A crying baby, a screaming adult, a teenager whose voice cracks: People could have sounded this shrill all along, a new study suggests, were it not for a crucial step in human evolution.
It is what we are missing that makes the difference. humans have vocal chordsmuscles in our larynx, or larynx, that vibrate to produce sound (Serial number: 11/18/15). But unlike all other primates studied, humans don’t have little bits of tissue above their vocal cords called vocal membranes. That uniquely human trait helps people control their voices well enough to produce the sounds that are the building blocks of spoken language, researchers report in the August 12 issue Sciences.
Vocal membranes act like the reed of a clarinet, making it easy for some animals to yell loud and shrill. Think about piercing calls of howler monkeys (Serial Number: 10/22/15). When the researchers used MRI and CT scans to look for vocal membranes in 43 different species of primates, the scientists were surprised by what they saw: All primates except humans had the tissue.
That loss of vocal membranes would have been a “very important and very revolutionary event in human evolution,” says Takeshi Nishimura, a paleontologist at Kyoto University in Japan.
Primates for the most part make sounds in the same basic way: they expel air from their lungs while vibrating the muscles of the larynx to create sound waves. To understand the role played by vocal membranes, Nishimura’s team studied videos of primate larynxes in action in chimpanzees, rhesus macaques and squirrel monkeys. The researchers also took larynxes from macaques and chimpanzees that had died of natural causes and, in what is common practice in the field, mounted the parts in tubes, pushing air through the larynxes to see how the vocal cords and membranes.
In both experiments, the larynxes made sounds that often fluctuated greatly in pitch. Nishimura’s team found that this happens only when an animal has both vocal membranes and vocal cords.
In humans, that type of screeching can occur when we put extreme pressure on our voice, such as when we scream, or when teenagers struggle to control their growing vocal cords and their voice cracks. But those are rare cases. Since humans don’t have vocal membranes, we generally make steadier sounds than other primates, the team concludes. Our mouths and tongues, the idea goes, can manipulate those stable tones into the complex sounds on which language is based.
“That’s a really elegant explanation,” says Sue Anne Zollinger, an animal physiologist at Manchester Metropolitan University in England who was not involved in the study. It’s almost counterintuitive, she says: “You lose complexity so you can produce more complex sounds.”
The loss of vocal membranes isn’t the only thing that makes humans more eloquent than other primates. Beyond anatomical differences, humans have specific genes which may have helped drive the evolution of language (Serial number: 8/3/18). And perhaps most importantly, human brains are structured differently from other primates in ways that also give us more control over our speech (Serial Number: 12/19/16).