Most everyone agrees the general answer to that question is "Africa" but recently there has been some debate as to where in Africa the earliest modern humans (H. sapiens) came from. In a previous post I discussed an argument that the source population might have been of North African origin. The paper arguing for a North African origin was actually a response to this paper by Henn and colleages, who conducted a genetic analysis of living hunter gatherers. Their results indicate that Southern African hunter gatherer populations have greater genetic diversity than hunter-gatherers in other parts of Africa, thus these populations are inferred to be older. I am slightly uncomfortable with this conclusion, because I don't understand how the authors can rule out the existence of more ancient modern human populations in other parts of Africa which have not survived to the present day. But, on second thought, wouldn't that same argument apply to any argument based on modern human genetic diversity?
Both the North African and Southern African arguments are in conflict with the most commonly accepted notion that modern humans evolved in East Africa. An East African origin is supported by the fact that Ethiopia is home to the oldest fossil evidence of modern humans from Omo Kibish (McDougall et al., 2005, news story), as well as the slightly younger fossils from Herto in the Middle Awash.