Final answer:
Prolactin is a pituitary hormone essential for milk production in mammals and has varied functions in different organisms, indicating its evolutionary role as a multi-functional regulator. In humans, it is involved in a positive feedback loop that stimulates milk production during lactation.
Step-by-step explanation:
Prolactin is a pituitary hormone that plays a crucial role in the establishment and maintenance of breast milk production but also exhibits a wide range of functions across different species. For instance, it regulates salt and water balance in freshwater fish and stimulates mammary gland growth and development in mammals. This hormone's diverse roles suggest that prolactin has evolved to perform multiple regulatory functions in various organisms. In humans, during lactation, the positive feedback loop involving the nipples' stimulation leads to prolactin secretion from the anterior pituitary, which in turn triggers milk production. This process is supported by other hormones such as oxytocin, which facilitates the milk let-down reflex, and is balanced by prolactin-inhibiting hormone, known as dopamine.