Dr. Beadle’s results showed that maize and teosinte were without any doubt remarkably and closely related. But to pinpoint the geographic origins of maize, more definitive forensic techniques were needed. This was DNA typing, exactly the same technology used by the courts to determine paternity.

In order to trace maize’s paternity, botanists led by my colleague John Doebley of the University of Wisconsin rounded up more than 60 samples of teosinte from across its entire geographic range in the Western Hemisphere and compared their DNA profile with all varieties of maize. They discovered that all maize was genetically most similar to a teosinte type from the tropical Central Balsas River Valley of southern Mexico, suggesting that this region was the “cradle” of maize evolution. Furthermore, by calculating the genetic distance between modern maize and Balsas teosinte, they estimated that domestication occurred about 9,000 years ago.

These genetic discoveries inspired recent archeological excavations of the Balsas region that sought evidence of maize use and to better understand the lifestyles of the people who were planting and harvesting it. Researchers led by Anthony Ranere of Temple University and Dolores Piperno of the Smithsonian National Museum of Natural History excavated caves and rock shelters in the region, searching for tools used by their inhabitants, maize starch grains and other microscopic evidence of maize.

In the Xihuatoxtla shelter, they discovered an array of stone milling tools with maize residue on them. The oldest tools were found in a layer of deposits that were 8,700 years old. This is the earliest physical evidence of maize use obtained to date, and it coincides very nicely with the time frame of maize domestication estimated from DNA analysis.

The most impressive aspect of the maize story is what it tells us about the capabilities of agriculturalists 9,000 years ago. These people were living in small groups and shifting their settlements seasonally. Yet they were able to transform a grass with many inconvenient, unwanted features into a high-yielding, easily harvested food crop. The domestication process must have occurred in many stages over a considerable length of time as many different, independent characteristics of the plant were modified.

The most crucial step was freeing the teosinte kernels from their stony cases. Another step was developing plants where the kernels remained intact on the cobs, unlike the teosinte ears, which shatter into individual kernels. Early cultivators had to notice among their stands of plants variants in which the nutritious kernels were at least partially exposed, or whose ears held together better, or that had more rows of kernels, and they had to selectively breed them. It is estimated that the initial domestication process that produced the basic maize form required at least several hundred to perhaps a few thousand years.

Every August, I thank these pioneer geneticists for their skill and patience.