Losing parts of our voice box may have helped humans evolve to speak

Unlike humans, 43 species of monkeys are known to have so-called vocal folds, which may prevent them from having precise vocal control

from Claire Wilson

The loss of small tissue structures from the voice box may have been essential to the evolution of human speech.

In a study of 43 nonhuman primates, all the animals had “vocal membranes,” a small expansion of the vocal cords in the throat that made their sounds louder and higher-pitched, but also more irregular and harder to control.

Because humans do not have vocal cords, this suggests that they were lost when our ancestors diverged from chimpanzees to allow for more precise vocal control, says Tecumseh Fitch at the University of Vienna in Austria.

While many animals call to communicate, the evolution of complex human speech appears to have required anatomical changes as well as changes in the brain. In humans, the vocal cords are webs of tissue in the throat that vibrate as air is pushed out of the lungs, allowing us to make “voiced” sounds, as opposed to breathing.

We already knew that several species of monkeys have vocal cords. To better understand the loss of these structures in humans, Fitch’s team looked at the voice box, also known as the larynx, of 43 species of monkeys and apes. This was done by performing magnetic resonance imaging (MRI) or computed tomography (CT) scans on dead or anesthetized animals in the first such large-scale primate study. The researchers found that all 43 species have this expansion of the vocal cords.

The team also analyzed videos that showed the voice box of an anesthetized chimpanzee with an endoscope down its throat as the animal grunted and growled as it woke up. They did the same with anesthetized rhesus macaques and squirrels who were stimulated to make sounds by placing an electrode in the part of their brain that causes them to produce vocalizations.

The researchers found that in all of these animals, the vibration and collision of the vocal membranes was the main source of their calls, as their vocal cords were in motion less often.

If humans still had vocal membranes, our speech would probably sound rougher and choppier, with sudden changes in pitch, like someone with laryngitis, Fitch says.

“A key thing that distinguishes human speech from animal sounds is our fine control over the sounds we make. This is only possible if our voice apparatus is easy to control by our brain,” he says Richard Futrell at the University of California, Irvine. “If the system is complex, then it will behave in a way that is chaotic and unpredictable.”

But Adriano Lameira from the University of Warwick in the UK says that many monkeys and apes make both loud and unusual sounds as well as some quieter and more controlled noises. “The supposed restraining effect [of vocal membranes] for primate vocal production seems exaggerated,” he says.

Journal reference: science, DOI: 10.1126/science.abm1574