The Out of Africa or African Replacement Hypothesis argues that every living human being is descended from a small group in Africa, who then dispersed into the wider world displacing earlier forms such as Neanderthal. Major proponents of this theory are led by Chris Stringer.
The Out-of-Africa theory was bolstered in the early 1990s by research on mitochondrial DNA studies by Allan Wilson and Rebecca Cann which suggest that all humans ultimately descended from one female: the Mitochondrial Eve.
Leaving Africa: Three Theories
Gabunia, Leo, et al. 2001 Dmanisi and dispersal. Evolutionary Anthropology 10:158-170.
Klein, Richard G. 2008 Out of Africa and the Evolution of Human Behavior. Evolutionary Anthropology 17:267-281.
Rightmire, G. P. 2000 Middle Pleistocene humans from Africa. Human Evolution 15(1-2):63-74.
Rose, Jeffrey I. 2004 The Question of Upper Pleistocene Connections between East Africa and South Arabia. Current Anthropology 45(4):551-555.
Straus, Lawrence G. 1999 Iberia: Bridge or cul-de-sac? Implications of the Iberian record for the debate on the middle to upper paleolithic transition. Human Evolution 14(1-2):139-149.