Geometric morphometric analysis of the ‘skeletal vocal tract’: A first step in involving osteoarchaeology in the search for a potential ‘genetic bias’ for language, using two Dutch historical skeletal collections

The capacity for speech/language is a distinctly human characteristic, with its emergence and causation not unequivocally pinpointed. Important in speech production are cognitive and morphological expressions of genetic makeup, finding phenotypic expression in combination with complex environmental factors, which together led to language differentiation. Current research at the Max Planck Institute for Psycholinguistics (Nijmegen, NL) into genetic bias for speech/language indicates that subtle differences in genotype, expressed as phenotype, can influence speech production at the individual level. A population-wide shift in phenotype can create subtle differences in speech, which may propagate and amplify through inter-generational cultural transmission of language. Using this information, this study attempts to analyse the morphological variation within and between two Dutch skeletal collections: Klaaskinderkerke (13th-17th centuries) and Middenbeemster (AD 1829-1866). Using 3D cranial models, geometric morphometric analyses are used to reveal and explain shape and size variation in a quantitative manner, resulting in highly visual output in original anatomical contexts. Multivariate statistics enable an assessment of differentiation between samples and sexes. Results show that overall, shape variation in the skeletal vocal tract between these samples and the sexes is not statistically significant. When comparing four sex-by-sample groups, some separation was found. Size variation was significant for both sexes and samples, reflecting sexual dimorphism and climatic circumstances or different lifestyles, respectively. Future research will have to further investigate the complex relationship between shape, size, allometry, sex, and external factors. In conclusion, at this time, quantifiable differences in vocal tract shape that could indicate morphological bias cannot be supported. The gathered morphological data can be added to a growing normative database for phenotypic variation around the world, to be used for this new approach. Osteoarchaeology can herein contribute direct assessment of morphological variation through time, aiding in the discovery of potential mechanisms behind the evolution of spoken language.