[roland00]

Agreed generally but as my own memory of reading these research papers and what wikipedia points out that lactase persistence is not caused by a single gene but there are several different gene variants that can produce this same effect.

And we know since the mid 2000s by a dozen or so genetic studies that these genes did not arrive from a single culture or single timeframe, but are an example of "convergent evolution" aka it was beneficial for humans generally and thus when independent random mutations occurred they persisted longer for those people were more likely to breed and thrive. This happened several times in human history and are not a singular event.

And thus individual humans may have Lactase Persistence in adulthood and other individual humans may not have the persistence. Furthermore how much Lactose you can process with the Lactase enzyme is a dependent process where some people can handle some quantities but not too much quantity of Lactose all at once.