Arising from differences in the archaeological record, debate continues as to whether anatomically modern humans were behaviorally modern as well. There are many theories on the evolution of behavioral modernity. These generally fall into two camps: gradualist and cognitive approaches. The Later Upper Paleolithic Model theorises that modern human behavior arose through cognitive, genetic changes in Africa abruptly around 40,000–50,000 years ago around the time of the Out-of-Africa migration, prompting the movement of modern humans out of Africa and across the world. Other models focus on how modern human behavior may have arisen through gradual steps, with the archaeological signatures of such behavior appearing only through demographic or subsistence-based changes. Many cite evidence of behavioral modernity earlier (by at least about 150,000–75,000 years ago and possibly earlier) namely in the African Middle Stone Age. Sally McBrearty and Alison S. Brooks are notable proponents of gradualism, challenging European-centric models by situating more change in the Middle Stone Age of African pre-history, though this version of the story is more difficult to develop in concrete terms due to a thinning fossil record as one goes further back in time.