Ancient DNA continues to rewrite the 9,000-year history of corn that shapes society

Three 2,000-year-old cobs in Honduras show that humans were returning varieties of corn to Mesoamerica, possibly boosting productivity and shaping civilization.

Corn as it is known today did not exist about 9,000 years ago. Ancient peoples of southwestern Mexico came across a wild grass called teosinte, which offered ears smaller than a little finger with only a handful of stony grains. But through genius or necessity, these native growers saw potential in the grain, added it to their diets, and set it on the path to becoming a domesticated crop that now feeds billions.

Despite how important maize or maize is to modern life, gaps remain in understanding its journey through space and time. Now a team led by Smithsonian researchers has used old DNA to fill in some of those gaps.

A new study, revealing details about maize’s 9,000-year history, is a great example of how basic research into ancient DNA can provide insights into human history that would otherwise be inaccessible, said co-lead author Logan Kistler, curator. archeogenomics. and archeobotany at the Smithsonian’s National Museum of Natural History.

“Domestication – the evolution from wild plants over thousands of years to the crops that feed us today – is arguably the most important process in human history, and corn is one of the most important crops grown on the planet today,” said Kistler. “Understanding more about the evolutionary and cultural context of domestication can provide valuable information about this food we rely so fully on and its role in shaping civilization as we know it.”

In the December 14, 2020 issue of the magazine the Proceedings of the National Academy of SciencesKistler and an international team of collaborators report the fully sequenced genomes of three approximately 2,000-year-old flasks from the El Gigante shelter in Honduras. Analysis of the three genomes reveals that these millennia-old varieties of Central American maize had South American descent and adds a new chapter to an emerging complex story of maize’s domestication history.

“We show that people returned maize from South America to the domestication center in Mexico,” said Kistler. “This would have provided an infusion of genetic diversity that may have increased resilience or productivity. It also underscores 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.

Then, in a groundbreaking 2018 study led by Kistler, scientists used ancient DNA to show that while Teosinte’s first steps towards domestication took place in Mexico, the process was not yet complete when humans first moved south to Central America. 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, a team of scientists, including Kistler, 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 .

Amazed to find fully domesticated corn in El Gigante side by side in a region not far from where partially domesticated corn had been discovered in Mexico, Kistler and project leader Douglas Kennett, an anthropologist at the University of California, Santa Barbara, teamed up to genetically determine where the El Gigante corn comes from.

“El Gigante rock shelter is remarkable for containing well-preserved plant remains from the past 11,000 years,” said Kennett. “More than 10,000 maize residues have been identified, from whole cobs to fragmentary stems and leaves. Many of these remains date back to late, but through an extensive radiocarbon study, we have been able to identify some that date back to 4,300 years ago. “

They scoured the archaeological layers around the El Gigante bomb shelter for cobs, kernels, or anything else that could yield genetic material, and the team began work on sequencing some of the site’s 4,300-year-old corn samples – the oldest traces of the harvest on El Gigante.

For two years, the team tried to sequence 30 samples, 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.

Using the three sequenced genomes of El Gigante corn, the researchers analyzed them against a panel of 121 published genomes of different corn varieties, including 12 derived from old corn cobs and seeds. The comparison revealed fragments of genetic overlap between the three samples from the Honduran rock shelter and corn varieties from South America.

“The genetic link with South America was subtle but consistent,” said Kistler. “We repeated the analysis many times with different methods and sample compositions, but always got the same result.”

Kistler, Kennett and their co-authors at collaborating institutions including Texas A&M University, Pennsylvania State University and the Francis Crick Institute and the University of Warwick In the UK, suppose 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.

While the results only pertain to the El Gigante corn samples dating back to about 2,000 years ago, Kistler said the shape and structure of the cobs of the approximately 4,000-year-old layer suggests they were nearly as productive as those he and his co-authors. can sequence. For Kistler, this means that the blockbuster improvement likely occurred earlier than during the intervening 2,000 years or so between these archaeological layers at El Gigante. The team further hypothesizes that it was the introduction of the South American corn varieties and their genes, likely at least 4,300 years ago, that increased the productivity of the region’s corn and the prevalence of corn in the diets of the people who lived. . in the wider region, as discovered in a recent study led by Kennett.

“We are beginning to see a convergence 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 increasingly important in the diet,” said Kennett.

Together with Kennett’s recent study, these latest findings suggest that something memorable may have happened in the domestication of maize in Central America about 4,000 years ago, and that an injection of genetic diversity from South America may have had something to do with it. This proposed timing is also consistent with the emergence of the first established agricultural communities in Mesoamerica that eventually gave rise to great civilizations in America, the Olmec, Maya, Teotihuacan, and the Aztec, although Kistler hastened to point out that this idea was still has consistently been relegated to speculation.

“We can’t wait to get into the details of what exactly happened around the year 4000,” said Kistler. “There are so many archaeological samples of maize that have not been genetically analyzed. If we started testing more of these samples, we would start by answering these lingering questions about how important this reintroduction of South American varieties was. “

Reference: “Archaeological Genomes of Central American Maize Suggest Ancient Gene Stream from South America” ​​By Logan Kistler, Heather B. Thakar, Amber M. VanDerwarker, Alejandra Domic, Anders Bergström, Richard J. George, Thomas K. Harper, Robin G. Allaby, Kenneth Hirth and Douglas J. Kennett, December 14, 2020, Proceedings of the National Academy of Sciences.
DOI: 10.1073 / pnas.2015560117

Funding and support for this research was provided by the Smithsonian, National Science Foundation, Pennsylvania State University, and the Francis Crick Institute.