[xyzzyz]

We don’t really know very well how many humans (including Neanderthals) lived in Europe. We have pretty good estimates of effective population sizes from genetic data, but these don’t tell us much about actual population sizes beyond giving us lower bounds.

What we do know, though, is that hunter-gatherer peoples have very low population densities, and that these low population densities are a result of resource scarcity, and violent inter-group competition for these scarce resources. Other hunter-gatherer groups impact you even if they are miles from you.

Oh, and we do know that anatomically modern humans met Neanderthals, at the very least because they interbred.