From about 6000 BC various communities of hunter-gatherers make the Nile the centre of their territory, around which they roam. But the drying of the sahara increasingly confines them to the river area. The unusual habit of this great river - flooding every year and depositing a layer of rich moist soil on the surrounding region - is ideally suited to the development of settled agriculture The river takes upon itself two otherwise laborious tasks, irrigation and the enriching of the soil.