Gurney, they're not the only ones. Japanese are often lactose-intolerant for the same reason.

Japanese are also often more susceptable to alcohol, and the hypothesis here is that Europeans spent a millennia or more using alcohol for providing a bug-free source of water (drinking "small beer" or wine), whereas the Japanese found tea and so purified their water by boiling. Europeans therefore bred a tolerance to alcohol which the Japanese never had to do. This is only a guess - telling a story to try to explain it - but it's a popular hypothesis for that genetic feature of the Japanese.

Graham.