Citation needed. Humans have been anatomically modern since 100k BC, including the brain. There is no reason to think there haven't been kings or slaves in that time, just that there is no history about them.
In fact much of the world was like this only a few centuries ago.
Migratory hunter-gatherer tribes have spawned some of the worlds "greatest" conquests - a good example would be the Mongols (who to this day hold the record for largest empire by geography). If it were just Genghis Kahn, that would be the exception that proves the rule, but there a lot more. https://en.wikipedia.org/wiki/Nomadic_empire has a nice list of friendly hunter-gathers and other nomadic tribes that decided to try to conquer their known worlds, genocide their enemies, enslave people, etc...