When did separation of powers start?

In the modern
world, separation of powers was first put into practice when the Constitution of the United
States was written and ratified. This was the first time that there was a government with a
clear separation between its legislative, judicial, and executive branches.

The idea of separation of powers did not come directly from the minds of those writing
the Constitution.  It had existed to some extent in Ancient Greece and in the Roman Republic. 
After that, it had been promoted by some of the most important political philosophers of the
Enlightenment era.  The man who is most clearly connected to this idea was
Montesquieu.

You could say, then, that separation of powers in the modern
world started with these philosophers and was put into practice by the United
States.