The Evolutionary Trait That May Have Led to Human Speech

“Vocal membranes” in primates make their speech grating and unpredictable, study suggests. Humans have no such thing

An orangutan sits open-mouthed
By studying the throats of 43 primate species, researchers found they all had vocal membranes that destabilized their voices. Humans, on the other hand, do not. seng chye teo/Getty Images

A new study suggests that humans’ lack of certain muscles might give us the ability to control our speech.

Humans and other primates make vocal noises using the larynx, a hollow tube connected to the throat that contains the vocal chords and helps with breathing. In the study, a group of scientists examined the larynges of 43 species of non-human primates. All of them had a set of muscles called vocal membranes, located above the vocal cords, reports the New York Times’ Oliver Whang. Humans, on the other hand, do not.

In a paper published Thursday in Science, researchers argue these vocal membranes make it harder for other primates to control the noises they make. This suggests that without them, humans were able to evolve more precise vocal control, Tecumseh Fitch, one of the paper’s authors and a biologist at the University of Vienna in Austria, tells New Scientist’s Clare Wilson.

Scientists already knew that some primates have vocal membranes, but this was the first large-scale study of primate larynges, per New Scientist. The researchers studied a broad range of species through magnetic resonance imaging (MRI) and computed tomography (CT) scans of dead or anesthetized primates.

“No one’s done a systematic evaluation like that,” Asif Ghazanfar, a psychologist at Princeton University who was not involved in the research, tells the Times. “We didn’t have a large sense of what primates had [vocal membranes] and what primates didn’t. We kind of had a guess, but this study nailed it.”

To find out how vocal membranes affected spoken sounds, the researchers attached larynges from three deceased chimpanzees and six rhesus macaques that had been euthanized for other experiments to simulated lungs, according to the Times. They found that the vocal membranes and vocal cords vibrated together. Mathematical models and video evidence also supported this finding, per the Times.

Without these membranes, humans’ vocal source is more stable, allowing better pitch control and production of long and even-toned sounds, reports Will Dunham of Reuters. “A key thing that distinguishes human speech from animal sounds is our fine-grained control over the sounds we make,” Richard Futrell, who studies language processing in humans at the University of California, Irvine, and was not involved in the study, tells New Scientist. “That is only possible if our vocal apparatus is easy for our brains to control.”

However, Adriano Lameira, an evolutionary psychologist at the University of Warwick in the U.K. who was not involved with the paper, tells New Scientist that “the alleged limiting effect [of vocal membranes] on primate vocal production seems exaggerated.” He points out that many apes and monkeys can make quiet and controlled noises.

Additionally, while the authors hypothesize that the loss of vocal membranes played a role in the development of human language, some experts want more evidence. “This study has shown that evolutionary modifications in the larynx were necessary for the evolution of spoken language,” Takeshi Nishimura, a primatologist at Kyoto University in Japan and lead author of the paper, tells the Times.

But Harold Gouzoules, an Emory University psychologist who wrote a commentary on the paper, says to the Times: “It might be a necessary step in the evolution of language, but whether it is absolutely critical remains to be seen.”