Britain's prehistory can be divided into three main phases: the Paleolithic, Mesolithic, and Neolithic periods. During the Neolithic period around 3,500 BC, farming was introduced to Britain and the first settlers established themselves. Archaeological sites from this time, such as Skara Brae, provide evidence of early settlements. The Bronze Age began around 2,400 BC with the arrival of the Beaker people, who introduced bronze technology. During the Iron Age starting 700 BC, the Celts arrived in Britain in waves and established tribal societies. The Romans invaded Britain in 43 AD under Emperor Claudius, occupying southern Britain and building Hadrian's Wall. They introduced writing and founded many towns before withdrawing from Britain