And he says the discovery of brain areas that give marmosets a vocal advantage over macaques could explain how humans took the next evolutionary step.

CHANG: This new paper suggests that it's the elaboration of these parts of the brain that might have evolved for humans to speak and have language.

HAMILTON: A skill that appeared at least 50,000 years ago.

Jon Hamilton, NPR News.