The Nile has been the lifeline of its region for much of human history. The fertile floodplain of the Nile gave humans the opportunity to develop a settled agricultural economy and a more sophisticated, centralized society that became a cornerstone in the history of human civilization. Nomadic modern human hunter-gatherers began living in the Nile valley through the end of the Middle Pleistocene some 120 thousand years ago. By the late Paleolithic period, the arid climate of Northern Africa became increasingly hot and dry, forcing the populations of the area to concentrate along the river region.