The vast majority of the adult population can not consume milk without experiencing a very unpleasant digestive problems. Once you stop being fed with breast milk, your body stops producing lactase, the enzyme needed to digest lactose, the main sugar component of milk.
However, if you drink milk without getting bloated, have abdominal pain and diarrhea, the typical accompanying signs of lactose intolerance, you’re happy mutants. Your ancestors were probably farmers who drank milk from their pets. Ok, we’re talking about mutants in scientific, generic way. At some point in evolutionary development, around 7500 years ago, our ancestors get the ability (with mutation on one chromosome) that makes the body produce lactase after weaning, which it firmly anchored in the genome of their offspring.
So, if you want to know what I’m talking about, take a look at this video. This guys explained that very nice: