Exploration of the covariation signal between cortical bone and dentine volumes across the upper limb bones and anterior teeth in modern humans and relevance to evolutionary anthropology

Cortical bone and dentine are two mineralized tissues sharing a common embryological origin, developmental, and genetic background, distinct from those of enamel. Understanding their relationship is crucial to decipher the factors acting on their postnatal development, and shedding light on the evolutionary patterns of tissue proportions. Here, we investigate the coordinated variation between cortical bone and dentine volumes measured from arm and forearm bones (humeri, ulnae, radii) and upper anterior teeth (central incisors, lateral incisors, canines) of modern humans. Given the shared characteristics of cortical bone and dentine, we expect similarities in their postnatal development, which may lead to covariation between their volumes. The degree of bone-dentine covariation may be influenced by the physiological response of upper limb bones to mechanical loading. No such covariation is expected with enamel volumes, due to the greater developmental independence of bone and enamel. Our sample includes 55 adults of African and European ancestries from South African osteological collections. Principal component analysis of cortical thickness variation along the shafts of paired humeri, ulnae, and radii is used to assess asymmetry. Bone regions with bilateral asymmetry in cortical bone thickness are considered sensitive to functional loads, while regions with minimal bilateral variation likely reflect genetic influences during bone postnatal development. Statistical analyses reveal strong positive correlations between cortical bone and dentine volumes across all bones and teeth, and weaker correlations between cortical bone and enamel. We outline a complex pattern of bone-dentine covariation that varies by skeletal location and tooth type. Contrary to our expectations, the presumed functional sensitivity of bone regions does not influence the covariation signal. Additionally, the strength of the covariation appears to align with the developmental sequence of the anterior teeth, with the upper canines showing the strongest correlation with cortical bone volumes, followed by lateral and central incisors. These results provide insights into the functional and biological factors influencing the coordinated variation of cortical bone and dentine volumes during postnatal development. Further research on the cortical bone-dentine covariation across different skeletal parts, including lower limb elements, would enhance our understanding of the effects of both endogenous and exogenous factors on the development of the mineralized tissues.