Thats interesting, I read quite some history on governments. I must admit that I mostly focused on Europe and Asia, since that were the places that were of most interest to me.

In these texts it was from the militia it usually evolved into guards (not a quote)

Although it does seem quite logical for the first policeesque system to be mostly for catching slaves, I do think(mostly guess) that most if not all slave holding societies already had a government system and with that guards.

In any way, if you would have some literature for me about that, do share it please!