Human sound systems are shaped by post-Neolithic changes in bite configuration

Abstract

INTRODUCTION Human 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.
RATIONALE Given 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.”
RESULTS Biomechanical 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.
CONCLUSION Our 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.