When the draft sequence of the Neandertal genome was analyzed it turned out that there was little difference across non-Africans in their proportion of admixture from this other human lineage. It was a rather strange finding as Neandertals seem to have flourished from Europe to the Altai, and from the ice sheets to the fringes of the Middle East. If Papuans had Neandertal admixture the logical conclusion was that that had to occur in the Middle East. Additionally, if Europeans didn’t have much more Neandertal admixture than Papuans, that means that after the initial absorption the modern humans simply swept the field as they pushed north and west.
But there’s a little problem here: the archeology indicates that Neandertals survived for nearly ten thousand years in Europe after the first arrival of moderns. So the necessary conclusion granting all the above is that after the initial hybridization some barrier prevented further leakage of Neandertal genes into Europeans (or, perhaps modern Europeans descend from recently arrived Middle Eastern farmers who lack the full Neandertal complement of Paleolithic Europeans?).
A new paper in PNAS overturns these paradoxes by re-dating the last Neandertals, allowing them to melt away rather quickly ~40,000 years B.P., …