The reason why most people migrate has to do largely with societal factors such as opportunities or economic factors in the modern world. Other factors can be environmental such is the case for the Nomad's, the Nomad's had to migrate to other areas due to the ice age which led to a lack of food and the extreme cold also made it hard for people to live comfortably. For the early Africans there were many environmental factors at play which led to them needing to leave, around northern africa the sahara desert constantly increases in size leading unbearable heat for most people and most importantly a lack of farming availablity.
