Mermaids and humans maybe.

I always thought they were the descendents of the original Atlanteans

I'm gonna have to go with magic, because they obviously weren't created by the Celestials. If I had to guess, i'd say they were created as a last resort to save the Atlantean race, which would explain why they are descendants of the original Atlanteans.

I think they are an evolution of the human race.

If I had to guess, i'd say they are water breathing Eternals, like the Lemurians are water breathing Deviants.