The creation of cities in ancient times is not without its snags. One of them is trying to make them last in tropical latitudes.
The rainforests of central Africa bear to mind. Humidity varies between 80 and 90%. It rains around 117 days per year with 70 inches of rain. Temperatures vary between 68 and 81 degrees Fahrenheit.
These three separate factors conspire together to leach away vital nutrients from the soil. This makes the agriculture necessary to create cities top priority day after day.
On the other side of the world, the Amazonian tribesmen seemed to have that priority solved.
This particular layer is called terra preta, an artificial mixture of the Amazon's native soil with charcoal, bone and manure. If the depth of that particular layer is any indication, it would seem that terra preta is so successful that it would have been used in multiple generations. And apparently, it works--its high concentration in charcoal, microbes and organic matter makes it very resistant against leaching by the heavy rains and floods.
It worked for the tribesmen of the Amazon. Could it have worked for the tribesmen of the Congo? If yes, would terra preta change African history in more ways than one?