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, published today in Science, 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.
Lead author of the study, Kyoto University Associate Professor Takeshi Nishimura, said “These results came from a multidisciplinary, international collaboration. Our work was largely based on the larynx collection established by the late Dr. Sugio Hayama half a century ago. This study has shown that evolutionary modifications in the larynx were necessary for the evolution of spoken language. Our research shows that the simplicity of vocal fold morphology is also linked to the ease with which its vibrations can be controlled.
Humans are able to speak through volitional regulation of the vocal fold actions and breathing, which are essentially involuntary, in accordance with a speech plan created by the brain. We argue that the more complicated vocal structures in nonhuman primates can make it difficult to control vibrations with precision. The results of this study reaffirmed the need to understand the biological basis and co-evolution of neural, physiological, and physical faculties for speech in order to further deepen our understanding of the evolutionary processes of speech."
Senior author Professor Tecumseh Fitch, University of Vienna, 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.”
Evolutionary biologist Dr Jake Dunn of Anglia Ruskin University and the University of Cambridge, co-author to the study, 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”.
Co-author Dr Jaap Saers, affiliated lecture in biological anthropology at the University of Cambridge, said "I used my experience with CT scanning and analysis of 3D data to investigate the first source of evidence: comparative anatomy. I travelled to the Primate Research Institute in Japan to CT scan a large collection of larynges established by the late Dr. Sugio Hayama half a century ago. This was a challenging task as chemical changes in the anatomical specimens due to being stored in jars for so long made them difficult to image. In the end we managed to get it done by staining them with iodine sourced from a local pharmacy. Based on these scans we could see that the one thing distinguishing the human larynx from those of other primates was a lack of vocal membranes.
Combining this finding with evidence from vocalisations in different species and mathematical modelling showed that, essentially, having a system with fewer moving parts means humans have a much more stable production of sounds. The beauty of this finding is that the increased complexity of human spoken language is thus the result of a simplification of our laryngeal anatomy."
Fitch further explained 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”
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Nishimura, T., Tokuda, I. T., Miyachi, S., Dunn, J. C., Herbst, C. T., . . . Fitch, W. T. Evolutionary loss of complexity in human vocal anatomy as an adaptation for speech Science.
