How long have mermaids been around?

Mermaids were born in the ancient myths of the Greeks and the Irish. These stories tell of people from the sea. The Irish tales call them Silkies and tell of one night a year when they come to shore on a full moon. They slip off their skins and become human, but if a human comes along and steals the skin they stay human until they get the skin back. There is one story about a man who fell in love with a Silkies and took her skin. She married him, had children, and after many years together she found her skin in a box under the bed. She then returned to the sea, but every so often she could be seen offshore watching the family she left.