Brito et al. compared the mobile DNA components of human microbiomes from North American and Fijian individuals. The dissemination of mobile genes between geographically separated microbiomes was not strongly restricted by dispersal limitation, as 62% of mobile genes were found in both sets of microbiomes. However, at least some mobile genes seemed to be subject to environmental selection, as the abundances varied between the two cohorts and even between neighbouring Fijian villages, despite similarities in overall microbiome compositions. Consistent with the dietary habits of agrarian societies, mobile genes that encode GH13 glycoside hydrolases, which degrade plant cell wall carbohydrates, were enriched in the Fijian cohort. Differences in commonly used antibiotics were also reflected in the data, with mobile genes that mediate resistance to quinolones enriched in the Fijian cohort and mobile genes that mediate resistance to cephalosporins enriched in the North American cohort.