|Amazonia today. From Hoorn et al. 2010 (Link)|
A recent review in Science magazine synthesizes what is known about the geological and biological past of the Amazon, to reconstruct a scenario where the uplift of the Andes mountains, which are relatively young, is responsible in part for the present-day patterns of diversity in the basin. Prior to the Andes uplift (which began in the Paleogene after the end-Cretaceous 65 Mya, and continues to the present day), the geology of Amazonia was 'cratonic', i.e. it was dominated by the rocks of the continental center. The rising mountain range changed both the climate, e.g. by intensifying rainfall to the East of the range, and also the landscape, as sediment eroded off the mountain faces and came to be deposited in the basin below. Extensive, inland wetlands (the Pebas system) first appeared in correlation with intensified uplift and then disappeared, replaced by a riverine, fluvial landscape.
The present-day biota of the Amazon is very much terrestrial. Therefore, the disappearance of the extensive wetlands, which formerly fragmented the terrestrial forests, was a prerequisite for the appearance of modern Amazonian fauna and flora. Furthermore, diversity as measured today for both mammals and tree species is highest where the soils are of Andean origin (swept down by erosion and deposited by the rivers), as opposed to where the original cratonic soils remain at the surface. Perhaps the nutrient levels are linked to both forest productivity and resultant diversity, but the climate engendered by the topographic relief may also have a role in patterns of diversity.
This is very much a 'big picture' review, and is worth reading to think about how the literal shape of the Earth has a role in defining the life that dwells as a thin film on its surface.