Here’s a freely accessible paper which amongst many considerations, discusses genetic diffusion in pastoral human populations at the Neolithic transition, and why Lactose Persistence, or specifically lactase persistence allele(s) (LCT*P), which allows for the digestion of fresh milk, was strongly selected for in northern Europe, at the start of agricultural domestication. This is the introduction:
Lactase is an enzyme that allows lactose digestion in fresh milk. Its activity strongly decreases after the weaning phase in most humans. However, in many European individuals and in people from various populations of other continents, lactase is still active throughout adult life . This dominantly inherited genetic trait is called lactase persistence and at least three mutations are tightly associated with it: −13,910 C/T (generally linked to −22,018 G/A) in northern Europe (100% association) , , −14,010 G/C in East Africa and −13,915 T/G in the Middle-East/North Africa , . In this study, we will use LP to refer to the lactase persistence phenotype, LCT to refer to the lactase gene and LCT*P to refer to the lactase persistence associated allele(s).
The particular distribution of lactase persistence throughout the world indicates that this trait evolved under strong positive selection , , . Two main hypotheses have been proposed: gene-culture coevolution (gcc) , , which suggests that lactose digestion confers a nutritional advantage to milk-consuming (e.g. pastoralist) populations; and calcium assimilation (cal) , which proposes that carriers of LCT*P are favoured in high-latitude regions, where lactose would substitute vitamin-D (deficient when sunlight is low) to allow accurate calcium assimilation, thus preventing rickets. Holden and Mace  studied the two mentioned hypotheses and a third one, proposing that LP was favoured in highly arid environments, where people would have drunk milk to prevent dehydration , . Their conclusion was that LP is an adaptation to pastoralism, in agreement with the gcc model, while no evidence for the other hypotheses was found. Compatible with this theory, Coelho et al.  suggested that −13,910*T originated in Eurasia before the Neolithic Era, and observed a significant departure from neutrality of this variant in the few populations examined. On the other hand, these studies do not explain the significant correlation found between LP frequencies and latitude in Europe , as predicted by the cal model.
Reference: Selection and Demography on the Diffusion of Lactase Persistence
Gerbault P, Moret C, Currat M, Sanchez-Mazas A, 2009 Impact of Selection and Demography on the Diffusion of Lactase Persistence. PLoS ONE 4(7): e6369. doi:10.1371/journal.pone.0006369
See also: Science Daily – Longer Life For Milk Drinkers, Study Suggests