Answer:
Only two backbones and thousands of ribs.
My first is equality; my second is inferiority; my whole is superiority.
I grow where no flower grows, where no light touches the walls, up or down, that I don't care, was here before people were.
You throw me out when you want to use me and you take me in when you don't want to use me. Who am I?
I do not breathe, but I run and jump. I do not eat, but I swim and stretch. I do not drink, but I sleep and stand. I do not think, but I grow and play. I do not see, but you see me everyday.
You use it between your head and your toes, the more it works the thinner it grows.