Human sound systems are shaped by post-Neolithic changes in bite configuration
In 1985, the linguist Charles Hockett proposed that the use of teeth and jaws as tools in hunter-gatherer populations makes consonants produced with lower lip and upper teeth (“f” and “v” sounds) hard to produce. He thus conjectured that these sounds were a recent innovation in human language. Blasi et al. combined paleoanthropology, speech sciences, historical linguistics, and methods from evolutionary biology to provide evidence for a Neolithic global change in the sound systems of the world's languages. Spoken languages have thus been shaped by changes in the human bite configuration owing to changes in dietary and behavioral practices since the Neolithic.Science, this issue p. eaav3218INTRODUCTIONHuman speech manifests itself in spectacular diversity, ranging from ubiquitous sounds such as “m” and “a” to the rare click consonants in some languages of southern Africa. This range is generally thought to have been fixed by biological constraints since at least the emergence of Homo sapiens. At the same time, the abundance of each sound in the languages of the world is commonly taken to depend on how easy the sound is to produce, perceive, and learn. This dependency is also regarded as fixed at the species level.RATIONALEGiven this dependency, we expect that any change in the human apparatus for production, perception, or learning affects the probability—or even the range—of the sounds that languages have. Paleoanthropological evidence suggests that the production apparatus has undergone a fundamental change of just this kind since the Neolithic. Although humans generally start out with vertical and horizontal overlap in their bite configuration (overbite and overjet, respectively), masticatory exertion in the Paleolithic gave rise to an edge-to-edge bite after adolescence. Preservation of overbite and overjet began to persist long into adulthood only with the softer diets that started to become prevalent in the wake of agriculture and intensified food processing. We hypothesize that this post-Neolithic decline of edge-to-edge bite enabled the innovation and spread of a new class of speech sounds that is now present in nearly half of the world’s languages: labiodentals, produced by positioning the lower lip against the upper teeth, such as in “f” or “v.”RESULTSBiomechanical models of the speech apparatus show that labiodentals incur about 30% less muscular effort in the overbite and overjet configuration than in the edge-to-edge bite configuration. This difference is not present in similar articulations that place the upper lip, instead of the teeth, against the lower lip (as in bilabial “m,” “w,” or “p”). Our models also show that the overbite and overjet configuration reduces the incidental tooth/lip distance in bilabial articulations to 24 to 70% of their original values, inviting accidental production of labiodentals. The joint effect of a decrease in muscular effort and an increase in accidental production predicts a higher probability of labiodentals in the language of populations where overbite and overjet persist into adulthood. When the persistence of overbite and overjet in a population is approximated by the prevalence of agriculturally produced food, we find that societies described as hunter-gatherers indeed have, on average, only about one-fourth the number of labiodentals exhibited by food-producing societies, after controlling for spatial and phylogenetic correlation. When the persistence is approximated by the increase in food-processing technology over the history of one well-researched language family, Indo-European, we likewise observe a steady increase of the reconstructed probability of labiodental sounds, from a median estimate of about 3% in the proto-language (6000 to 8000 years ago) to a presence of 76% in extant languages.CONCLUSIONOur findings reveal that the transition from prehistoric foragers to contemporary societies has had an impact on the human speech apparatus, and therefore on our species’ main mode of communication and social differentiation: spoken language.Labiodentals depend on bite configuration.Biomechanical modeling shows that labiodental sounds like “f” are easier to produce (and to accidentally arise) under overbite and overjet (A) than under the edge-to-edge bite (B) that prevailed before the Neolithic (C). Overbite and overjet persisted only when exposed to the softer diets that became characteristic with food production (D versus E) and more recently with intensified food processing (F). Both developments led to a spread of labiodental sounds.Linguistic diversity, now and in the past, is widely regarded to be independent of biological changes that took place after the emergence of Homo sapiens. We show converging evidence from paleoanthropology, speech biomechanics, ethnography, and historical linguistics that labiodental sounds (such as “f” and “v”) were innovated after the Neolithic. Changes in diet attributable to food-processing technologies modified the human bite from an edge-to-edge configuration to one that preserves adolescent overbite and overjet into adulthood. This change favored the emergence and maintenance of labiodentals. Our findings suggest that language is shaped not only by the contingencies of its history, but also by culturally induced changes in human biology.
