What other groups played roles as intermediaries between civilizations in early global history?

The answer to this question is The Parthian and Sassanid empires
The parthian empire is the ancestor of what we known as the ottoman empire, which control a large part of europe, and africa while the sassaniaf empires is the ancestor of the persian empire which control a large part of Asian continent and Africa.

Ifound this on google “separation of powers. a fundamental principle of the united states government, whereby powers and responsibilities are divided among the legislative branch, executive branch, and judicial branch.” i hope this you for what your looking for