The anatomical transformation that allowed human beings to speak

Research has delved into the details of the anatomical transformation by which evolution led to the appearance of the ability to speak in humans. The results of the study include a striking finding.

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 exhibits several distinctive features. The oscillations of our vocal folds are much more stable and lack 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 range of sounds that speech and spoken language allow.

However, identifying the evolutionary adaptations that gave rise to human speech has been a challenge.

Using MRI and CT scans, Takeshi Nishimura of Kyoto University in Japan and colleagues examined the larynxes of 29 genera and 44 species of primates, finding in all non-human taxa a vocal membrane that is entirely absent in humans.

Artistic recreation of a visual representation of the sound of human voices. (Illustration: Amazings/NCYT)

After observing the activity of this membrane during primate vocalizations, Nishimura’s team developed anatomical and phonal models to compare the acoustic effects of vibrating this membrane. This vocal membrane (specifically its evolutionary loss) resulted in the stable vocal source of humans and enables the ability to produce the diverse and harmonic-rich sounds that characterize human speech.

What makes this study especially remarkable is its finding that it was a simplification, rather than an increase in complexity, that allowed the laryngeal anatomy to acquire the characteristics necessary to enable complex speech in humans. Specifically, the loss of vocal membranes and air sacs common to all nonhuman primates allowed for the increased acoustic complexity required for human speech.

The study is entitled “Evolutionary loss of complexity in human vocal anatomy as an adaptation for speech”. And it has been published in the academic journal Science. (Source: AAAS)