Every culture on this planet has enslaved people at some point, including their own race.
Why the past tense?https://www.globalslaveryindex.org/2018/data/maps/#prevalenceSlavery was never about race
Not because of race. Slavery was/ is a way how to maintain a successful civilization successful. In the world, there are that many resources. If you want to have a high standard of living, you need to drain them from others. Ancient civilization drained war captives, we drain Africa, India, and Southeast Asia. Oh, and also animals.
Racism was just one of the ways how to maintain a stable slavery system. Not the cause, but I wouldn't say slavery was never about it.