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.

The median is 4 because 2 x’s out 9, 3 x’s out 7, 3 x’s out 5, and 4 is the only remaining number.

1.world in spatial terms2.places and regions 3.human systems4.physical systems5.geography and it's uses6.environment and society