The most cause of human migration out of Africa into other areas of the world is due to climate change. The climate change resulted in harsh conditions such as drought. This also led to a lack of food and water in the region thereby leading to poor living conditions in the region.
They then decide to seek better living conditions and environment by moving to other parts of the world to search for greener pastures.
If you read in the passage the Spanish overpowered the Inca and the Aztecs in many ways. One of them being the Spanish had more troops or more people to fight for them, while the Inca or the Aztecs never did.