Human history
When humans started to grow crops and domesticate certain animals such as dogs, goats, sheep, and cattle; their societies changed. Because people now grew crops and raised livestock, they started to stay in the same place and build permanent settlements. In most places, this happened between 10,000 and 12,000 years ago. Their diet also changed. People ate more cereals and vegetables. They started to keep extra foods and seeds for later. In some years there were surpluses (extras) that could be traded for other goods. People who could tolerate milk had an advantage. Originally, milk drinking could only be done by babies. Farming societies gave a great advantage to the few people who could tolerate milk. This tolerance eventually became almost universal.