My favorite theory for the development of lactose tolerance is that drinking fresh milk offers protection from smallpox.
As a sanity check, http://books.google.com/books?id=GyE8Qt-kS1kC&pg=PA151&dq=&l... includes an estimate that approximately 10% of children in the 1700s died from smallpox. The first successful smallpox vaccine was made from cows. And the inspiration for the vaccine was the realization that the reason why milkmaids had good complexions was that something about their job gave them some level of protection against smallpox.
In mammals of all kinds, including humans and cows, milk delivers antibodies to protect babies. Therefore having fresh milk, and the fresher the better, provides protection from smallpox. This both helps survival and, as was apparent in the complexions of the milkmaids, resulted in more attractive adults.
There you have a very strong selection pressure to be able to drink fresh milk.
Wasn't the theory not that milk provides protection from smallpox, but proximity to cows transmits cowpox, which itself provides protection from small pox?
Then again, in a world where >95% of people are farmers and there's no refrigeration (so milk has to be consumed fast), I'm not sure the difference between handling cows and drinking milk is much of a difference.
The reason it comes to mind is not because I've ever been near a farm, but I was just reading a collection of letters by a writer in the late 1940s, who had a small farm on an island. He was dying very slowly of tuberculosis (it seems implied people didn't distinguish between regular TB and bovine) and he mentions wanting to get a cow, but one that's been tested for TB to make sure his son doesn't get sick like him. Because humans can and do catch it from cows.
As a sanity check, http://books.google.com/books?id=GyE8Qt-kS1kC&pg=PA151&dq=&l... includes an estimate that approximately 10% of children in the 1700s died from smallpox. The first successful smallpox vaccine was made from cows. And the inspiration for the vaccine was the realization that the reason why milkmaids had good complexions was that something about their job gave them some level of protection against smallpox.
In mammals of all kinds, including humans and cows, milk delivers antibodies to protect babies. Therefore having fresh milk, and the fresher the better, provides protection from smallpox. This both helps survival and, as was apparent in the complexions of the milkmaids, resulted in more attractive adults.
There you have a very strong selection pressure to be able to drink fresh milk.