Researchers sequencing the DNA of ancient maize from dozens of archaeological sites have found that early North Americans carried corn from Mexico into the southwestern U.S. on two different routes at different times: One took the high road, the other took the low road, thousands of years later. The findings were published this week in the first-ever issue of Nature Plant.
We know that maize (Zea mays ssp. Mays) was domesticated from a wild grass called teosinte found in central and southern Mexico, but its journey into what’s now the American Southwest has been up for debate. Comparing the DNA of modern crops with their wild contemporary cousins wouldn’t take into account the transitional plants along the way between those eaten by early human ancestors and those in the markets and county fairs today.
For the whole story, a large international team led by Rute da Fonseca and Thomas Gilbert from the University of Copenhagen sequenced the DNA of ancient maize—including long-disappeared varieties—from 28 archaeological sites in the Southwest and Mexico, which were dated from 5,910 to 740 years ago. About a dozen of their samples came from excavations of multiple stratigraphic layers of the Tularosa Cave in New Mexico, allowing them to look at genes from different time periods. The team then compared these results with modern corn, as well as genetic material from four museum specimens of ancient Chilean maize.
The first phase of maize introduction into the Southwest, they found, occurred through the highlands of the Sierra Madres about 4,000 years ago. That was followed by a gene flow from a lowland route along the Pacific coast 2,000 years later. “When considered together, the results suggest that the maize of the U.S. Southwest had a complex origin,” study co-author Jeffrey Ross-Ibarra from UC Davis says in a news release.
Additionally, the team found evidence of evolutionary pressure applied to the plant by human migrations and the diverse tastes of various cultures. These included adaptations for growing in dry climate, as well as genes responsible for changes in starch and sugar composition, which made it possible to develop sweet corn.
“It’s only really recently that maize has become what you would recognize as maize if you went to a grocery store,” Gilbert tells Science. In the evolution of corn on the cob, the first thing farmers bred out of ancient maize was an annoying feature called “shattering,” where wild plants scatter seeds every which way. Then, over several centuries, farmers bred drought-resistant maize. Finally, only about a thousand years ago, came corn with improved nutritional content and easy-to-process kernels that made making treats like tortillas easier.
