They'd also have the situational awareness to decide that life there in France sandwiched between coastal tribes A and B and inland tribe C sucks, so they're go

They'd also have the situational awareness to decide that life there in France sandwiched between coastal tribes A and B and inland tribe C sucks, so they're going to try their chances on that fuzzy bit of land that looks promising on the horizon. It only takes one small group over a timescale of thousands of years to make that decision. Dispersal is even more likely if people or stories are able to get back and forth; the issue of "we only have 8 people and not enough genetic diversity" is moot if the story or the rumor spreads that there's lots of land and lots of food on that fuzzy spot over there.