By Will Dunham
WASHINGTON — Scientists have identified evolutionary modifications in the voice box that distinguish humans from other primates that may underlie an ability essential to humanity — speech.
Researchers said Thursday that a study of the voice box, known as the larynx, in 43 primate species shows that humans differ from monkeys and apes in lacking an anatomical structure called the vocal cords — small, band-like extensions of the vocal cords.
Humans also lack the balloon-like laryngeal structures called air sacs, which may help some monkeys and apes produce loud and resonant calls and avoid hyperventilation, they found.
The loss of these tissues, the researchers say, led to a stable vocal source in humans that was critical to the evolution of speech—the ability to express thoughts and feelings using articulate sounds. This simplification of the larynx allows people to have excellent pitch control with long and stable speech sounds, they said.
“We argue that the more complex vocal structures in non-human primates may make precise vibration control difficult,” said primatologist Takeshi Nishimura of Kyoto University’s Center for the Evolutionary Origins of Human Behavior in Japan, lead author of the study, published in the journal Science http://www.science.org/doi/10.1126/science.abm1574.
“Vocal membranes allow other primates to make louder, higher-pitched voices 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 in Austria.
The larynx, a hollow tube in the throat that is connected to the upper part of the trachea and contains the vocal cords, is 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, like monkeys and apes. The evolutionary line that gave rise to our species, Homo sapiens, diverged from that which gave rise to our closest living relatives, the chimpanzees, approximately 6-7 million years ago, with changes in the larynx occurring sometime after that.
Only living species were included in the study, as these soft tissues are not suitable for preservation in fossils. This also means that it is not clear when the changes occurred.
Fitch said it is possible that laryngeal simplification arose in a human ancestor called Australopithecus, which combined ape-like and human-like features and first appeared in Africa approximately 3.85 million years ago, or later in our genus Homo, which first appeared in Africa about 2.4 million years ago. Homo sapiens arose more than 300,000 years ago in Africa.
Researchers have 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 titas.
While this evolutionary simplification of the larynx was key, it “didn’t give us speech by itself,” Fitch noted, pointing out 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 from the lungs driving the vibrations of the vocal cords. The acoustic energy thus generated then passes through the pharyngeal, oral and nasal cavities and appears 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 Guzules of Emory University in Atlanta, who wrote a Science commentary accompanying the study. “Speech is the sound expression of language – and humans, alone among primates, can produce it.”
Paradoxically, the increased complexity of human spoken language followed evolutionary simplification.
“I think it’s quite interesting that sometimes in evolution, ‘less is more’ — that by losing a trait, you can open the door to some new adaptations,” Fitch said.
Add Comment