Some mutations have a positive effect on the organism in which they occur. They are called beneficial mutations . They lead to new versions of proteins that help organisms adapt to changes in their environment. Beneficial mutations are essential for evolution to occur. They increase an organism’s changes of surviving or reproducing, so they are likely to become more common over time. There are several well-known examples of beneficial mutations. Here are just two:
Milk is good for us we all know that. While babies can digest milk there was a time when adults could not. But in areas where humans domesticated cattle and other animals that they could obtain milk from milk became an important part of their diet. This happened because a small population of people developed the ability to digest lactose and  they passed it onto their children and now most people can digest milk even as adults.
Who evolved the cattle or humans?