Smithsonian.com

The Incan walled complex of Sacsayhuamán near Cuzco, Peru (courtesy of flickr user Altamar)

In 1532, when the Incas first met a European, their empire stretched from what is now northern Ecuador to central Chile. The largest empire of the Americas numbered more than eight million people. But the Incas didn’t exist until about A.D. 1100. Before than, the Wari and Tiwanauku occupied the central Andes.

Archaeologists suspected a worsening environment led to the disappearance of the Wari and Tiwanauku. But what about the Incas’ rise? To get a better idea of the factors that shaped these early South American civilizations, a group of French-led scientists examined a 26-foot-long mud core taken from a Peruvian lake. Their analysis appears in the journal Climate of the Past.

The mud core trapped pollen, seeds, charcoal and other bits in layers for 4,000 years. By analyzing the contents of this debris, the archaeologists developed a picture of the region’s changing climate, particularly during the time of the Wari, Tiwanauku and Incas.

For the 3000 years before A.D. 1000, the region had cool temperatures. But around 880, a drought began and lasted for at least 100 years. This corresponds with the declines of the Wari and Tiwanauku.

Then around A.D. 1150, the climate began to warm by several degrees. That would have extended the land that could be planted by about 300 yards in elevation. In addition, melting glaciers could have provided more water for irrigation.

With all the extra land to be cultivated, the Incas could have had large surpluses of food (in fact, when the Spanish arrived, they found a 10-year supply of food in the Incan warehouses). More food would have meant more freedom to build roads and monuments and create an army big enough to conquer neighbors.

Of course, all of this is speculation, and more work is needed to match up the archaeological and climate records. As archaeologist Warren Church of Columbus State University in Georgia told the Los Angeles Times: “It is important to remember that climates do not make empires. People do.”