The current trend is for dispersed lineages to be assimilated. For example, colonization had its share of wars and genocides, but more often the population that was conquered gave up its culture and language and became part of the conquering population. This of course assumes they are genetically compatible. So for a species there are two options for a "lineage", it can die out or be assimilated. Darwin stated that a beseiged species that is not genetically compatible, but competed for the same resources would only have one option, extinction. So, if you call them "human", then you are assuming genetic compatibiltiy already. If you call them "hominids" then some of them might have been compatible but not all. If you use the term primates then you have to assume alot of them were not genetically compatible. I wonder if all chimpanzees can also trace their genetic history back to Africa? What about baboons or makaks? Are they all out of africa as well?