Early civilizations, such as those in egypt, sumer, and harappa, were all

These three civilizations were all centered around rivers that flooded regularly.

Explanation:

In ancient times, to prosper, civilization should be closer to rivers. Most important, it was necessary to understand how the floods work and, then, be adaptative to each situation. In the case of the civilzations that you asked:

the answer is: the sesquicentennial of the american civil war is an occasion to consider the legacy of that great conflict. this legacy is our inheritance, and despite many indelible features, we continue to negotiate the resolution of that conflict. the civil war was fought over slavery, but not always on behalf of slaves

the name of the theory is landmass theory.