This event must have happened in prehistoric rather than historic times, but even in historic times, peaceful blending has happened. Not that everything was pea

This event must have happened in prehistoric rather than historic times, but even in historic times, peaceful blending has happened. Not that everything was peaceful; human history is riddled with war and violence, even within homogenous groups of humans. But there are plenty of examples where one group conquered another and then assimilated into the population they conquered. We're quite often not genocidal. So it's entirely possible that Neanderthals were assimilated into Sapiens tribes.