For most humans, speech flows naturally. A child's first words are celebrated, but once we are lingual, we just open our mouths and speak. But how did we come to have smooth, spoken language?
Scientists have turned to our relatives, non-human primates, for answers. In the hunt for what makes us unique, researchers have some ideas. One distinction they have long highlighted is the position of humans' voice box. Humans have a low larynx, while other primates have a high larynx. And scientists have thought that a low larynx is required to produce a variety of distinct vowel sounds, thought to be necessary for spoken language.
But now scientists are shifting their thinking. Vowel sounds might not be such a unique characteristic of human vocalizations, as a new study of baboons finds that they, too, produce five vowel sounds in their calls.
Not only could this upend researchers' ideas of what makes human vocalization unique, it could also suggest the roots of human speech lie much further back in the primate family tree.
To study baboon calls, the researchers first recorded baboon vocalizations for a year. Then they analyzed the recorded calls in search of formants – frequencies of sound that are distinct characteristics of vowels.
Then, using measurements of the baboons' vocal tracts, the researchers mapped these formants onto the vocal space of baboons and inferred that the animals can produce five different kinds of vowel-like sounds. Their results are detailed in a paper published Wednesday in the journal PLOS ONE.
"We know that any tube can make something that looks like a vowel. But what is the difference here is that we have five," says Joël Fagot, a cognitive scientist at the French National Center for Scientific Research and one of the study authors. And that vocal richness suggests that the vowel-like sounds fall into a system of vocalization.
This research resolves a kind of paradox in baboon vocalization, Dr. Fagot says, in a phone interview with The Christian Science Monitor. Non-human primates, like baboons, are known to use different calls for different kinds of communication. But, he says, "there was a very prevalent idea that these animals are just unable to make sounds that would be similar to vowels. So there was this kind of mismatch" and it hinged on vowels.
Philip Lieberman, a linguist and cognitive scientist, and professor emeritus at Brown University, isn't so sure that the researchers are picking up formant frequencies in their acoustic analysis of the baboon recordings. Instead, he says in a phone interview with the Monitor, they might be picking up mostly the pitch of a baboon's voice.
W. Tecumseh Fitch, a professor of cognitive biology at the University of Vienna, agrees that the methods Fagot and his colleagues used are "tricky." Using that technique in animals takes some trial and error, he explains in an email to the Monitor, but "fortunately, David Reby (the editor of this paper) knows all about this – that's why I basically trust the result."
The baboons' anatomy may be physically capable of producing vowel sounds, Dr. Lieberman says, but that doesn't mean they are capable of speech. Identifying formant frequencies among a bunch of calls doesn't mean they can split up the calls and rearrange them to form sounds more similar to human spoken language.
Fagot agrees. "To be able to speak, you need much more than this," he says. Like, say, cognitive ability.
"But clearly they seem capable of at least having some of the building blocks of speech, which is the ability to form vowel-like sounds," Fagot says.
Vowels are produced by manipulating the vocal tract and are thought to be particularly difficult to produce for animals with vocal tracts distinct from ours, explains Thore Jon Bergman, an evolutionary biopsychologist at the University of Michigan who was not part of the baboon study. "The idea was that, based on their anatomy, primates just didn't have the ability to produce a wide range of formant positions, which are needed to make the vowel sounds – which is clearly not true."
That's not to say that a baboon can actually speak smoothly like a human and has just been holding out on us. This paper is probably suggesting that "if you somehow were able to wire a human brain up to a baboon vocal tract, you would be able to probably make yourself understood," Dr. Bergman tells the Monitor in a phone interview, though the sounds could be slurred and mushy.
This baboon study follows a report published in December that found that the macaque vocal tract is capable of making the same vowel sounds as a human one, despite structural differences.
"This study provides additional evidence, complementary to our recent findings, that scientists have underestimated the flexibility of the primate vocal tract," says Dr. Fitch, who was the lead author of the macaque study. "This corroborates our argument that primitive vocal anatomy would not have been a hindrance to evolving spoken language."
Taken together, the two studies could change scientists' ideas of how human speech evolved.
If the low larynx was a necessary prerequisite for speech, as previously thought, that would mean the roots of human speech could only go back within our own lineage, say, 100,000 years. But without the restraint of that vocal anatomy, it's possible the building blocks, as Fagot put it, of speech could have been around much longer than modern humans.
With macaques and baboons displaying some of those building blocks, Fagot says, the roots of human speech could actually go back to the common ancestor of humans and monkeys, which is thought to have lived some 25 million years ago.
"I agree that [this study] suggests that the ability to produce this variety of sounds, especially this vowel space, the different formant placements, probably is fairly evolutionarily old and is not a recent adaptation to speech," Bergman says. "Rather than speech driving some new ability to make new sounds, it probably just built off of and refined some of the abilities that have been in the primate lineage for a long time."