The Amazon once ran the other way. Fifteen million years ago, before the Andes rose and blocked its course, the proto-river flowed westward, possibly draining into a system that continued across what is now Africa. When the South American Plate collided with the Nazca Plate and the Andes pushed upward, the water backed up into a vast inland sea. Marine creatures adapted: freshwater stingrays of the Amazon today, over twenty species strong, descend most closely from Pacific cousins stranded when the continent shifted. About ten million years ago the water worked eastward through sandstone, the rainforest was born, and when the Ice Ages dried out the tropics, the forest retreated into scattered islands of green. Species diverged on those islands and rejoined when the rains returned. The mind-bending biodiversity of the Amazon is in part a consequence of that long separation and reunion.

For most of the colonial era, the upper Amazon lay inside the Spanish Viceroyalty of Peru. The Portuguese were supposed to stay on the far side of the Tordesillas line, but a river system this navigable could not hold an imaginary boundary. Beginning with the founding of Belem at the river's mouth in 1616, Portuguese settlers, Jesuit and Carmelite missionaries, and slave-hunting bandeirantes pushed west up the Solimoes. Pedro Teixeira led 70 soldiers and 1,200 Indigenous paddlers in 47 great canoes all the way to Quito and back between 1637 and 1639. Smallpox epidemics in 1661, 1695, 1724 and the 1740s emptied whole river stretches of their Indigenous inhabitants. By the Treaty of Madrid in 1750, Portugal had effectively taken what Spain could not hold. The captaincy of Sao Jose do Rio Negro was formalized in 1755, and in 1850 Amazonas split from Para to become a province of its own.

The late 19th century brought a commodity that the rest of the world suddenly could not do without. Automobile tires, telephone lines, bicycle tubes and industrial belts all needed latex from the Hevea brasiliensis tree, and Amazonas had Hevea in abundance. Migrants poured in from Brazil's drought-stricken northeast to tap wild trees scattered across the forest. Enslaved and debt-bonded Indigenous seringueiros cut the bark, caught the white sap and carried it out. Untold thousands died of disease and overwork. Meanwhile, Manaus built itself a Belle Epoque city in the middle of the jungle: the famous Amazon Theatre, a new port, electric trams, a Palace of Justice, European luxury shops. The rubber boom collapsed after Brazilian seeds smuggled to British Malaya in 1876 grew into plantations that undercut every Amazonian producer. What remained were the opera house, the empty warehouses and the memory of black gold.

In 1957, with the old economy gone and the region depopulating, Brazil's government tried something new. Act 3.173 created the Manaus Free Trade Zone, later expanded under military-government decree in 1967. The idea was to pull industry into the Amazon by waiving import taxes on components assembled there. Televisions, motorcycles, air conditioners and later electronics came to be manufactured in factories surrounded by jungle. The policy was partly a security calculation, an attempt to populate a region the generals feared could be lost to neighbors or rebellion, and partly an attempt to slow deforestation by giving the Amazon a reason to exist besides cattle and logging. It worked unevenly. Manaus grew to 2.1 million people, nearly half the state's population, a modern skyline rising from a river port 1,500 kilometers from the ocean.

Amazonas divides into 62 municipalities, but most of them do not connect to each other by road. Parintins, Manacapuru, Itacoatiara, Tefe and Coari string along the Amazon in the state's eastern half. The western reach runs to the tri-border at Tabatinga where Brazil, Peru and Colombia meet, territory where Ticuna people live and speak their own language alongside Portuguese and Spanish. The far northwest rises into the Guyana Shield and Pico da Neblina, at 2,995 meters the highest mountain in Brazil, hidden most days under a cap of cloud that gives the peak its name, meaning Mountain of the Mists. Between the rivers lies dense terra firma forest; between the flooded igapo of the black-water Negro and the varzea of the white-water Solimoes lie thousands of square kilometers where the only way in is by boat and the only way out is patience.