Tropical rainforests are a very ancient biome. When the earth was much warmer, 240 million years ago, the ancient “coal forests” of primitive non-vascular plants died out and the succeeding forest, mainly tropical, consisted of ferns, conifers, cycads and other lesser-known groups. Insects were the dominant animal form on land; dinosaurs appeared only later during this period. Fossils of flowering plants (angiosperms) first appear in rocks dating from one hundred and fifty million years ago. These plants were enormously successful and eventually became the dominant land vegetation, forcing the ferns and other earlier groups to a marginal status. We know that rainforests originated sometime after 100 million years ago, and that they were the dominant forest type at the time of the disappearance of the dinosaurs (65 million years ago). They represent the world’s oldest extant biome, although current tropical forests almost certainly differ in many respects from the earlier ones. However, angiosperms still form the majority of the plant species in tropical forests.
Some scientists believe, as indicated by recent fossil data from South America, that many tropical species arose as long as 14 million years ago or more. A South American monkey skull, which has been dated as 20 million years old, has anatomical similarities to Old World monkeys, and supports the hypothesis that some New World fauna originated in Africa. Also, fossil teeth of Old World rodents which resemble those of New World porcupines have been found. These animals presumably migrated to South America by floating on “rafts” of vegetation. Many other very ancient tropical mammal fossils – marsupials, herbivores, rodents, edentates – have been found in the Andes. The Isthmus of Panama arose about seven million years ago, allowing exchanges of fauna between North and South America. Fish fossils are correspondingly ancient, and have been found far from the Amazon and Orinoco Rivers, where it was previously thought that fish species in this region evolved. There are fossils of catfishes, rays, piranhas, carnivores (flesh-eaters) and piscivores (fish-eaters), even fossils of a fish which ate fruit (and which is very similar to a fish presently living in these rivers). What does this mean? These data imply that these species arose many millions of years ago, at a time prior to the uplifting of the Andes as a mountain chain, and when this area was continuous with the region which is now the Amazon basin and covered with rainforests. Most of these fish fossils are related to those of the current tropical regions (Amazon and Orinoco basins) and not to present-day fish of the region where the fossils were found.