They could organize and innovate in ways that even their country cousins, the erecti that had spread out before them hundreds of thousands of years earlier, could not emulate. There were very few of them at first, but their numbers increased rapidly. Part of it was natural increase, a sapient male and a sapient female producing sapient children. But a lot of the increase came about through intermixing with their less evolved cousins. The first and more comprehensible form of genic exchange is migration.
People move across considerable distances and settle someplace.
If there isn't anybody there to begin with, or if the newcomers exterminate the older population, the issue is simple, for no gene mixture occurred, the new new genes are there, in a new place, that's it. But if there is a population already there and admixture occurs, the newcomers' genes have joined in what is now a new genetic pool. The second form of genic exchange is gene flow. This differs from migration in that nobody moves around very much. People are like the individual drops of water bobbing up and down in the ocean.