Fascinating true(?) story about how lactose intolerance changed the course of history, as recorded in the ancient sagas[1]:
When the Vikings established their colony in Vinland, they wished to establish peaceable relations with the Native Americans. They invited the local chiefs to a party at their longhouse, in which they served an amazing new drink -- milk -- which the Americans had never seen before. The following morning, suffering from intense abdominal pains, the natives accused the vikings of trying to poison them, and promptly laid siege to the colony until the Vikings packed up and buggered off.
But for this incident, it's entirely possible that the Vikings might have established a durable colony in the Americas, leading to contact between the old and new world 500 years earlier.
"What drove the two groups apart was a killing. Some of the Norse men killed a skraeling who was attempting to steal weapons, which Thorfinn had forbidden as trade items."
The Skraelings, being mammals, and observers of other mammals such animals they hunted, would have been familiar with milk, even without owning livestock.
If they bought raw milk, they probably intended to use it for their children, but it seems a rather difficult trade item, as a liquid, and having a shelf life of only a day or two.
They probably were trading butter or cheese, neither of which contains enough lactose to cause problems. Fresh cheese would have been known from the stomachs of suckling animals, and might have been a prized delicacy before they were aware it could be manufactured. Butter would have been completely new to them.
When the Vikings established their colony in Vinland, they wished to establish peaceable relations with the Native Americans. They invited the local chiefs to a party at their longhouse, in which they served an amazing new drink -- milk -- which the Americans had never seen before. The following morning, suffering from intense abdominal pains, the natives accused the vikings of trying to poison them, and promptly laid siege to the colony until the Vikings packed up and buggered off.
But for this incident, it's entirely possible that the Vikings might have established a durable colony in the Americas, leading to contact between the old and new world 500 years earlier.
1: http://www.ancientworlds.net/aw/Places/District/1009811