The roots of mermaid mythology are more varied than one would expect. In modern myth we tend to see mermaids in a singular way - kind and benevolent to humans who keep to their own kind in the deep waters of the ocean. Not all stories go this way, though, and in most cases the most ancient tales of mermaid mythology follow quite a different view.
The earliest known mermaid legends come from Syria around 1000 B.C. where the Syrian goddess Atargatis dove into a lake to take the form of a fish, but the powers there would not allow her give up her great beauty, so only her bottom half became a fish and she kept her top half in human form.