http://news.bbc.co.uk/1/hi/health/6397001.stm
Dr Mark Thomas, from UCL, said: “The ability to drink milk is the most advantageous trait that's evolved in Europeans in the recent past.
â€Although the benefits of milk tolerance are not fully understood, they probably include the advantage of a continuous supply compared with the 'boom and bust' of seasonal crops, its nourishing qualities, and the fact that, unlike stream water, it's uncontaminated with parasites, making it safer.
“All in all, the ability to drink milk gave some early Europeans a big survival advantage.â€