There is no history.
Mermaids are not human – they are half-human and half-fish.
They are mythological, fictional creatures.
Ariel is the Disney version of Hans Christian Andersen’s Little Mermaid character, designed in 1989. She’s a red-head because Disney did not want people to confuse her with Daryl Hannah’s blonde character from Splash (1984), and the rest of her design was influenced heavily by Alyssa Milano.
There is precisely zero reason to lose one’s mind because Disney cast a black actress to play Ariel in their live action version.
Your iconic red-head animated version still exists and will continue to appear in various media – most recently in Ralph Breaks the Internet.
This is a new take on the fictional character of a mythological creature featured in their 1989 cartoon.
Halle Bailey is a talented performer and will do well in the role. Breathe. Let her be part of your world.