If there had been multiple independent evolutions from a precursor - H. erectus, for instance, who also migrated to Asia from Africa - to what we collectively call modern humans, in geographic isolation and/or at distinctly different times, they would be different sub-species, at minimum, and perhaps not able to interbreed at all. That this is not the case; i.e., that wide variations in human phenotype have no influence on the viability of offspring - the definition of species - pretty much seals the single origin hypothesis. The reports I've seen on mitochondrial DNA research does nothing but reinforce my conviction.

Consider, as counter-examples, both American continents and Australia: same total available time, similar climates, and in the case of South America at least, related species, particularly monkeys. Nothing like hominids evolved to fill analogous niches in those isolated environments.

I wouldn't care to argue against "Out of Africa", either (beautiful movie, btw). The same global palaeoclimate that made Africa perfect for the evolution of prehominids to hominids mitigated against their independent development elsewhere... if for no other reason than by occasionally re-paving the surface with a kilometer or two's thickness of ice*. The presence of modern occupation sites, while limiting the possibility of exploration somewhat, has not prevented palaeontologists from establishing a comprehensive picture of the prehistoric fauna of Eurasia, neither before nor contemporary with human migration and settlement.

To characterize the commonly accepted idea of "the African cradle" as a guilt-driven sop to African peoples (enslaved, oppressed, or otherwise), is a bizarre projection of motive onto a straight-forward, logical interpretation of physical evidence... the absence of which is, most decidedly, "the evidence of absence".

Finally - your claim of no great African civilizations other than Egypt is unusually ill-informed; try The Great Zimbabwe to start with.

Fox

* - It may be tempting to shift blame for 'absence of evidence' in the northern latitudes to glacial scouring of the surface and subsequent reforestation, rather than overbuilding by later humans... but it won't hold up.