Scientists have identified evolutionary modifications in the voice box that distinguish humans from other primates that may support an ability indispensable to humanity: speech.
A study of the voice box, or larynx, of 43 primate species showed that humans differ from monkeys and apes in lacking an anatomical structure called the vocal fold: small, ribbon-like extensions of the vocal cords, researchers said Thursday.
They found that humans also lack balloon-like laryngeal structures called air sacs, which may help some monkeys and apes make loud, ringing calls and avoid hyperventilation.
The loss of this tissue, the researchers say, led to a stable sound source in humans that was critical to the development of speech—the ability to express thoughts and feelings using articulate sounds.
Simplifying the larynx, they said, allowed people to have excellent pitch control with long, steady speech sounds.
“We argue that the more complex vocal structures in non-human primates may make it difficult to accurately control vibration,” said primatologist Takeshi Nishimura of the Center for Evolutionary Origins of Human Behavior in Japan, lead author of the study. Published in Science.
“Vocal membranes allow other primates to produce louder, higher-pitched voices than humans — but make vocal breaks and loud vocal irregularities more common,” said evolutionary biologist and study co-author W Tecumseh Fitch of the University of Vienna.
The larynx is a hollow tube in the throat attached to the upper part of the trachea and containing the vocal cords used for speaking, breathing and swallowing.
“The larynx is the voice organ that creates the signal we use to sing and speak,” Fitch said.
Humans are primates, just like the great apes. The evolutionary line that gave rise to our species, Homo sapiens, diverged from that which gave rise to our closest living relative, the chimpanzee, approximately 6-7 million years ago, sometime after which changes occurred in the larynx.
Only living species were included in the study, as these soft tissues are not suitable for fossil preservation. This also means that it is not clear when the changes occurred.
It’s possible, Fitch said, that laryngeal simplification may have originated in a hominid called Australopithecus, which combined ape-like and human-like features and first appeared in Africa about 3.85 million years ago, or later in our species, which for first appeared across Africa about 3 years ago. 2.4 million years ago. Homo sapiens appeared more than 300,000 years ago in Africa.
Researchers have studied the anatomy of the larynx in apes, including chimpanzees, gorillas, orangutans, and gibbons, as well as Old World monkeys, including macaques, guinea pigs, baboons, and mandrills, and New World monkeys, including capuchins, tamarins, marmosets, and goats.
Although this evolutionary simplification of the larynx was key, it “did not give us speech by itself,” notes Fitch, noting that other anatomical features were important for speech over time, including the change in the position of the larynx.
The mechanisms of sound production in humans and non-human primates are similar, with air coming from the lungs causing the vocal cords to vibrate. The sound energy thus generated then passes through the cavities of the pharynx, mouth and nose and emerges in a form governed by the filtering of specific frequencies dictated by the vocal tract.
said zoologist and psychologist Harold Gozol of Emory University in Atlanta, who wrote a Science Commentary accompanying the study.
“Speech is the auditory, sound-based method of linguistic expression—and humans, alone among primates, can produce it.”
Paradoxically, the increasing complexity of human spoken language follows evolutionary simplification.
“I think it’s interesting that sometimes in evolution it’s ‘less is more’ — that by losing a trait, you can open the door to some new modification,” Fitch said.
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