Mesopotamia--this early civilization traces connections to trade as early as 2300 BC connecting to Egypt and the Indus Valley.
Through trade, ideas and collective learning occurred spreading to these Bronze Age civilizations. Eventually this spread to more growing civilizations to the north (Greece) and east (Persia).
I hope this helps you and answers your question correctly. i believe the answer is the Dark Age.
Slavery was not as prevalent in the North as it was in the South due to economic and geographic factors.
The key factor was the type of economy in the North versus the South.
Very quickly, the Northern economy became based on industry while the Southern economy was largely based on large scale plantation style farms.
The end result was that the South needed slaves more for their type of economy to be profitable.
