I understand this question's a bit Euro-centric and that feudalism tends to prefer to what society was like post-Roman but how and why did humans find themselves in a society that was reigned by a monarchy and operated on a class system of warriors and farmers and serfs? I want to say it seems that such a system would exist before the Roman Empire but maybe on a much smaller scale.

What was the alternative? How did we go from hunter-gatherers to kingdoms being made? And why were kingdoms made, then expanded and battling others?