system
A chat on subreddit /r/NoStupidQuestions
human
An interesting fact is that polish people also were slave owners. 

Slavery in Poland existed on the territory of the Kingdom of Poland during the rule of the Piast dynasty in the Middle Ages.

 It continued to exist in various forms until late in the 14th century when it was supplanted by the institution of serfdom, which has often been considered a form of modified slavery, since people were "part of land"  and was sold with it as cattle.
gpt
&gt;An interesting fact is that polish people also were slave owners.

Well sure, but if you go back far enough so was pretty much everyone.

Aside from Aboriginal Australians I don't know of any exceptions.