3) high adult lactose digestion capacity is an adaptation to dairying:
High lactose digestion capacity in adults is common in populations of European and circum-Mediterranean origin where an evolutionary adaptation to millennia of drinking milk from domestic livestock could have occurred.

The mutation may have been present in a few individuals everywhere But it was in northern climes, with one harvest a year, or in nomadic populations with a strong nutritional dependence from milk that such people would have found a survival advantage in the year-round nutrition of cow, goat and sheep milk.