Answer:
The fact that all mammals have mammary glands that produce milk for their young is evidence that all mammals have a common ancestor.
Step-by-step explanation:
This is because the presence of mammary glands is a shared characteristic that evolved in the common ancestor of all mammals, and has been passed down to all of their descendants through the process of evolution by natural selection. This shared characteristic is an example of a homologous trait, which is a trait that is similar in different species because it was inherited from a common ancestor.