How and why did early humans migrate out of africa and across the earth's varied landscapes?

Human migration occurred because of several factors. The reasons are the following:
1.food shortages
2.War
3 Seasonal changes
4.Disease and epidemic.

Africans are nomadic people. They thrive more from their environment. They only had to stop migrating during the agricultural era, wherein they learn to plant and tend crops and started cattle herding.