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Anatomical Simplifications Allowed Vocal Complexity in Human Speech


Humans simplified the larynx to gain vocal stability

Scientists have long known that chimpanzees and other nonhuman primates cannot speak or sing like humans, but the reasons have been debated.  Although the most important changes during human evolution were in our brain, anatomical changes in vocal anatomy have also been suggested to play a role in our capacity to produce complex sounds.  A new study by a large international team of researchers in Japan and Europe reveals that changes to human vocal anatomy, specifically to the larynx or “voice box,” provided the stable, clear voices we use to communicate. 

Surprisingly, these changes involve a loss of a specific part of the vocal folds (or “vocal cords”) in our larynx: most primates have thin, ribbon-like vocal membranes rising out of their vocal folds, and humans have lost this feature.  This simplification of our vocal anatomy, along with our loss of the air sacs seen in chimpanzees and other apes, seems to have provided the more stable and controllable voice pitch, and less noisy voice quality, that we humans use during singing or speech. Thus, seemingly paradoxically, the increased complexity of human communication involved a simplification of our vocal anatomy.

The senior author of the study, University of Vienna Professor Tecumseh Fitch, said that these results came as a great surprise. “People have been talking about evolutionary changes in our throats and oral cavity for many years, but this is the first time we took a close look at the larynx in a large selection of monkeys and apes. To our surprise, we found that virtually all nonhuman primates have these thin vocal membranes, indicating that we humans lost them in our recent evolution.  We were able to look inside the larynx of vocalizing chimpanzees and monkeys, to see that unstable, noisy calls like screams involve active vibrations of their vocal membranes.”

First author Takeshi Nishimura of the University of Kyoto explained that the study started long ago “when we were using endoscopy to look down the throat of an anesthetized chimpanzee. When the chimpanzee started to wake up, he made a long series of vocalizations that allowed us to literally see his vocal folds vibrating”.

Evolutionary biologist Jake Dunn at Anglia Ruskin University explained that “we can use the comparative method to reconstruct our evolutionary past. When virtually all primates have a trait, such as these vocal membranes, we can assume it was present in their common ancestor, and if humans alone lack the trait, it means we must have lost it in our recent evolution”.

Austrian voice scientist Christian T. Herbst, co-author to the study, thinks that “it is really amazing to have a glimpse into the workings of human evolution: The reduced anatomical complexity in our voice production anatomy seems to coincide with our increased ability to create and transmit extremely rich verbal information that relies on enhanced cognitive and neural predisposition. What we look at here is a tradeoff, where an adaptation in the voice box might have helped to develop the capacity to speak and sing. Very simply put, the ability to produce complex vocal information moved from the throat to the brain”.

Co-author Ole Næsbye Larsen at University of Southern Denmark notes that “a comparison of extant species is often used to infer the evolution of traits such as animal behavior that do not leave a fossil record. When we first videorecorded the workings of the squirrel monkey voice box during vocalization, we had no idea that these recordings would someday support a hypothesis on the evolution of the human ability to talk”.

Fitch explains that “Computer modelling shows that vocal membranes allow primates to create loud, high-pitched vocalizations, but also make their voices unstable and shrieky.  We think that the more melodious quality of the human voice is a direct result of our loss of these membranes. Of course, other changes (especially in our brains) were also needed to gain language, but this anatomical simplification probably improved the speed and accuracy with which we can sing and speak”

Publication in Science

Nishimura, T., Tokuda, I. T., Miyachi, S., Dunn, J. C., Herbst, C. T., . . . Fitch, W. T. (2022). Evolutionary loss of complexity in human vocal anatomy as an adaptation for speech. Science.

DOI: 10.1126/science.abm1574

Scientific contact: 

Prof. W Tecumseh Fitch
Department of Behavioral and Cognitive Biology
Djerassiplatz 1, 1030 Wien

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