Mermaids are human and fish composite beings found in many later European stories and in related forms across the world. There is no single historical moment when every mermaid tradition began, and similar water beings do not always share the same ancestry.
Ancient and medieval backgrounds
Ancient Mediterranean art includes fish-tailed gods and goddesses, including figures associated with Syria and the Near East. Atargatis is often named as the first mermaid, but surviving descriptions and images do not support one simple line from her worship to every later tale.
Greek Sirens were originally bird-bodied singers, not fish-tailed women. Medieval artists increasingly represented Sirens with fish tails, helping the names siren and mermaid overlap in later European languages and art.
Folklore and reported sightings
Medieval and early modern mermaids could warn of storms, represent temptation, offer prophecy, or marry humans. Sailors also reported marine animals as mermaids, but the repeated claim that every story came from mistaking a manatee or dugong does not explain the full symbolic tradition.
Selkies, ningyo, Mami Wata, and other water beings should not be relabeled as mermaids without attention to their own cultures. Comparison is useful, but it should preserve local names, sources, and meanings.