The Dink Network

For mermaids, there is no clear definition of a fish (They contain vertebrates and invertebrates, along with other dipolar characteristics which make them difficult to classify in one group that doesn't cover other clearly separate groups.) where as there is for mammals. Mammals have mammary glands and generally give birth the live young (but not always, see platypus and echidna) and often have hair or fur for warmth due to warm blood. A mermaid has mammaries, and hair, and their blood temperature and birthing practices aren't generally agreed on. As such, mermaids are clearly fish-like mammals and not mammal-like fish. They would probably have evolved like whales and dolphins, but from humans.