 Some Iron Age migration does seem to have occurred but the nature of the interactions with the indigenous populations of the isles is unknown.According to this model, by about the 6th century (Sub-Roman Britain), most of the inhabitants of the Isles were speaking Celtic languages of either the Goidelic or the Brythonic branch.Since the late 20th century, a new model has emerged (championed by archaeologists such as Barry Cunliffe and Celtic historians such as John T. Koch) which places the emergence of Celtic culture in Britain much earlier, in the Bronze Age, and credits its spread not to invasion, but due to a gradual emergence "in situ" out of Proto-Indo-European culture (perhaps introduced to the region by the Bell Beaker People, and enabled by an extensive network of contacts that existed between the peoples of Britain and Ireland and those of the Atlantic seaboard.