The Amazon Rainforest, also known in English as Amazonia or the Amazon Jungle, is a moist broad-leaf forest that covers most of the Amazon Basin of South America. This basin encompasses seven million square kilometers (1.7 billion acres), of which five and a half million square kilometers (1.4 billion acres) are covered by the rainforest. This region includes territory belonging to nine nations. The majority of the forest is contained within Brazil, with 60% of the rainforest, followed by Peru with 13%, Colombia with 10%, and with minor amounts in, Venezuela, Ecuador, Bolivia, Guyana, Suriname and French Guiana. States or departments in four nations contain "Amazonas" in their names. The Amazon represents over half of the planet's remaining rainforests, and it comprises the largest and most species-rich tract of tropical rainforest in the world.
The rainforest likely formed during the Eocene era. It appeared following a global reduction of tropical temperatures when the Atlantic Ocean had widened sufficiently to provide a warm, moist climate to the Amazon basin. The rain forest has been in existence for at least 55 million years, and most of the region remained free of savanna-type bio-mes at least until the current ice age, when the climate was drier and savanna more widespread.
Based on archaeological evidence from an excavation at Caverna da Pedra Pintada, human inhabitants first settled in the Amazon region at least 11,200 years ago. Subsequent development led to late-prehistoric settlements along the periphery of the forest by 1250 AD, which induced alterations in the forest cover. Biologists believe that a population density of 0.2 inhabitants per square kilometer (0.52 /sq mi) is the maximum that can be sustained in the rain forest through hunting, with agriculture needed to host a larger population.
The first European to travel the length of the Amazon River was Francisco de Orellana in 1542. The BBC documentary Unnatural Histories presents evidence that Francisco de Orellana, rather than exaggerating his claims as previously thought, was correct in his observations that an advanced civilization was flourishing along the Amazon in the 1540s. It is believed that the civilization was later devastated by the spread of diseases from Europe, such as smallpox.
As the largest tract of tropical rainforest in the Americas, the Amazonian rainforests have unparalleled biodiversity. One in ten known species in the world lives in the Amazon Rainforest. This constitutes the largest collection of living plants and animal species in the world. The region is home to about 2.5 million insect species, tens of thousands of plants, and some 2,000 birds and mammals. To date, at least 40,000 plant species, 2,200 fishes, 1,294 birds, 427 mammals, 428 amphibians, and 378 reptiles have been scientifically classified in the region. One in five of all the bird species in the world live in the rainforests of the Amazon, and one in five of the fish species live in Amazonian rivers and streams. Scientists have described between 96,660 and 128,843 invertebrate species in Brazil alone.
The green leaf area of plants and trees in the rainforest varies by about 25% as a result of seasonal changes. Leaves expand during the dry season when sunlight is at a maximum, then undergo abscission in the cloudy wet season. These changes provide a balance of carbon between photosynthesis and respiration.
The rainforest contains several species that can pose a hazard. Among the largest predatory creatures are the black caiman, jaguar, cougar, and anaconda. In the river, electric eels can produce an electric shock that can stun or kill, while piranha are known to bite and injure humans. Various species of poison dart frogs secrete lipophilic alkaloid toxins through their flesh. There are also numerous parasites and disease vectors. Vampire bats dwell in the rainforest and can spread the rabies virus. Malaria, yellow fever and Dengue fever can also be contracted in the Amazon region.