Biology Question related to Evolution?

A protein similar to the mammalian hormone prolactin is found in amphibians and birds. In amphibians, such as newts, it causes the animals to seek water as they do when they lay their eggs. In birds, such as pigeons, it causes them to regurgitate material from the crop to feed their young. Mammals respond to prolactin by producing milk. How does the theory of common descent explain these similarities?
This is for my lab so it does matter, religon people leave no comment.
When I say religion people no comment im not insulting them i have my religious beliefs, i just strictly wanted a science answers

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