The Sahara west of the Nile in southern Egypt was hyperarid and unoccupied during most of the Late Pleistocene epoch.About 11,000 years ago  the summer monsoons of central Africa moved into Egypt, and temporary lakes or playas were formed. The Nabta Playa depression, which is one of the largest in southern Egypt, is a kidney-shaped basin of roughly 10 km by 7 km in area [2-4]. We report the discovery of megalithic alignments and stone circles next to locations of Middle and Late Neolithic communities at Nabta, which suggest the early development of a complex society. The southward shift of the monsoons in the Late Neolithic age rendered the area once again hyperarid and uninhabitable some 4,800 radiocarbon years before the present (years BP). This well-determined date establishes that the ceremonial complex of Nabta, which has alignments to cardinal and solstitial directions, was a very early megalithic expression of ideology and astronomy. Five megalithic alignments within the playa deposits radiate outwards from megalithic structures, which may have been funerary structures. The organization of the megaliths suggests a symbolic geometry that integrated death, water, and the Sun. An exodus from the Nubian Desert at [approximately] 4,800 years BP may have stimulated social differentiation and cultural complexity in predynastic Upper Egypt.