Final answer:
Early complex societies in Mesopotamia and Egypt interacted through mechanisms like trade, warfare, and diplomacy, leaving behind evidence such as trade goods and artistic influences.
Step-by-step explanation:
Interaction between early complex societies in places like Mesopotamia and Egypt occurred through mechanisms such as trade, warfare, and diplomacy. These interactions were facilitated by sustained contact between the regions through the exchange of goods, military campaigns, and diplomatic relations. For example, the Mesopotamian city-states commonly waged war against each other and foreign invaders, while ancient Egypt, protected by its geography, developed a single unified state. Scholars have evidence of these interactions through archaeological findings, such as trade goods and artistic influences, as well as written records.