They left no written records and are known only through archaeological finds and the oral history of their successors in their territory. It had been suggested that the Sao were the descendants of the Hyksos who conquered Ancient Egypt and that they moved south from the Nile valley into middle Africa in several waves under pressure from invaders, or that they originated in the Bilma Oasis north of the lake Chad.[5] A more widely accepted theory is that the Sao were simply the indigenous inhabitants of the Lake Chad basin and that their ultimate origins lie south of the lake.[6] And recent archaeological research indicates that the Sao civilization developed indigenously from earlier cultures in the region (such as the Gajiganna culture, which began at around 1,800 BCE and began to build fortified towns by about 800 BCE), gradually increasing in complexity.[7][8][9][10] Sao artifacts show that they were skilled workers in bronze, copper, and iron.[11] Finds include bronze sculptures and terra cotta statues of human and animal figures, coins, funerary urns, household utensils, jewelry, highly decorated pottery, and spears.[12] The largest Sao archaeological finds have been made south of Lake Chad.