Tropical rainforests cover about 6% of the earth’s land surface. Tropical America contains about 50% of the world’s tropical rainforest, approximately four million km2 of forest cover. Southeast Asia has about 2.5 million km2, and Africa, with the smallest area, about 1.8 million km2. This does not mean, however, that rainforests have lain within their present contours for many thousands of years. For example, recent evidence indicates that the Bolivian rainforest has been gradually expanding over the past three thousand years; for the previous 50,000 years it had not extended so far south because of drier conditions (Mayle, et al., 2000).