I think you are entering into dangerous territory here, afterall the nazis used pseudo science to justify their aryan racial theories, now you are doing it from the other extreme. We need africans because we are not diverse enough in our dna is bogus rubbish, europe has a population of circa 400 million surely there is huge diversity already within that population. Some animal species only have a few thousand individuals and conservationists are happy that they have enough diversity to sustain the species. You could also use your very same argument to justify colonisation, africans are not diverse enough therefore they need europeans to invade in order to increase the dna diversity for the african population. Also farming and breeding of livestock where most of our livestock are descended from a relatively small number of individual bulls refutes the theory about having too small a gene pool. Of course it is true in agriculture that some pure bred animals are too highly bred , but most of our farm animals are perfectly healthy.