Unlike people, 43 species of monkeys and apes are known to have so-called vocal membranes, which may prevent them from having precise voice control
11 August 2022
The loss of small tissue structures from the voice box may have been essential for the evolution of human speech.
In a study of 43 non-human primates, all the animals had “vocal membranes”, a small extension of the throat’s vocal cords that makes their sounds louder and higher but also more irregular and harder to control.
As humans lack vocal membranes, this suggests they were lost when our ancestors diverged from chimpanzees to allow more precise voice control, says Tecumseh Fitch at the University of Vienna in Austria.
While many animals make calls to communicate, the evolution of complex human speech seems to have required anatomical changes, as well as changes in the brain. In humans, the vocal cords are flaps of tissue in the throat that vibrate as air is expelled from the lungs, allowing us to make “voiced” sounds, as opposed to breathy ones.
We already knew that a few species of monkeys and apes have vocal membranes. 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 apes and monkeys. This was done by carrying out magnetic resonance imaging (MRI) or computed tomography (CT) scans on dead or anaesthetised animals in the first such large-scale study of primates. The researchers found that all 43 species had this vocal cord extension.
The team also analysed video footage that showed the voice box of an anaesthetised chimpanzee with an endoscope in its throat while the animal made grunts and growls as it was waking up. They did the same for anaesthetised rhesus macaques and squirrel monkeys that were stimulated to make noises by having an electrode put into the part of their brain that causes them to produce vocalisations.
The researchers found that in all these animals, vibration and collision of the vocal membranes are the primary source of their calls, as their vocal cords were in motion less often.
If humans still had vocal membranes, our speech would probably sound more rough and variable, with abrupt pitch changes, like someone with laryngitis, says Fitch.
“A key thing that distinguishes human speech from animal sounds is our fine-grained control over the sounds we make. That is only possible if our vocal apparatus is easy for our brains to control,” 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 at the University of Warwick in the UK says many apes and monkeys make both loud and irregular calls as well as some quieter and more controlled noises. “The alleged limiting effect [of vocal membranes] on primate vocal production seems exaggerated,” he says.
Journal reference: Science, DOI: 10.1126/science.abm1574
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