The canyons and mesas of the Four Corners region are filled with thousands of ruins built by eary Anasazi Indian inhabitants. Some of the most visited are here in the Mesa Verde National Park in the Southwest corner of Colorado. Though dendrochronology, the study of tree rings, the constuction of these dwellings can be dated very precisely.
The best preserved ruins are those built in caves or cliff dwellings sheltered by rock overhangs. As we drive the rim of the mesa we can see a half dozen examples of these.
The Indians here lived on top of the mesas for thousands of years. During a 20 year period ending about 1276 A.D., they retreated to cliff dwellings and then abandoned these within a ten year period. The reason for the retreat and exodus is thought to be a 23 year drought which decreased food supplies and increased warfare over scarce resources.
A major source of food was corn. They used a "dry farming" (no irrigation) method, planting in wet, low-lying areas where water pooled in the spring after the eight-foot snowfall melts.
The development of Maize (corn) by an early Mesoamerican geneticist probably happened sometime between 10,000 and 6,000 BC in southern Mexico. This development has done more to enable a population explosion to six billion than any other human invention.
From the 1938 to the 1960s, Mangeldorf's 'Tripartite Hypothesis' on the origin of corn was widely accepted. He believed that corn was developed from a cross between an undiscovered wild Maize and the plant Tripsacum.
In 1968, George Beadle, in retirement, began to provide convincing evidence for his own 'Teosinte Hypothesis', which simply believed that Maize was developed from the plant Teosinte. Today scientists generally accept the Teosinte Hypothesis because of advances in the study of genomes.