Scientists have identified evolutionary modifications to the voice box that distinguish humans from other primates that may underpin an ability indispensable to humanity: speech.
Researchers said Thursday that an examination of the voice box, or larynx, in 43 species of primates showed that humans differ from apes and monkeys by lacking an anatomical structure called the vocal cords — small extensions of the vocal cords vowels in the form of tapes.
Humans also lack balloon-like laryngeal structures called air sacs that may help some apes and monkeys produce loud, resonant screams and avoid hyperventilation, they found.
The loss of these tissues, the researchers say, resulted in a stable vocal source in humans that was central to the evolution of speech: the ability to express thoughts and feelings through articulate sounds.
This simplification of the larynx allowed humans to have excellent pitch control with long, stable speech sounds, they said.
“We argue that the more complicated vocal structures of nonhuman primates may make it difficult to control vibrations precisely,” said primatologist Takeshi Nishimura of the Center for the Evolutionary Origins of Human Behavior at Kyoto University in Japan, lead author of the research published in the journal. science
“Vocal membranes allow other primates to make louder, higher-pitched cries than humans, but they make voice interruptions and noisy vocal irregularities more frequent,” said evolutionary biologist and study co-author W Tecumseh Fitch from the University of Vienna.
The larynx, a hollow tube in the throat that is connected to the top of the trachea and contains the vocal cords, is used for speaking, breathing, and swallowing.
“The larynx is the organ of the voice, which creates the signal we use to sing and speak,” Fitch said.
Humans are primates, just like monkeys and apes. The evolutionary lineage that gave rise to our species, Homo sapiens, diverged from that which led to our closest living relatives, the chimpanzees, approximately 6 to 7 million years ago, and laryngeal changes occur some time later.
Only living species were included in the study because these soft tissues are not suitable for preservation in fossils. This also means that it is unclear when the changes occurred.
Fitch said it’s possible that the simplification of the larynx arose in a human precursor called Australopithecus, which combined ape- and human-like traits and first appeared in Africa about 3.85 million years ago, or later in our genus Homo, which first appeared in Africa ca. 2.4 million years ago. Homo sapiens originated more than 300,000 years ago in Africa.
The researchers studied the anatomy of the larynx in apes such as chimpanzees, gorillas, orangutans and gibbons, as well as Old World monkeys such as macaques, guenoons, baboons and mandrills and New World monkeys including capuchins, tamarins , marmosets and marmosets.
While this evolutionary simplification of the larynx was critical, “it didn’t give us speech by itself,” Fitch noted, noting that other anatomical features mattered for speech over time, including a change in the position of the larynx.
The mechanisms of sound production in humans and non-human primates are similar, with air in the lungs driving the oscillations of the vocal cords. The acoustic energy generated in this way then passes through the pharyngeal, oral and nasal cavities and emerges in a form governed by the filtering of specific frequencies dictated by the vocal tract.
“Speech and language are critically related, but they are not synonymous,” said primatologist and psychologist Harold Gouzoules of Emory University in Atlanta, who wrote a Science commentary accompanying the study.
“Speech is the audible, sound-based form of language expression, and humans alone among primates can produce it.”
Paradoxically, the increase in complexity of human spoken language followed an evolutionary simplification.
“I think it’s pretty interesting that sometimes in evolution ‘less is more’ — that by losing a trait you can open the door to some new adaptations,” Fitch said.
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