It was caused by a mix of different scenarios. Rome was constantly attacked by Barbarian tribes; they were over-expanded, which made it really hard for them to keep their entire empire under control; they had a lot of economic problems and relied too much on slave labor; and they spent way too much money on their military, just to name a few factors.
Answer:
little rainfall
Mesopotamians depended on the annual flooding of Tigris and Euphrates rivers
this was similar to the Egyptians
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Answer:
As humans migrated, they had to adapt to many different environments
