natural selection
A long time ago, Europeans were only able to digest lactose in their first few years. Now, they are able to digest lactose for their entire lives! What is the likely path that this mutation took?
A person was born able to digest lactose. They began to drink milk, so they got big and strong with thick bones. All the people wanted strong babies, so this person had many children. Now there is a bunch of babies, most of whom can digest lactose, and the babies get strong. The babies have many children, and soon most of Europe could digest lactose. And its all because some person who drank milk had kids.