At least in some cases, when the geographically expanding population of anatomically modern human beings encountered local groups of premodern varieties of humans, especially the Neandertals, the premoderns:

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Quizplus · Accepted Answer

Studies have shown that modern humans interbred with Neandertals and other premodern varieties of humans, with some populations carrying up to 4% of Neandertal DNA in their genomes. Therefore, it is most likely that they mated with them and produced offspring rather than killing them off, cannibalizing them or enslaving them.

At Least in Some Cases, When the Geographically Expanding Population