Read
this sentence aloud, if you are able.
As you do, a
cascade of motion begins, forcing air from your lungs through two muscles,
which vibrate, sculpting sound waves that pass through your mouth and into the
world. These muscles are called vocal cords, or vocal folds, and their
vibrations form the foundations of the human voice.
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They also speak to
the emergence and evolution of human language.
For several years,
a team of scientists based mainly in
Japan used imaging technology to study the
physiology of the throats of 43 species of primates, from baboons and
orangutans to macaques and chimpanzees, as well as humans. All the species but
one had a similar anatomical structure: an extra set of protruding muscles,
called vocal membranes or vocal lips, just above the vocal cords. The exception
was Homo sapiens.
The researchers
also found that the presence of vocal lips destabilized the other primates’
voices, rendering their tone and timbre more chaotic and unpredictable.
Animals with vocal lips have a more grating, less controlled baseline of communication,
the study found; humans, lacking the extra membranes, can exchange softer, more
stable sounds. The findings were published Thursday in the journal Science.
“It’s an
interesting little nuance, this change to the human condition,” said Drew
Rendall, a biologist at the University of New Brunswick who was not involved in
the research. “The addition, if you want to think of it this way, is actually a
subtraction.”
That many primates
have vocal lips has long been known, but their role in communication has not
been entirely clear. In 1984, Sugio Hayama, a biologist at
Kyoto University,
videotaped the inside of a chimpanzee’s throat to study its reflexes under
anesthesia. The video also happened to capture a moment when the chimp woke and
began hollering, softly at first, then with more power.
People have been talking about evolutionary changes in our throats and oral cavity for many years, but this is the first time we took a close look at the larynx in a large selection of monkeys and apes
Decades later,
Takeshi Nishimura, a former student of Hayama and now a biologist at
Kyoto University and the principal investigator of the recent research, studied the
footage with renewed interest. He found that the chimp’s vocal lips and vocal
cords were vibrating together, which added a layer of mechanical complexity to
the chimp’s voice that made it difficult to fine-tune.
Nishimura and his
colleagues wondered whether vocal lips played a significant role in primate
communication generally, so they set out to study the throats of as many
primate species as they could. The presence of an extra membrane in all the
animals was surprising and vindicating.
“People have been
talking about evolutionary changes in our throats and oral cavity for many
years, but this is the first time we took a close look at the larynx in a large
selection of monkeys and apes,” said William Tecumseh Fitch, a biologist at the
University of Vienna and one of the authors of the paper.
Asif Ghazafar, a
psychologist at
Princeton University who was not involved in the research,
said: “No one’s done a systematic evaluation like that. We didn’t have a large
sense of what primates had it and what primates didn’t. We kind of had a guess,
but this study nailed it.”
The ubiquity of
the vocal lips did not necessarily mean that they affected the sounds their
possessors produced. So Nishimura’s group removed the larynges from three
deceased chimpanzees and attached them to simulated lungs; they did the same
with six rhesus macaques that had been euthanized for other approved
experiments. In all the simulations, the vocal lips and the vocal cords
vibrated in unison. Mathematical models of other primates’ larynges yielded
similar results.
In their paper,
the researchers propose that the absence of vocal lips — and their complicating
vibrations — in humans was a key factor in the evolution of language in our
species. Vibrating in splendid isolation, our vocal cords allowed for subtle
changes in inflection and register that characterize our own speech. We reason
and cajole, plead and suggest, all in a controlled manner.
“This study has
shown that evolutionary modifications in the larynx were necessary for the
evolution of spoken language,” Nishimura said.
Rendall added: “It
suggests, or reinforces, that there’s a completely different change in tactic
from human communication to nonhuman primate communication. Human language
doesn’t target the emotional response, but you’re trying to change their mind —
you’re hitting the cognitive and inferential systems.”
Still, Rendall
said, primates often speak softly and subtly, and humans often communicate
through screams and yells. He recommended a “healthy skepticism” in
extrapolating from the anatomical finding the origins of complex speech and
language. “I think they’ve just highlighted the fact that this loss of membrane
in humans is probably centrally important to our ability to produce these
stable vocal fold vibrations, which underlies the production of speech sounds,”
he said.
Harold Gouzoules,
a psychologist at
Emory University who wrote an accompanying commentary to the
recent paper, agreed. “Establishing causality here is essentially impossible,”
he said. “It might be a necessary step in the evolution of language, but
whether it is absolutely critical remains to be seen.”
Gouzoules said that the
research was most noteworthy for its comparative analysis of primates and its
ability to draw evolutionary insights, to a degree, from simple anatomy, which
often hides in plain sight. “Language is clearly more than the sum of its
parts,” he said. “It’s just not likely that we’re ever going to have a
completely satisfactory explanation.”
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