Increased abilities are often based on anatomical expansion – but the opposite may have been the case in the evolution of human speech: a structural simplification in the larynx may have played an important role. Researchers conclude this from the finding that primates have membrane structures on the vocal folds that humans lack. According to the research results, these elements cause acoustic instability in the monkeys. The loss of these structures could therefore have contributed to the fact that humans were able to produce the clear sounds that are characteristic of language.
No other being can communicate in such a complex way as we can: the ability to convey complex information to our fellow human beings via certain sound sequences has become a key element of the success of our species. Scientists have therefore long been investigating the basics of human language ability. Often the focus is on cognitive abilities, but it is also known that anatomical adaptations play a role.
Targeting the larynx
Basically, human voice production is based on the same acoustic and physiological principles as in other land vertebrates: A current of air causes the vocal folds in the larynx to vibrate. However, it is already assumed that the comparatively deep-seated larynx and set of tongues in humans are linked to the ability to speak. As part of the current study, the international research team has now taken a closer look at the anatomy of the larynx in primates. To do this, the scientists first examined the larynxes of 44 species in detail using magnetic resonance imaging (MRI) and computed tomography (CT).
The researchers found that, with the exception of humans, all primates have certain structural elements in the vocal tract: the vocal folds have additional membrane structures. The scientists then observed the activity of these elements during the vocalizations of some of the primates. It became clear that they have a special vibration behavior. Finally, to characterize the acoustic effects of these vibrations more precisely, the team developed computer models based on the data obtained.
Less is more
It was shown that the membrane structures are linked to the development of characteristic features of animal voices: they oscillate irregularly, causing abrupt frequency transitions. “In the larynx of vocalizing chimpanzees and other monkeys, we see active vibrations of these membranes causing loud and unstable scream-like calls,” says senior author Tecumseh Fitch from the University of Vienna. The loss of these structures in humans seems plausible against the background of these results, the scientists explain: “By avoiding the acoustic instability, humans could possibly produce clearer initial sounds, which accelerated the evolution of human language,” says co-author Isao Tokuda von from the Japanese Ritsumeikan University in Kusatsu.
“If only humans lack these structures that all other primates possess, it suggests that we have lost them in our recent evolutionary history,” says co-author Jacob Dunn of Anglia Ruskin University in Cambridge. Interestingly, the adaptation to speaking in this case was not characterized by the addition of features, emphasizes lead author Takeshi Nishimura from Kyoto University: “Paradoxically, the increasing complexity of human communication went hand in hand with a simplification of our vocal anatomy”. In conclusion, however, the scientist emphasizes: “Of course, other changes, including those in our brains, were necessary to develop language skills, but this anatomical simplification probably increased the accuracy with which we can communicate,” says Nishimura.
Source: Kyoto University, professional article: Science, doi: 10.1126/science.abm1574