Given the importance of subsistence farming in this realm, the green revolution has had great impact here

Subsistence farming involves farmers growing just enough food that will be adequate for them and their families. So no surplus is farmed for sale. The Green Revolution allowed larger crop yield for farmers by introducing resistance strains, fertilizers and pesticides. As you can imagine, the wide spread use caused a very negative impact on the environment. Subsistence farming was basically revamped by the Green Revolution to allow surplus be produced for sale in developing nations.

Migration people get up and go cause they feel it is best for them 

Iam pretty sure it is c. 2000 bce