The Multiregional Hypothesis argues that our earliest hominid ancestors radiated out from Africa and Homo sapiens evolved from several different groups of Homo erectus in several places throughout the world.
The main proponent of the multi-regional hypothesis is Milford Wolpoff. But, growing genetic and archaeological evidence seems to suggest that of all the different evolutionary pathways, the Multiregional Hypothesis is looking less and less likely. There actually are more than three theories, but there are three main strains of the argument about how first Homo erectus and then Homo sapiens left Africa.
Leaving Africa: Three Theories
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Rose, Jeffrey I. 2004 The Question of Upper Pleistocene Connections between East Africa and South Arabia. Current Anthropology 45(4):551-555.
Soriano, Sylvain, Paola Villa, and Lyn Wadley 2007 Blade technology and tool forms in the Middle Stone Age of South Africa: the Howiesons Poort and post-Howiesons Poort at Rose Cottage Cave. Journal of Archaeological Science
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.