It's by Mathiesen

See:
http://mathii.github.io/2019/10/12/t...istence-allele

" the reason for selection remains somewhat unclear and none of the proposed hypotheses (access to energy content, hygiene, vitamin D or calcium, etc… ) seem completely convincing (See Szpak et al. 2019 for example)."

"Before 5000 BP, there are only a couple of occurrences of the allele. It’s quite possible that these are genotyping or dating errors. By 2500 BP, the allele is present over a band stretching from Ireland to Central Asia at around 50 degrees latitude. This probably reflects the spread of Steppe ancestry populations in which the allele originated. However, the allele is still rare (say <1% frequency) over this entire range. It does not become common anywhere until some time in the past 2500 years - when it reaches its present-day high frequency in Britain and Central Europe. It’s also at high frequency today in Scandinavia. I don’t have any ancient DNA from the past 2500 years from the region, but Margaryan et al. 2019 suggests that the trajectory was very similar to Britain and Ireland. The allele is relatively rare in Iberia in this period, but intermediate frequency today (45% frequency in 1000 Genomes IBS).""Timing and location of selection

Since we have relatively dense sampling in Britain & Ireland, Central Europe and Iberia (and a few samples in the Indus valley), we can look at the trajectory of the allele in each of these regions separately. The figure below shows a logistic growth model fitted to the ancient and modern (from 1000 Genomes) data in these regions. In Britain, the allele doesn’t start to increase in frequency until perhaps 3000-4000 BP. It then increases rapidly until around 1500 BP, when it seems to level off at around the present-day frequency. There’s a similar pattern in Central Europe, consistent with measurements of the allele frequency in Medieval Germany (Kruttli et al. 2014). In contrast, in Iberia, the increase starts much later—perhaps 1000-2000 BP—but doesn’t obviously level off. What does this tell us about the things that might be driving selection? It’s possible that the start time of the increase is limited by the absence of the allele, rather than the absence of selection. But if selection really stops at 1500 BP, then it would suggest that whatever is driving selection stops in Britain, Ireland and Central Europe at that time, but is still present in Iberia. Another possibility might be some kind of frequency-dependent selection although I have no idea how that would work in practice."


As he says, the very earliest samples could well be errors, and as far as the maps are concerned, it looks to me perhaps like a pick up in central Europe by the steppe people and then they spread it both west and all the way east.

Not married to that idea either. Let's say that imo it's still an open questions.