Ireland was first inhabited around 7500 BC by Mesolithic hunter-gatherers who travelled across the land bridge that connected southwest Scotland with the northern part of Ireland (it was submerged around 6700 BC). Farmers from the Middle East arrived in Ireland around 3500 BC. Their elaborate graves are also a feature of Neolithic communities in Brittany and the Iberian peninsula. From the 6th century BC, the island was invaded by waves of Celtic tribes from central Europe, including the Gaels, who established pastoral communities within massive stone forts. By AD 200 the Gaels dominated the island, though there was no central control: society was based on a complex structure of hundreds of small kingdoms. The Romans, who dominated much of northern Europe, never reached Ireland. The Gaels traded with other Celtic peoples and sent raiding parties to form settlements in Scotland (Dál Riata) and west Wales.