Odd. I always viewed it the other way around.
A Constitutional Democratic Republic can degenerate into a Democracy,
which can degenerate into Socialism, which can degenerate into
communism and worse.
It’s a downhill trajectory.
(Shorts wadding desisitization mission complete)
Before wading into ancient Greek and Roman governance, which the subject calls for, consider this. Most things evolve toward increasing complexity. It’s as hard to believe the complexity of republicanism arose alone from primary conjecture as to believe arithmetic arose from calculus.
More likely, simple democracy came first and was later modified in some places into something resembling republican government. To go the other way is, IMO, to say that there was complexity before simplicity.