If you can tolerate milk in your diet, you have your cattle-herding ancestors to thank.
All humans digest mothers' milk as infants, but until cattle were domesticated 9000 years ago, weaned children no longer needed to digest milk. As a result, many shut down the enzyme lactase, which breaks down the lactose into sugars. After cattle were domesticated, however, it became advantageous for their keepers to digest milk, and lactose tolerance evolved.On the other hand, Asians have been shown to be more tolerant of Jackie Chan movies.
Not everyone can stomach milk equally, however. Northern Europeans, who tend to be descended from cattle farmers, are much more lactose tolerant than most Asians, who were not as dependent on cattle. [Link]