In Africa: A Biography of the Continent, John Reader asks why "the out-of-Africa [human] population grew from just hundreds to 200 million in 100,000 years, and rose to just over 300 million by AD 1500" while the African population itself only "increased from 1 million to no more than 20 million in 100,000 years, and rose to only 47 million by AD 1500". Dr. Science at Obsidian Wings answers:
The only part of the world to which human beings are truly native is sub-Saharan Africa. Ecologically, there are no "native peoples" anywhere else in the world, because outside of Africa Homo sapiens is always an invasive alien species. You'd think that the fact that we're adapted to Africa in a way we aren't adapted to anywhere else would be an advantage, but it turns out not to work that way. The overwhelming factor, for H. sapiens as well as stink bugs, is that our native range is adapted to us -- humans or bugs become dangerously invasive when we can escape not just the limited space of our native range, but the constraints on our population that come from other co-native species: predators or parasites (including diseases).