Ecological and evolutionary dynamics of the oral microbiome across childhood

Childhood represents a critical period for oral microbiome development, yet evolutionary trajectories and the relative roles of host and environment remain unclear. Using a large longitudinal metagenomic dataset of 920 samples from a twin cohort spanning the first decade of life, we characterised microbial shifts and population dynamics of key bacterial groups. Microbiome diversity was initially reduced and highly heterogeneous and became increasingly complex and convergent with age. Microbial community state was associated with developmental age, environment and in late childhood was surprisingly strongly associated with host genotype. Strain-level analyses revealed species-specific temporal patterns of genetic variation particularly within <i>Streptococcus</i>, reflecting adaptive responses to host and environmental pressures. <i>Fusobacterium</i> exhibited consistently high replication rates, indicating sustained growth dynamics. Phylogenetic reconstruction further revealed host and niche specific genomic diversification of Saccharibacteria lineages. These findings establish childhood as a decisive period of oral microbial evolution and highlight the role of host-microbiome and epithelial interactions in shaping community structure, providing guidance for oral management strategies that promote lifelong oral health.