Scientists have identified evolutionary changes in the voice box that distinguish humans from other animals in supporting a skill essential to humanity: speaking.
After examining the voice box, or larynx, in 43 species of animals, researchers reported Thursday that humans differ from monkeys and apes in that they lack an anatomical structure called the vocal cords: small, ribbon-like extensions of the vocal cords.
Humans also lack the balloon-like laryngeal structures called air sacs that may help some monkeys and apes produce loud and resonant calls and avoid hyperventilating.
The loss of this tissue, according to the researchers, left humans with a stable vocal cord that was critical to the evolution of speech — the ability to express thoughts and feelings using clear sounds.
They suggested that this simplification of the larynx enabled humans to have better pitch control with long and steady speech sounds.
“We argue that the highly complex vocal structures of nonhuman primates make it difficult to accurately control vibrations,” said lead author Takeshi Nishimura of Kyoto University’s Center for the Evolution of Human Behavior. Published in the journal Science.
“Vocal membranes allow other animals to call louder, more vocally than humans — but they also make vocal disturbances and noiseless vocal dissonance more common,” said W Tecumseh Fitch, an evolutionary biologist at the University of Vienna and study co-author.
The larynx is a hollow tube in the throat that is attached to the trachea and contains the vocal cords, used for speaking, breathing, and swallowing.
“The larynx is the vocal organ that produces 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, diverged from the one that led to our closest relatives, the chimpanzees, about 6m to 7m years ago, after which laryngeal changes sometimes occurred.
Because these soft tissues are not suitable for preservation in fossils, only living species were included in the study. It is unclear when the changes occurred.
Fitch said it’s possible that the larynx was simplified in a human precursor called Australopithecus, which combined ape- and human-like traits and appeared in Africa about 3.85 million years ago, or later in the first African species, Homo. 2.4 million years ago. Homo sapiens appeared in Africa about 300,000 years ago.
The researchers studied laryngeal anatomy in primates including chimpanzees, gorillas, orangutans, and gibbons, as well as Old World monkeys including macaques, kunas, baboons, and mandrills, and New World monkeys including capuchins, tamarins, marmosets, and tadpoles.
While this evolutionary simplification of the larynx was important, it “did not give us speech by itself,” Fitch noted, pointing out that other anatomical features were important for speech over time, including changes in the position of the larynx.
The mechanisms of sound production are similar in humans and nonhuman animals, with air from the lungs driving the oscillations of the vocal cords. The sound energy generated in this way then passes through the larynx, 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 not identical,” said Harold Gousholz, a psychologist and principal at Emory University in Atlanta. A Commentary on Science with a Study Companion.
“Speech is an audible sound-based method of language expression – and humans are the only animals capable of producing it.”
Paradoxically, the increased complexity of human spoken language followed evolutionary simplicity.
“I think it’s really interesting that sometimes ‘less is more’ in evolution — that by losing a trait you can open the door to some new adaptations,” Fitch said.
“Total coffee junkie. Tv ninja. Unapologetic problem solver. Beer expert.”