Which came first?

I was a little confused when we were talking about race and how it was created for political and social means. I know that it is a social construct with no biological significance, but I always thought that some sentiment of "race" had pretty much always been in existence. When we learned that race really came into existence and become more prevalent because it was easier to excuse slavery this way, I began to think. I didn't understand how it could have happened in this way. I truly thought that slavery could have only came about if there was already an existing notion that other races, such as blacks, were inferior.