Scientists have identified evolutionary modifications to the voice box that distinguish humans from other primates that can support an ability vital to humanity: speech.
Researchers said Thursday that a study of the voice box, or larynx, in 43 primate species showed that humans differ from apes and apes in the lack of an anatomical structure called a vocal membrane: small, ribbon-like extensions of the vocal cords.
Humans also lack balloon-like laryngeal structures called air sacs, which can help some monkeys and apes produce loud and resonant calls and prevent hyperventilation, they found.
The loss of these tissues resulted in a stable vocal source in humans that was crucial to the evolution of speech — the ability to express thoughts and feelings using articulate sounds, according to the researchers.
This simplification of the larynx allowed people to have excellent pitch control with long and stable speech sounds, they said.
“We argue that the more complicated vocal structures in non-human primates can make it difficult to control vibrations with precision,” said primatologist Takeshi Nishimura of the Center for the Evolutionary Origins of Human Behavior at Kyoto University in Japan, lead author of the study. published in the magazine. Science.
“Vocal membranes enable other primates to make louder, higher-pitched sounds than humans — but they make vocal breaks and noisy vocal irregularities more common,” said evolutionary biologist and study co-author W Tecumseh Fitch of the University of Vienna.
The larynx, a hollow tube in the throat that connects to the top of the windpipe and contains the vocal cords, is used for talking, breathing, and swallowing.
“The larynx is the vocal organ, which creates the signal we use to sing and speak,” Fitch said.
Humans are primates, just like monkeys and apes. The evolutionary lineage that led to our species, Homo sapiens, split from that that led to our closest living relatives, chimpanzees, about 6 to 7 million years ago, with the laryngeal changes occurring some time after.
Only living species were included in the study because these soft tissues are not suitable to be preserved in fossils. As a result, it is also unclear when the changes took place.
Fitch said it’s possible the laryngeal simplification originated in a human precursor called Australopithecus, which combined ape-like and hominin traits and first appeared in Africa about 3.85 million years ago, or later in our genus Homo, which first appeared. in Africa about 2.4 million years ago. Homo sapiens originated in Africa over 300,000 years ago.
The researchers studied the anatomy of the larynx in great apes, including chimpanzees, gorillas, orangutans and gibbons, as well as Old World monkeys including macaques, guenons, baboons and mandrills, and New World monkeys including capuchins, tamarins, marmosets and titis.
While this evolutionary simplification of the larynx was crucial, it “didn’t give us speech on its own,” Fitch noted, noting that other anatomical features were important for speech over time, including a change in the position of the larynx. .
Sound production mechanisms in humans and non-human primates are similar, with air from the lungs driving oscillations of the vocal cords. Acoustic energy generated in this way then passes through the pharynx, oral cavity and nasal cavity and emerges in a form determined by filtering specific frequencies determined by the vocal tract.
“Speech and language are critically related, but not synonymous,” said primatologist and psychologist Harold Gouzoules of Emory University in Atlanta, who wrote a commentary in Science on the study.
“Speech is the audible, sound-based mode of language expression — and humans, only among the primates, can produce it.”
Paradoxically, the increased complexity of human spoken language followed an evolutionary simplification.
“I think it’s kind of interesting that sometimes in evolution ‘less is more’ — that by losing a trait you could open the door to new adaptations,” Fitch said.