There is evidence that human habitation in the Arabian peninsula dates back to about 15,000 to 20,000 years ago. However, the harsh climate historically prevented much settlement. In pre-Islamic Arabia, apart from a small number of urban trading settlements, such as Mecca and Medina, located in the Hejaz in the west of the peninsula, most of what was to become Saudi Arabia was populated by nomadic tribal societies or uninhabitable desert.
Archaeology has, however, revealed some early settled cultures: the Dilmun on the Persian Gulf, and Thamud north of the Hejaz. The earliest known events in Arabian history are migrations from the peninsula into neighbouring areas.