Human speech and language are highly complex, consisting of a large number of sounds. The human larynx (voice box) has acquired the capability to create a wider array of sounds, even though previous work has revealed many similarities between our larynx and those in other primates. Looking across a wide range of primates, researchers from Japan and Europe used a combination of anatomical, phonal, and modeling approaches to characterize sound production in the larynx. The authors found that instead of the human larynx having increased complexity, it has actually simplified relative to other primates, allowing for clearer sound production.
The larynx is an organ in the top of the neck involved in breathing, producing sound and protecting the trachea against food aspiration. Image credit: OpenStax College / CC BY 3.0.
Human vocal production is based on the same acoustic and physiological principles as vocal production in other terrestrial vertebrates — air from the lungs drives the oscillation of vocal folds in the larynx.
However, human speech has several distinctive features.
The oscillations of human vocal folds are far more stable, lacking the irregular oscillations and abrupt frequency transitions commonly seen in most other mammals.
Combined with enhanced neural control, these attributes allow humans to create the wide array of sounds that enable speech and spoken language.
Yet identifying the evolutionary adaptations that gave rise to human speech has been challenging.
“Paradoxically, the increased complexity of human communication involved a simplification of our vocal anatomy,” said Dr. Takeshi Nishimura, a researcher in the Center for the Evolutionary Origins of Human Behavior and the Primate Research Institute at Kyoto University.
Using magnetic resonance imaging and computed tomography, Dr. Nishimura and his colleagues examined the larynges from 29 genera and 44 species of primates.
They discovered that all nonhuman species possessed a vocal membrane that is wholly absent in humans.
After observing the activity of this membrane during primate vocalizations, they developed anatomical and phonal models to compare the acoustic effects of vibration of this membrane.
This vocal membrane — specifically its evolutionary loss — resulted in humans’ stable vocal source and enables the ability to produce the diverse and harmonic-rich sounds that characterize human speech.
“The thin vocal membranes found in the larynx in our large selection of monkeys and apes are specific to nonhuman primates,” said Dr. Tecumseh Fitch, a researcher in the Department of Behavioral and Cognitive Biology and the Cognitive Science Hub at the University of Vienna.
“Inside the larynx of vocalizing chimpanzees and monkeys, we see active vibrations of their vocal membranes causing loud and unstable scream-like calls.”
“The presence of vibrating tissues to the vocal folds may increase the vibrational degrees of freedom, causing frequent vocal instability,” said Dr. Isao Tokuda, a researcher in the Department of Mechanical Engineering at Ritsumeikan University.
“By avoiding this instability, humans possibly achieved stable source sounds, accelerating the evolution of human language.”
“Using the comparative method to reconstruct our evolutionary past has shown that, if humans lone lack the vocal membranes that virtually all nonhuman primates have had as a trait, we may have lost it in our recent evolution despite sharing a common ancestor,” said Dr. Jake Dunn, a researcher in the Behavioural Ecology Research Group at Anglia Ruskin University, the Department of Archaeology at the University of Cambridge, and the Department of Behavioral and Cognitive Biology at the University of Vienna.
“Other changes, including those in our brains were also needed to gain language, of course, but this anatomical simplification probably accelerated the accuracy with which we sing and speak,” Dr. Nishimura said.
The results were published this week in the journal Science.
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Takeshi Nishimura et al. 2022. Evolutionary loss of complexity in human vocal anatomy as an adaptation for speech. Science 377 (6607): 760-763; doi: 10.1126/science.abm1574
Source link: https://www.sci.news/biology/human-larynx-11090.html
