I have another source of nutrition thanks to me drinking milk!
Haha I am Superior to you!
We can all drink milk now!
The advantage of this mutation was that, if there was a drought and water was needed for the crops, the certain people could drink milk instead. It also had calcium advantages by boosting their bone strength and vitamin D.
When the kids of these two parents had their kid, the gene continued to spread which eventually led to our population now being 65% lactose intolerant. It doesn't seem like a big change from 100% to 65% but this is because natural selection is still going through all of us as we live right now.
Overall, this mutation in one person spread to a large portion of the population since it had a reproductive advantage. This natural selection process has forever changed the human population.