Why can't everyone digest milk?

The dairy industry is significant in Yankee territory, and a natural question is: Why can't everyone digest milk? The science journal Nature recently published a fascinating article on this subject. The article says that almost all young children can digest milk, but worldwide only about 35% of adults can. Why?

I recommend reading the whole article (link below), but here are some highlights:

• The lactase gene is needed to digest the lactose in milk.
• Babies are born with the lactase gene, but it switches off in most people around age 7 or 8.
• In prehistorical times, it switched off in everyone.
• About 7,500 years ago, a genetic mutation appeared in certain populations that allowed the lactase gene to function even in adults.

The article also explains that even in lactose intolerant populations, and more than 7,500 years ago, humans fermented milk into cheese and yogurt as a way of reducing lactose to tolerable levels. Why? Because the nutritional advantages of dairy products were so great. Those nutritional advantages likely influenced how civilizations developed, and perhaps even which civilizations survived.

Here is the article: The Milk Revolution

Some of the conclusions in the article are controversial. I recommend reading the comments, too. I like this comment: "I think we're only beginning to understand what's going on here."

UPDATE 4/01/15: This article in the Economist is about a similar theme:
No use crying: The ability to digest milk may explain how Europe got rich