Humans have inhabited the territory of modern Niger for millennia; stone tools, some dating as far back as 280,000 BC, have been found in Adrar Bous, Bilma and Djado in the northern Agadez Region. Some of these finds have been linked with the Aterian and Mousterian tool cultures of the Middle Paleolithic period, which flourished in northern Africa circa 90,000 BC–20,000 BC. It is thought that these early humans lived a hunter-gatherer lifestyle. In prehistoric times the climate of the Sahara desert was much wetter and more fertile than it is today, a phenomenon archaeologists refer to as the 'Green Sahara', which provided favourable conditions for hunting and later agriculture and livestock herding.