How Old? How Long?
Our textbooks once told students that there were relatively few inhabitants in the New World when Europeans began colonization. We now know that was untrue! The best estimates tell us that at peak, as many as 25 million people lived in North and South America. While there's still a lot of debate about the exact dates, those peoples represented at least 4 separate migrations and could have lived there as long as 20,000 years ago.
When Francisco Pizarro founded Lima (The "City of Kings") in 1535, the Inka had held an empire larger than Europe for only about 100 years. It was divided into four large provinces, ruled by a strictly hierarchical royalty. Pizarro found cities with vast underground water and sanitary infrastructures, gold-plated temples and palaces, and highly developed agriculture. The farming in Meso-America was so successful that warehouses were stuffed with goods. The Inka had a federalist society where conquered peoples contributed labor in return for food, infrastructure and security. They also had amazing amounts of gold and silver.
But what Pizarro didn't find was the high population that seems to have existed just a hundred years earlier. Experts estimate that from the first landing of Columbus, disease began to decimate the native populations. As many as 95% of the population in some areas may have died from smallpox, hantavirus and other communicable diseases. Pizarro did not conquer Peru as much by the power of his swords as by the shadow of the diseases that preceded him. Read more about Inka history here.
The first genetically engineered crop (squash) seems to have been developed in Peru about 6000 years ago. Maize was genetically engineered in Meso-America about the same time, in an amazing series of crosses that would have challenged Mendel! Residents of the Amazon terraformed soils (adding charcoal and compost to make the clay soil more water-absorbent and fertile). They may also have been responsible for much of the biodiversity in the forest, deliberately introducing fruit and nut (mast) trees for their own good.