Anarchism

Well it really depends on what you call a hierarchy.

If it's a social organization where one person can take a decision that other peoples follow, then yeah, it probably emerged far before we evolved into humans. But it's not necessarily opposed to anarchism.

If you begin to add other elements of definition, like the fact that it happens in all aspects of life, that the organization is not agreed to by some or any of the people it applies to, that it is derived from spiritual, religious, economical or political principles instead of just emerging from people interactions, that it is tied with differences in ways of living, etc., then you get a quite stricter conception of hierarchy, and i think most people will agree that those emerged or crystallized with the switch from nomadic to sedentary societies and the rise of institutionalization of religion and politics.

So i'd say there's no clear moment because there's no clear definition of hierarchy, but i'd say there is a consensus to say that there has been a process of hierarchisation of societies at various steps of prehistory.