Your simple throat is the reason you don't sound like a chimp - Science

When it comes to the plumbing required to produce human speech, less is more. A new study suggests our larynx evolved to have much simpler vocal anatomy than that of our ancestors. These simplifications may have allowed our species to produce stable, even-toned, and comprehensible speech instead of the rough, warbling vocalizations of other primates.

“It’s a fair conclusion,” says Bart de Boer, a linguist who studies the evolution of speech at the Free University of Brussels who wasn’t involved in the work. “Researchers … have been wondering about the evolution of the vocal folds for a long time.” Yet he and others say there aren’t enough data yet to prove these simplifications evolved specifically for speech.

Mammals vocalize by forcing air through their larynges, which causes folds of tissue to oscillate and produce a wide repertoire of sounds. In humans, a twin pair of such folds known as the vocal cords is responsible for creating these sounds. The vocal tracts of nonhuman primates, meanwhile, hold an additional feature: thin flaps known as vocal membranes, or vocal lips, often found near or connected to the vocal folds.

Just what these lips do has long been unclear. “We didn’t really know the function, and we really didn’t know how widespread they were,” says William Tecumseh Fitch, an evolutionary biologist at the University of Vienna who studies why primates sound the way they do.

He and colleagues started with dead chimpanzees donated by local zoos. They hooked up three larynges that had been removed from the bodies of these apes to a device that blows compressed air through the organ, simulating vocalization. High-speed footage revealed how the vocal folds and membranes vibrated in response.

The patterns matched those in live animals, as seen in an endoscopic video of a chimp waking from anesthesia and other similar recordings of the larynges of rhesus macaques and squirrel monkeys.

Fitch and colleagues used data from the experiments with the dead and live animals to build a computer model of a larynx. When they gave the virtual voice box the vocal folds and vocal membranes of, say, a chimpanzee, and simulated what its voice would sound like, the acoustic signal was chaotic and uneven. Its virtual vocal membranes oscillated chaotically and the soundwaves were unstable, swinging wildly between acoustic frequencies. By contrast, the acoustic measurements from a humanlike virtual voice box—with only vocal folds and no vocal membranes—resembled those of a stable, humanlike voice.

Fitch says vocal membranes may have evolved to help nonhuman primates loudly and efficiently produce a wide range of frequencies, but at the cost of vocal stability. “They make you sound like Kurt Cobain and not an opera singer,” Fitch says. (You can listen to simulated primate and human speech from one of his team's earlier experiments here.)

Deep scans of the larynges of 44 primate species revealed that every nonhuman primate—from baboons to marmosets to orangutans—possessed these vocal membranes, the researchers report today in Science.

The results suggest humans lost our vocal membranes in order to make our speech more stable and intelligible as the need to communicate ever more sophisticated information grew, Fitch says. “I think people have a tendency to think of evolution as always getting more complex,” he says. “This is a nice counterexample.”

Vocal membranes don’t fossilize, however, so we can’t say when our species lost them, explains the study’s first author, Takeshi Nishimura, a primatologist at Kyoto University. Yet if scientists can one day identify the genes responsible, he says, they might be able to show if they disappeared, say, only after humans evolved.

“I think the data are really convincing in showing that these membranes were lost in humans,” says Katarzyna Pisanski, who studies bioacoustics and vocal communication at CNRS, the French national research center. “But why they were lost is harder to support.” The researchers haven’t presented solid evidence that having vocal membranes would invariably lead to rough, harsh speech, she argues, nor have they demonstrated that such speech would be difficult to understand.

Correction, 11 August, 3:04 p.m.:A previous version of this article incorrectly stated that vocal membranes evolved to make our speech more stable.

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Your simple throat is the reason you don't sound like a chimp - Science
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