In other species that intermix like that, you see stretches of DNA where all admixture is excluded, because it’s instantly selected out. In humans, the such a stretch exists, and has to do with sex determination, which is a pretty much a smoking gun. I’m finding stuff about the Y chromosome, but I thought there was something on the X chromosome to do with testicle development as well.

Obviously, female hybrids were fertile at least some of the time, since there is admixture. But, it’s possible every half-and-half male hybrid ever ended up sterile, and later generations would probably have had higher rates of sterility.

The general idea that humans are irrationally obsessed with categories and tribes holds, but this doesn’t seem like a clear-cut example.

Those “hybrids” would have children with “human” X’s even if they were daughters, but be introducing neanderthal DNA back into a “double human X” mother, ensuring her daughter still had the reproductive advantages over a neanderthal mother still, but retaining neanderthal genes adaptive to the northern climate.

Shake and bake a couple generations, you get white people.

Anyone outside of Africa has similar-ish admixture; skin colour has little to do with it. European hunter-gathers at the end of the ice age were what we’d consider black, and they were replaced by Middle Eastern looking and originating agriculturalists. The light skin colour is from Eurasian steppe nomads that rode in on the first horses in the bronze age. Other unrelated groups, like from the Caucuses area or pre-modern Japan, also have/had light skin.