Fig. 1. Early development of the gut microbiome in healthy children.

The development of the gut microbiome during early childhood consists of four different stages (acquisition, developmental, transitional and stable) that are characterized by stereotypic changes in diversity and composition³⁰. a, During the first 3 years of life, the gut microbiome exhibits increasing richness and evenness within individual samples (α-diversity)^50,56 and decreasing compositional differences compared with adults (β-diversity)⁶¹, followed by gross stabilization. Different early-life exposures can either advance or delay the maturation trajectories of the gut microbiome. b, Recent longitudinal studies of healthy children have also revealed evolutionary-conserved changes in the relative abundance of major bacterial phyla during the first 3 years of life^30,50. Depicted trends in different gut microbiome metrics are estimates on the basis of the results of three recently published studies (Supplementary Table 1).