One of the fastest evolving regions of the human genome is an enhancer region near the lactase gene, where new mutations allow people to digest lactose -- and thus animal milk -- into adulthood. One wave of such a mutation spread out from the Indo-European homeland, another throughout eastern Africa (both of these related to cows), and still another from the Arabian peninsula (related to camels).
These three independent waves were all started by nomadic pastoralists, not farmers or hunter-gatherers. Most of the latter don't own livestock to milk in the first place, and when farmers do, they can find other ways around the problem of lactose in their animals' milk -- like letting it age into cheese. Fine for sedentary farmers, but not for on-the-go herders who may get milk into the butter stage but can't stay put long enough to culture it into 5-year aged cheddar. They need to consume it now.
I wonder if there isn't another independent mutation among Tibetans, who have historically led a nomadic and herder lifestyle and whose diet apparently is chock full of dairy from the yak. Check this out: dozens of cups per day of butter tea, which has not only yak butter but yak milk -- and salt! Somehow I doubt all those hummus-and-granola hippies making their pilgrimage to Tibet will share the news of how much saturated fat and salt their favorite oppressed people wolf down all day.
The lactose content of yak milk is the same as for other dairy animals, so any of those three waves of mutations in western Eurasia and eastern Africa would have thrived if they'd gotten into Tibet. Only problem is getting into Tibet. Thus, if lactose tolerance is spreading there, it's probably an indigenous mutation. Just googling around, I couldn't find anything, but Tibetans are not a popular group for genetic study, except for genetic adaptation to the air pressure at such high altitudes.
But air quality isn't the only thing that affects your Darwinian fitness in the mountains. Because jackshit grows that high up, you're probably going to rely a lot more on herding robust animals and, not wanting to kill them too often for their meat, stealing their more replenishable store of milk.