She claimed that a population density of 0.2 inhabitants per square kilometre (0.52/sq mi) is the maximum that can be sustained in the rainforest through hunting, with agriculture needed to host a larger population.However, recent anthropological findings have suggested that the region was actually densely populated.States or departments in four nations contain "Amazonas" in their names.The Amazon represents over half of the planet's remaining rainforests, The rainforest likely formed during the Eocene era.Terra preta is found over large areas in the Amazon forest; and is now widely accepted as a product of indigenous soil management.The development of this fertile soil allowed agriculture and silviculture in the previously hostile environment; meaning that large portions of the Amazon rainforest are probably the result of centuries of human management, rather than naturally occurring as has previously been supposed. This region includes territory belonging to nine nations.
A possibility causing the variation is the Sahel, a strip of semi-arid land on the southern border of the Sahara.
When rain amounts in the Sahel are higher, the volume of dust is lower.
The higher rainfall could make more vegetation grow in the Sahel, leaving less sand exposed to winds to blow away.
More than 56% of the dust fertilizing the Amazon rainforest comes from the Bodélé depression in Northern Chad in the Sahara desert.
The dust contains phosphorus, important for plant growth.