The question of when brown rats became commensal with humans remains unsettled, but as a species, they have spread and established themselves along routes of human migration and now live almost everywhere humans are.
A cuestión de cando as ratas pasaron a facerse especie comensal dos seres humanos non está aínda esclarecida, pero a especie foise espallando e establecendo ao longo das rutas de migración humanas e agora vive practicaemtne en todos os sitios onde vive o ser humano.