Inhabited since the Palaeolithic era, the territory of Metropolitan France was settled by Celtic tribes known as Gauls during the Iron Age. Rome annexed the area in 51 BC, leading to a distinct Gallo-Roman culture that laid the foundation of the French language. The Germanic Franks arrived in 476 and formed the Kingdom of Francia, which became the heartland of the Carolingian Empire. The Treaty of Verdun of 843 partitioned the empire, with West Francia becoming the Kingdom of France in 987.