Do you know why we drink milk after infancy? Because we evolved to. It was when man domesticated the Auroch (the predecessor to the cow) that it is thought they started holding on to lactase production farther into life. Milk allowed the humans to live better and fatter when times were lean. As such, there was an evolution towards continue lactase production. There's some really interesting work in this area done by McCracken out of UCLA. There's also some cool studies since then around the cultural migrations of lactase and lactose intolerance by Enattah and Tishkoff.

Pretty neat stuff seeing how evolution affects all of us.