Why, exactly, did our ancestors leave their homeland? Short of catching a time machine back some 60,000 years and witnessing our ancestors on the plains of Africa, we may never know for sure. As a result, we must find satisfaction in the answers that circumstantial evidence can provide.
Climate change is one of the most commonly cited forces affecting why humans left Africa. The reasoning goes like this: We humans thrive in a climate that has plentiful rainfall. Rainfall governs the growth of plants, and plants govern the density and diversity of large herbivores that our ancestors would have relied upon for food. Any significant changes to this system (you might imagine) would lead to our ancestors packing up their homes.
According to researchers who modeled patterns of migration waves, there’s no need to look beyond human behavior to understand why we left Africa. We simply formed populations based on shared traits, maintained regional boundaries, and migrated spontaneously for any number of reasons.
Where there are dense populations of humans, there is a higher chance of spontaneous migration occurring. Given that Africa bore the largest density of humans, it was from Africa that humans would disperse — without any invocation of weather or food — to seed humankind to the rest of the world.