There's a fascinating article in the current Natural History magazine about lactase persistence. It's a story of independent evolution in different human populations, in Europe and in Africa, of the ability of adults to digest milk. It shows how we evolve with our food supply, and how those changes spread through populations. Lactase persistence seems to have developed between 7000 and 9000 years ago, depending on the locale.
It's also an account of the racism involved in speaking of "lactose intolerance" and indeed, in finding ways to speak about varying traits that are not disparaging of peoples not like us.
The article is not posted on the magazine's site, but if you google <"Follow the Drinking Gourd" natural history> you may be able to find it. I'm not linking to it because it's unclear to me who or what holds the copyright.