It could be a lot more simple, such as a bacteria that grew on cheese, or a virus that milk protected you from. This would support massive switch to tolerance

It could be a lot more simple, such as a bacteria that grew on cheese, or a virus that milk protected you from. This would support massive switch to tolerance (simple survival of the fittest). It also supports the spread, as a bacteria or virus would not have made it out of the "islands" (himalayas, oceans) and so tolerance wouldn't have been an advantage.