The domestication of maize (Zea Mays ssp. mays), a global food staple of great economic and cultural importance, began 9,000 years ago in southwestern Mexico, and humans spread this important grain to South America at least 7,000 years ago as a partial domestication. South America served as a secondary improvement center where domestication syndrome was remedied and improved varieties emerged in parallel with similar processes in Mesoamerica. Now, researchers from the United Kingdom and the United States have extracted and sequenced DNA from 2,000-year-old corn cobs found in El Gigante Rock Shelter, Honduras, and found that hybrids of some South American varieties were likely reintroduced to Central America. .
Three roughly 2,000-year-old corn on the cob from the El Gigante shelter in Honduras. Image credit: Thomas Harper.
“We show that people returned maize from South America to the domestication center in Mexico,” said co-lead author Dr. Logan Kistler, Curator of Archeogenomics and Archeobotany at Smithsonian’s National Museum of Natural History.
“This would have provided an infusion of genetic diversity that may have increased resilience or productivity.”
“It also underlines that the process of domestication and crop improvement is not just a straight line.”
Humans began selectively breeding the wild ancestor of maize teosinte about 9,000 years ago in Mexico, but partially domesticated varieties of the crop did not reach the rest of Central and South America until after 1,500 and 2,000 years, respectively.
For years, the conventional thought among scholars had been that maize was first fully domesticated in Mexico and then distributed elsewhere.
However, after 5,000-year-old cobs found in Mexico were found to be only partially domesticated, scientists began to rethink whether this thinking covered the full story of maize domestication.
In 2018, Dr. Kistler and his colleagues ancient DNA to show that while Teosinte’s first steps towards domestication took place in Mexico, the process wasn’t complete when humans first started carrying it south to Central and South America.
In each of these three regions, the process of domestication and crop improvement ran in parallel, but at different speeds.
In an earlier effort to hone in on the details of this richer and more complex domestication story, the researchers found that 4,300-year-old corn remains from the Central American El Gigante rock shelter came from a fully domesticated and highly productive variety.
Surprised to find fully domesticated corn in El Gigante that co-existed in a region not far from where partially domesticated corn had been discovered in Mexico, they tried to genetically determine where the El Gigante corn came from.
“El Gigante rock shelter is remarkable for containing well-preserved plant remains from the past 11,000 years,” said Dr. Kennett.
For two years, the authors attempted to sequence 30 samples from the El Gigante hideout, but only three were of suitable quality to sequence an entire genome.
The three viable samples were all from the more recent layer of the rock shelter occupation – carbon dating between 2,300 and 1,900 years ago.
With the three genomes whose sequence was new, the team analyzed them against a panel of 121 published genomes from different corn varieties, including 12 derived from old corn cobs and seeds.
The comparison revealed fragments of genetic overlap between the El Gigante samples and South American maize varieties.
“The genetic link with South America was subtle but consistent,” said Dr. Kistler.
“We repeated the analysis many times with different methods and sample compositions, but always got the same result.”
The researchers hypothesize that the reintroduction of these South American varieties to Central America may have kick-started the development of more productive hybrid varieties in the region.
They also think it was the introduction of the South American corn varieties and their genes, probably at least 4,300 years ago, that influenced the productivity of the corn from the region and the prevalence of corn in the diets of people living in the wider region.
“We are beginning to see a confluence of data from multiple studies in Central America that indicate that between 4,700 and 4,000 years ago, corn was becoming a more productive staple crop and gaining importance,” said Dr. Kennett.
The research was published this week in the Proceedings of the National Academy of Science.
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Logan Kistler et al. Archaeological genomes of Central American corn suggests an ancient gene flow from South America. PNAS, published online December 14, 2020; doi: 10.1073 / pnas.2015560117