Vestigial Vocal Organ Muffles Human Speech

July 18, 2008 -- All four great apes -- humans, chimps, gorillas and orangutans -- have vocal tract air sacs evolved for calling out to others over long distances. In humans, new research suggests, the anatomical structures have shrunk, leaving us with the vestiges of the sacs and much quieter voices as a result.

The find highlights how human evolution sacrificed volume for a better ability to speak with others, one on one.

Such "private talk" allows an individual to exclude unwanted listeners, such as eavesdropping prey in the wild or business rivals in modern life.

For many animals, explained lead author Tobias Riede, "an amplifying device is helpful."

"Unfortunately, it comes with a cost," added Riede, who is a researcher at the National Center for Voice and Speech in Denver. "You have to fine-tune it in order to keep the voice from breaking."

He and his colleagues came to that conclusion after studying models of mammalian air sacs, made "Myth Busters"-style out of PVC pipe, an inflatable urinary bladder from a pig, and other items. All experiments were conducted at the Japan Institute of Science and Technology, where Riede was a visiting researcher.

The scientists found that the larger air sacs in our ape ancestors, as well as certain other mammals, accomplish three things when an individual vocalizes. First, they make sounds louder.

"This happens if the acoustic resonance frequency of the air sacs meets the vibration frequency of the vocal folds," explained Riede.

Second, they change the spectral characteristics of sounds, meaning that the timbre or pitch can vary among individuals.

Finally, the air sacs cause vocalizations to break at times, giving the voice a hoarse, uncontrolled quality, not unlike a singer trying to reach a high note whose voice instead cracks.