Where does the myth of mermaids originate from?

In Mesopotamian mythology , Ea was an amphibious being (merman) who taught mankind wisdom. As described by the Babylonians 7,000 years ago, Ea had the form of a fish but with the head of a man under his fish’s head and under his fish’s tail the feet of a man. In the daytime he came up to the seashore of the Persian Gulf and instructed mankind in writing, the arts, and the sciences.
Ea would later be called Oannes by the Greeks. Greek mythology also has stories of the god Triton, the merman messenger of the sea.
The earliest known mermaid legends come from Syria around 1000 B.C. The Syrian goddess Atargatis dove into a lake to take the form of a fish out of shame for accidentally killing her human lover, but the powers there would not allow her give up her great beauty; only her bottom half became a fish and she kept her top half in human form.
Some Greek myths described the Sirens as mermaids; the Sirens had beautiful voices that lured unsuspecting sailors and fishermen to their deaths on rocky shores. (Sirens were more frequently depicted as half-woman, half-bird in Greek myths.) The Arabian Nights tales also described sirens as mermaids.
Some of the legends of the Pacific Islands said that human beings are descended from both mermaids and mermen. Somewhere back in time, their tails somehow dropped off, and people were magically able to walk on land.
All sea-faring cultures have mermaid tales. Reports of ‘mermaid’ sightings continued right into the 1900s.