By the late 5th century BC, La Tène influence spreads rapidly across the entire territory of Gaul. The La Tène culture developed and flourished during the late Iron Age (from 450 BC to the Roman conquest in the 1st century BC) in France, Switzerland, Italy, Austria, southwest Germany, Bohemia, Moravia, Slovakia and Hungary. Farther north extended the contemporary pre-Roman Iron Age culture of northern Germany and Scandinavia. A major archaeogenetics study uncovered a migration into southern Britain in the Bronze Age, during the 500-year period 1,300–800 BC. The newcomers were genetically most similar to ancient individuals from Gaul. The authors describe this as a "plausible vector for the spread of early Celtic languages into Britain". The major source of materials on the Celts of Gaul was Poseidonios of Apamea, whose writings were quoted by Timagenes, Julius Caesar, the Sicilian Greek Diodorus Siculus, and the Greek geographer Strabo. In the 4th and early 3rd century BC, Gallic clan confederations expanded far beyond the territory of what would become Roman Gaul (which defines usage of the term "Gaul" today), into Pannonia, Illyria, northern Italy, Transylvania and even Asia Minor. By the 2nd century BC, the Romans described Gallia Transalpina as distinct from Gallia Cisalpina. In his Gallic Wars, Julius Caesar distinguishes among three ethnic groups in Gaul: the Belgae in the north (roughly between the Rhine and the Seine), the Celtae in the center and in Armorica, and the Aquitani in the southwest, the southeast being already colonized by the Romans. While some scholars believe the Belgae south of the Somme were a mixture of Celtic and Germanic elements, their ethnic affiliations have not been definitively resolved. One of the reasons is political interference upon the French historical interpretation during the 19th century. In addition to the Gauls, there were other peoples living in Gaul, such as the Greeks and Phoenicians who had established outposts such as Massilia (present-day Marseille) along the Mediterranean coast. Also, along the southeastern French Mediterranean coast, the Ligures had merged with the Celts to form a Celto-Ligurian culture. Initial contact with Rome In the 2nd century BC Mediterranean Gaul had an extensive urban fabric and was prosperous. Archeologists know of cities in northern Gaul including the Biturigian capital of Avaricum (Bourges), Cenabum (Orléans), Autricum (Chartres) and the excavated site of Bibracte near Autun in Saône-et-Loire, along with a number of hill forts (or oppida) used in times