Scientists have long assumed that this process took place gradually, perhaps over thousands of years. When geneticist Svante Paabo in Leipzig, Germany, turned the tools of genetic analysis to this ancient question, though, he got a surprise. His study of maize samples from the Balsas River Valley in southern Mexico, published last week in the journal Science, reveals that modern maize appeared on the scene far earlier than scientists had thought—perhaps as early as 9,000 years ago, almost 3,000 years before the earliest archeological evidence. The findings suggest that ancient humans were capable of causing rapid and decisive changes in the genetic makeup of staple crops, even without the tools of modern genetics. And they raise the possibility that ancient Mexicans may have benefited from their own maize-fed green revolution, similar to the one fueled in the 1960s and 1970s by high-yielding strains of wheat and rice.
Paabo and his colleagues performed their DNA analysis on 4,300-year-old cobs. They focused on three genes that control essential characteristics of modern maize: one that represses the formation of branches, making the plant easier to harvest, and two that reduce the protein and change the quality of the starch in the kernels, making them more valuable as food. The ancient cobs showed much less natural variation of all three genes than teosinte—clear evidence of alteration by human selection.
John Doebley, a geneticist at the University of Wisconsin-Madison and a coauthor of the study, speculates that the transformation took place in a mere 100 years—a flash by evolutionary standards—though scientists won’t know for sure until they’ve tested further. Apparently genetically modified maize has been shaking things up for thousands of years.
