Sussex emerged as a distinct entity, the territory of the South Saxons, in the 5th Century. Before that, there is archaeological evidence for hominid and early human settlement of this region of south east England. In historic times, the area was invaded and settled by various continental European peoples: the Atrebates from Gaul, the Romans and their Celtic clients, and the Saxons.
In medieval times, the county was known for its iron mining and working which only began to decline at the dawn of the industrial revolution. The arrival of the railways and the fashion for seaside bathing and holidays transformed Sussex from an isolated rural county of farms and small market towns.
The geography and size of Sussex meant that there were linguistic and other differences from east to west. With the Parliamentary Reform Act these divisions were formalised into an Eastern and a Western Division which became the basis of the county councils of East Sussex and West Sussex.
There is evidence of hominid presence at Boxgrove near Chichester, West Sussex dating back to the Lower Paleolithic period. The first modern humans began arriving in the Mesolithic Age when Britain was still connected to Europe. There is evidence of land use near the River Ouse in the Neolithic period as well as a flourishing flint mining and trading activity together with a pottery industry. Human occupation of Sussex continued through the Bronze Age and into the Iron Age. Over fifty Iron Age sites have been found in the Sussex Downs including a small number of farmsteads supporting a view of mixed farming with ploughed crop cultivation, animal husbandry (cattle and sheep), and fishing including harvesting marine shellfish.