“French fries” might not be on the menu if not for ancient farmers, and not because we can now grow plenty of potatoes, but because it would be harder to enunciate the f sounds needed to order them. The ability to make labiodental sounds—which are sounds that require you to put your lower lip on your upper teeth, such as f and v sounds—may not have fully developed until agriculture introduced softer foods to the human diet, changing our jaws, according to an intriguing and controversial study published today in Science.
Orthodontists know that overbite, and the human jaw’s horizontal overlap called overjet, are common among people all over the world. But the study’s authors assert that such jaw structures were rarer in the Paleolithic Period, when hunter-gatherer’s rough diets demanded more force from teeth that met edge to edge. Agriculture softened our ancestors’ diets with processed gruels, stews and yogurts, and this fare led to gradually shrinking lower jaws to produce today’s overcrowded mouths. This diet-driven evolution of the human bite over the last 10,000 years might have shaped some of the sounds we use to communicate today.
University of Zurich linguist Balthasar Bickel hypothesizes that less wear and stress on teeth and jaws allowed overbite to persist more often, creating a close proximity between the upper teeth and lower lip that made it a bit easier to utter f and v sounds. (Try making a “fuh” sound, first with your upper and lower teeth aligned edge to edge and then, probably more successfully, with your bottom jaw pulled back so your lower lip can more easily touch your upper teeth.)
“One of the take-home messages is really that the landscape of sounds that we have is fundamentally affected by the biology of our speech apparatus,” Bickel said at a press conference this week. “It’s not just cultural evolution.”
Each time ancient humans spoke, there was only a small chance of their slowly changing jaw configurations producing labiodental sounds, but like a genetic mutation, it could have caught on over time. “Every utterance that you make is a single trial. And if you think of this as going on for generations over generations, you have thousands and thousands of trials—with always this probability of changing—and that leaves the statistical signal we find in the end,” Bickel said.
Bickel and colleagues tested the idea that overbite helped produce labiodentals by building biomechanical models and making them talk. Their data suggest that making f and v sounds takes 29 percent less muscular effort when the speaker has an overbite/overjet configuration. The researchers then searched for real-world evidence of where labiodental sounds became more common over time.
“We looked into the distribution of labiodental sounds across thousands of languages and their relation to the characteristic sources of food of the people speaking those languages,” Damián Blasi, also of the University of Zurich, said at the press conference. The survey showed that languages spoken by modern hunter-gatherers use only about one-fourth as many labiodental sounds as other languages.
Tecumseh Fitch, an expert on bioacoustics and language evolution at the University of Vienna who was not involved in the new study, says the interdisciplinary approach of biomechanics, bioacoustics, comparative and historical linguistics came to him as a surprise. “This is probably the most convincing study yet showing how biological constraints on language change could themselves change over time due to cultural changes,” he says via email. “The study relies, inevitably, on various assumptions and reconstructions of unknown factors (especially bite structure in current and ancient populations), but I think the authors build a very plausible case that will open the door to future detailed research.”
Still, the evolutionary process remains far from clear. Despite today’s ubiquitous modern human dental orientations around the world, half of about 7,000 existing languages never started to regularly use labiodental sounds at all. And the correlation of the sounds with softer foods doesn’t always hold up. Cooking has been around for hundreds of thousands of years, easing the stress on human teeth and jaws. Ancient Chinese agriculture produced easy-chewing rice, yet f and v sounds aren’t as common in Chinese as they are in Germanic or Romance languages.
Bickel, Blasi and colleagues argue that the evolution of overbite simply meant labiodentals would be produced more often. “That doesn’t mean that labiodentals will emerge within all languages. It does mean that the probability of producing labiodentals increases slightly over time, and that means that some languages are likely to acquire them but not all languages will,” says co-author Steven Moran.
Not everyone is convinced that diet reshaped our tooth alignment in the first place, however. “They haven’t established even that a soft diet would give you an overbite,” says Philip Lieberman, a cognitive scientist at Brown University. “To relate that to diet it has to be epigenetic,” meaning chemical compounds that become attached to genes can change gene activity without altering the DNA sequence. “There has to be some sort of regulatory mechanism that is triggered directly from the environment or diet, and I don’t know of any data on an epigenetic effect restructuring [tooth and jaw position].” Even such a link wouldn’t convince Lieberman that the change prompted the rise of f and v sounds. “We can produce these sounds whether we have overbite or not,” he says. “There’s arbitrariness in language. People have different words for the same things, and I don’t think we can relate any of it to changes in teeth.”
Evolutionary biologist Mark Pagel at the University of Reading found some of the authors suggestions more plausible. “If their argument that having that overbite or overjet has become more prominent in recent fossils is actually true, if you get a developmental change actually changing the shape of our mouths, then there’s a real plausibility to it,” he says, adding that sounds tend to develop via the path of least resistance. “We make more readily the sounds that are easier to make. We’re constantly introducing tiny little variants. And if the shape of your mouth means you are more likely to introduce some kind of variant … then they are just a bit more likely to catch on.”
Despite the correlation between mouth shape and sounds, paleoanthropologist Rick Potts of Smithsonian’s Human Origins Program has reservations about the study’s conclusion that changing diets caused a rise of labiodentals. “In my view they don’t provide sufficient reasons for us embracing diet as the reason for producing [more] v and f sounds because they don’t deal at all with the anatomy of producing those sounds.”
Making v and f sounds, Potts says, requires only very slight retraction of the temporal muscle on the side of the head, which draws the jaw backward with a very subtle movement. “How does a harder diet limit the retraction of the jaw?” he asks. “That’s the essence of being able to make the v and f sounds. They do not in any way demonstrate how a bite-to-bite configuration of the teeth inhibits or makes it more expensive to make these sounds. I can’t see anything in the way teeth are oriented toward one another that would limit the retraction of the jaw.”
Potts says the study identifies some intriguing correlations but falls short in demonstrating likely causation. As an example, he says that if researchers found that the color red was favored by equatorial peoples like the Masai, and they also found that such people had a lower density of light receptors in their retinas than Arctic people, they might conclude that lack of light receptors was a biological cause for preferring the color red.
“But how would you possibly discount the fact that it’s just cultural history why the Masai wear red whereas Arctic people tend not to?” he asks. “It’s just the way people distinguish themselves and it becomes passed on in ways that are geographically oriented. I’m just concerned that [the study] hasn’t given enough credit to the idea of the accidents of cultural history and identity being part of why v and f sounds are less frequent in certain groups of people worldwide than others.”
Balthasar Bickel, on the other hand, says that language has been too often regarded as a purely cultural or intellectual phenomenon, and he hopes his group’s work will help to open new lines of scientific inquiry. “I believe there is a huge potential out there for studying language as part of the biological system it really is embedded in.”