Read the passage.
Pet Cats
The cats we keep as pets today were once wild animals. Thousands of years ago, rats and mice followed humans, hoping to find scraps of food to eat. Wild cats wanted to hunt the rats and mice. So, they started spending more time around humans, too. The cats ate well, and the people were happy to have them killing pests. Later, cats were taken on ships to hunt the mice there, too. Sailors welcomed cats on board and brought them to other parts of the world. After some time, people were used to having cats around. They wanted to keep them as pets, not just hunters.
What is the main idea of the passage?