Science Daily: Ancient 'Out Of Africa' Migration Left Stamp On European Genetic Diversity.
Researchers from Cornell University (USA) have sampled more than 10,000 sequenced genes from 35 people of European (20) and Sud-Saharan (15) ancestry. The result is quite conclusive: Europeans have more "deletereous" (harmful, less likely to survive natural selection) genes than Africans.
The rationale for this result (coincident with other studies) is that Europeans (or our Asian ancestors) went through one or more population bottlenecks when diversity was pretty small, allowing these defects to persist, even many milennia later. In contrast African populations have always been pretty large, forcing these potentially harmful genes to become less frequent through normal natural selection.