For decades, scientists thought that most primates could not produce vowels, sounds fundamental to human speech. That’s because nonhumans supposedly lacked the necessary vocal anatomy. But now, researchers report that Guinea baboons, monkeys that inhabit the forests and savanna of West Africa, make five vowellike sounds similar to those used by humans. The findings bolster a recent study showing that Japanese macaques are also anatomically capable of speech. Together, the work suggests that the basic elements of spoken language began to evolve much earlier than suspected, at least 25 million years ago.
“It perfectly complements our own results,” says William Tecumseh Fitch, an evolutionary biologist and cognitive scientist at the University of Vienna and the lead author of the macaque study. “But they’re looking at what baboons actually do,” not a simulation as in his team’s research, he adds. The discovery “provides additional evidence that scientists have underestimated the flexibility of the primate vocal tract.”
That error stems from a fundamental misunderstanding of the monkey larynx, says Joel Fagot, a primatologist at Aix-Marseille University in France and an author of the new study. “It was thought that in order to pronounce vowels, you had to have a low larynx [voice box], as humans do,” he says. Because monkey larynxes are set much higher than our own, scientists thought this anatomical difference explained why primates could not utter vowels, which are “critical for language,” Fagot says. “You can’t have language without them.” Yet human babies with high larynxes can also pronounce vowels, a phenomenon that perplexed Fagot and his colleagues.
So why couldn’t other primates do the same? Although some studies have mentioned vowellike sounds in baboons and other monkeys as well as chimpanzees, no one had carried out an in-depth study to see how the animals made them or whether they combined them, a key element of speech. To find out, Fagot’s colleague and the study’s lead author, Louis-Jean Boe from Grenoble Alpes University in France, recorded 1404 vocalizations of 15 Guinea baboons (Papio papio) living at a primate center in France. The baboons’ vocal repertoire included grunts, barks, copulation calls (made only by females), distress calls known as yaks, and wahoos, long-distance contact calls most often made by males.
Their analysis of the calls revealed something the other research had not: that the baboons produced at least five distinct sounds that correspond to vowels in the International Phonetic Alphabet, the authors report today in PLOS ONE. That’s enough to put them on par with many human languages, most of which have three to five vowels, though some have as many as 24. Further, the baboons regularly combined two vowels in rapid succession into a single call: “Wahoo!” And that means they have “some kind of system for combining and using the sounds,” says Fagot, another skill once thought unique to humans. That doesn’t mean they have a language, which requires a structure with rules for combining those sounds, but, says Fagot, they have some of the building blocks for it.
The scientists also dissected the vocal tracts of two baboons that died of natural causes. They found that the monkeys’ tongues have the same muscles as human tongues, which indicates they can make precise movements to form each vowellike sound—something scientists had not looked at in such detail before. It is this ability to control the tongue, rather than the position of the larynx, that is key to producing vowellike sounds, the researchers note.
“This is extremely significant research,” says John Esling, a linguist at the University of Victoria in Canada. “There’s no justifiable reason,” he adds, for these past assumptions about what’s necessary for human speech sounds.
Other researchers have argued that human language appeared only when we evolved our low larynx about 70,000 to 100,000 years ago. But the new study, coupled with Fitch’s work, shows that monkeys have the necessary anatomical features for speech. And that means the last common ancestor of baboons and humans also possessed some of these capabilities. Rather than being recent, says Esling, the talent for speech has likely been around “for many more millions of years than previously imagined.”