I don't think anyone knows how exactly lactase persistence arose in Europe. Here is a brand new paper describing a DNA analysis of 83 ancient European individua

I don't think anyone knows how exactly lactase persistence arose in Europe. Here is a brand new paper describing a DNA analysis of 83 ancient European individuals: http://biorxiv.org/content/early/2015/03/13/016477.article-info http://biorxiv.org/content/early/2015/03/13/016477.article-i... The relatively late appearance (earliest known example of lactase persistence gene is from 4,300 years ago) and relatively small selective advantage (1.5%) are surprising. > Mutations are typically recessive so you might have households where one parent is lactose tolerant, and none of the children are, due to the lottery of gene expression in sexual reproduction. Maybe some intolerant children are carriers and some aren't, but it's still a roll of the dice. Lactase persistence is a dominant mutation. If you only have one copy of the gene you still produce enough lactase to fully digest milk.