Answer:
1) Prehistoric people of Asia were able to emigrate to North America because the climate was colder. The earth was going through a period of glaciation as show in the popular film the ice age. This caused a the bering strait to become frozen, and it worked as a land bridge that allowed migration. This is why native americans have straight black hair like east asians.
Early people of the Middle East migrated into Mesopotamia because there are two important rivers in the region: the Tigris and the Eufrates. In fact, Mesopotamia is a word of greek origin which means "land between two rivers".
2) Societies become a civilization as they become sedentary. They become sedentary because they learn how to grow crops and domesticate animals. These crops and animals generate economic growth which is appropriated by a ruling class. This ruling class establish themselves as rulers and impose their law and will on the people.