Remains of Stone Age settlements of hunters and fishermen suggest that the first inhabitants crossed from the low countries of Continental Europe on one or more wide causeways. By the time their successors had turned to subsistence farming, the land links to the continent had disappeared under the sea. These offshore islands created at the ending of the Ice Age shared, with nearside Europe, a slowly evolving agricultural economy using bronze and iron tools. The Ancient Britons were Celts, whose ancestors had migrated from the valleys of the Rhine, the Rhône and the Danube. Having asserted their command of northern Italy and France (Gaul), the Celts established a bridgehead to Ireland and thence to Britain. By 600 BC they were the undisputed dominant force of Western Europe and were to remain so until challenged by the Romans.