Just want to add, the movie seems to strongly suggest that the Tethered are human.
Red also mentions it during her final confrontation with Adelaide.

What defines the Tethered, is the conditions in which they happen to have been born - or raised.
Their limited access to resources, or level of education (that we, people from the Surface, would think is low or even nil), gives them no other choice but develop these characteristics (difficulty to communicate verbally, a tendency for violent expression? - yet when cultivated, an ability to efficiently overthrow whole regions of the Surface overnight).

The story of Adelaide shows it: even if you were born Tethered, you can have chances to integrate the Surface society.

The fact that the Tethered are human, feels to me as a parallel with the arbitrary of our societies: where and how you were born (something that you cannot control), decides what you can and cannot become later in your life.

Remember also the dialogue about a potential athlete career at the beginning of the movie, Adelaide tells her daughter she can become whatever she wants if she wills it. It might be a hint how Adelaide and her family integrated the privileges and feeedoms of living Above.

So there are no strict rules about being a Tethered, one can join the Surface society with a lot of luck.
Now there are no clues in the movies that Adelaide family had the same experience as her, and we can only imagine that they have only ever known life above the Surface.

Just want to add, the movie seems to strongly suggest that the Tethered are human.
Red also mentions it during her final confrontation with Adelaide.

What defines the Tethered, is the conditions in which they happen to have been born - or raised.
Their limited access to resources, or level of education (that we, people from the Surface, would think is low or even nil), gives them no other choice but develop these characteristics (difficulty to communicate verbally, a tendency for violent expression? - yet when cultivated, an ability to efficiently overthrow whole regions of the Surface overnight).

The story of Adelaide shows it: even if you were born Tethered, you can have chances to integrate the Surface society.

The fact that the Tethered are human, feels to me as a parallel with the arbitrary of our societies: where and how you were born (something that you cannot control), decides what you can and cannot become later in your life.

Remember also the dialogue about a potential athlete career at the beginning of the movie, Adelaide tells her daughter she can become whatever she wants if she wills it. It might be a hint how Adelaide and her family integrated the privileges and freedoms of living Above.

So there are no strict rules about being a Tethered, one can join the Surface society with a lot of luck.
Now there are no clues in the movies that Adelaide family had the same experience as her, and we can only imagine that they have only ever known life above the Surface.