Mammals usually develop lactose intolerance when being grown up to a certain extent, so they get independent from it and switch to other food. That included humans, until they started making use of milk from other animals. From that point on, those with lactose intolerance had worse chances to survive and produce offspring and after a hundred generations, a majority could digest it. [/smartass]
But who cares what might be natural and what might not? I'm doing as much of what I'm feeling like as possible.