Nature has in interesting article on Neolithic dairying, lactose intolerance, cheese, yogurt, and the spread of genes that allow for lactose digestion. The ability to digest milk in adulthood—called lactase persistence—is less than 40% in Greece and Turky, but higher than 90% in the UK and Scandinavia.
During the most recent ice age, milk was essentially a toxin to adults because — unlike children — they could not produce the lactase enzyme required to break down lactose, the main sugar in milk. But as farming started to replace hunting and gathering in the Middle East around 11,000 years ago, cattle herders learned how to reduce lactose in dairy products to tolerable levels by fermenting milk to make cheese or yogurt. Several thousand years later, a genetic mutation spread through Europe that gave people the ability to produce lactase — and drink milk — throughout their lives. That adaptation opened up a rich new source of nutrition that could have sustained communities when harvests failed.
Paleo Diabetic 
That is fascinating information about the AP allele mutation. I found it interesting that amylase developed well after grain was introduced as a food source. A genotype/phenotype study using different haplotype backgrounds (Eur/African) was performed to research lactase persistence using the LTT (Lactase Tolerance Test) where BG is tested after ingesting lactose. A rise of greater than 1.7 mM is classified as lactase persistent. A rise of less than 1.1 mM is classified as lactase non-persistent. In-between this is intermediate persistent. Those on a strict Paleo diet don’t really care, but it would be interesting to know if one wanted to indulge occasionally. (doi:101038/ng1946)
Steve, why do you say lactose is a toxin if you can’t digest it? It’s soluble fiber! OK,
it might be unpleasantly gassy like some other soluble fibers, but if you can’t digest it, at
least it won’t raise blood glucose!
Hey, Jim.
My writing was unclear. The “toxin” remark was a direct quote from the linked article, not my choice.
I think of lactose not as a soluble fiber, but a dissacharide sugar (glucose plus galactose). But maybe I’m being too serious.
-Steve