" it’s hard to see what the evolutionary advantage might be for lactase persistence in the absence of a regular supply of fresh milk. And so we think of this as a classic example of how we have invoked shifts in our genome with our own practices—a gene-culture coevolution—experienced only in communities that were practicing dairy farming with domesticated milky beasts. What advantage having both access to milk and the ability to process it might seem obvious: In fact, it’s really the realm of intelligent but speculative guesswork. A regular supply of nutritionally rich food is one; avoiding the boom and bust cycles of seasonal crops is another possibility. By 6,000 years ago, milk had become a part of Neolithic life. "

― Adam Rutherford , A Brief History of Everyone Who Ever Lived: The Stories in Our Genes