The ethnogenesis of the Germanic tribes remains debated. However, for author Averil Cameron "it is obvious that a steady process" occurred during the Nordic Bronze Age, or at the latest during the Pre-Roman Iron Age. From their homes in southern Scandinavia and northern Germany the tribes began expanding south, east and west during the 1st century BCE, and came into contact with the Celtic tribes of Gaul, as well as with Iranian, Baltic, and Slavic cultures in Central/Eastern Europe.