Here is a site with some discussion of the various theories and the evidence on which they are based.
The Recent African Genesis of Humans Multiregional Evolution Population Bottlenecks and Volcanic Winter
The debates have been going on for quite some time.
African Exodus by Christopher Stringer and Robin McKie (Henry Holt and Company, 1996) discusses the theories.
Also, the already mentioned
The Seven Daughters of Eve by Bryan Sykes is informative.