Most of humanity's time on earth was spend in small pre-historic tribes, I don't see any reason to assume those societies were monarchies. They probably didn't have the resources to support deeply entrenched power structures.

Humans come with built-in abilities to establish small ad-hoc social structures. You'd probably have some members of the community that were generally charismatic or bright (or particularly good at violence) who'd generally tend to get their way, but it is a complex web of social pressures.

I mean, just think of your friends -- you've probably got somebody who is broadly the "ideas guy" or kind of a natural leader, but they don't just automatically over-rule everybody.