What if humans colonized Africa at the same time as Europe?

If you look at the climate map, the tropics are the worse than anywhere in Europe in the early 15th centurary. In the tropics, you have heat so bad that you have to sleep on top of your house, and tons of disease. Inversely, the areas near the equator in Africa have the same climate, but many societies already had ways of dealing with them. If the Portuguese never setup their tradeposts in West Africa in the 15th century, only an area on the northern half of Africa would be colonized, where there are no rainforests and disease would not be as big of a problem. By the time that the Europeans get to the Sahel in Africa, they would already have tons of disease resistance, meaning that they would have an advantage over the native societies of tropical Africa. What would happen?