First time I finally read that gut bacteria (and not genetic traits) could be what helps humans digest milk.
OK, I'm wildly interpreting this article : http://www.upenn.edu/pennnews/news/penn-team-links-africans-ability-digest-milk-spread-cattle-raising, about a study led by University of Pennsylvania researchers on genes and lactase persistence in African populations. It ends with "Another possibility is that commensal bacteria in the gut could offer humans a helping hand in digesting milk."
Those discussions about prehistory, that link lactase persistence, genetic mutations and migration/admixture events will change. And they will include more considerations about the evolution of gut bacteria in human populations. (Nowadays there seem to be a strong trend, in biology and medicine, to explore the complexity and importance of gut bacteria, and this will also have an impact in studies about prehistory).
By the way, this is not only for humans... The perfect example of unscientific observation :
In my garden, several visiting cats have progressively become milk addicts. At first, they were interested in milk as much as they were fan of salad. My guess is that they are progressively sharing their gut bacteria with a cat that already had the right set of gut bacteria. After all, they eat and drink from the same bowls everywhere in the neighborhood, and their hygiene behavior is questionable ... :-) ...