The origin of maize (or commonly corn) into the American Southwest has been contentious among archaeological researchers supporting two different routes of entry. New Research into the DNA of ancient maize now supports entrance via both routes a highland route and later a low land coastal route. The researchers studied 32 archaeological samples spanning from 6,000 years ago of evolution to modern landraces of Mexico today. They looked for genetic similarities that would indicate the geographic origins. The findings indicate that about 4,000 years ago maize traveled north along a highland route, then later about 2,000 years other varieties traveled north along the coastal roads. The oldest corn cobs in the US, dating to more than 3,000 years ago, were much more genetically similar to those from the Mexican highlands.
The younger the samples get, however, the more genes they share with coastal varieties. That suggests that maize “initially went up the highland route,” Gilbert says. But the crop, which was still in its early stages of domestication, probably wasn’t thriving at high altitudes or in the dry American Southwest. As the centuries passed, Gilbert says, farmers “were crossing in varieties from elsewhere to keep improving it through time.”
Researchers were also able to look at how traits were selected during the domestication process. The first traits to be bred out were shattering, a common wild seed dispersal mechanism. Later varieties were bred to be more drought tolerant, a necessary trait for plants in the dry American Southwest. In was only about a 1,000 years ago that traits for involved taste, more nutritional qualities and easier processing were selected for in varieties of maize.
The work “indicates that our understanding of the spread of maize is a far more complex story than previously thought,” says Robert Hard, an archaeologist at the University of Texas, San Antonio, who studies early agriculture in the southwest and northern Mexico and was not involved in the research. But in order to complete the picture, more archaeological data are needed. “We have almost no data from the vast reaches of northern Mexico, the land between the origin of maize in Mexico and the American Southwest,” Hard notes. That represents “a huge gap in our understanding.”