Scientists use genomic testing to trace the history of corn

In a new study from UC Santa Barbara, scientists have uncovered new details about the 9,000-year-old history of corn by analyzing the genomes of ancient cobs.

Maize was first partially domesticated in southwestern Mexico before spreading throughout Central and South America. However, the analysis of three 2,000-year-old cobs from Honduras showed that they were more related to varieties from South America than to varieties from Mexico. 

“I was most surprised that improved maize varieties developed by Indigenous South Americans were reintroduced northward into Central America,”said study co-lead author Professor Douglas J. Kennett. “We could only know this through whole genome sequencing.”

The research suggests that the arrival of corn from South America may have played a role in the development of more productive varieties and greater consumption of corn in Central America about 4,700 years ago.

“We show that humans were carrying maize from South America back towards the domestication center in Mexico,” said study co-lead author Logan Kistler. “This would have provided an infusion of genetic diversity that may have added resilience or increased productivity. It also underscores that the process of domestication and crop improvement doesn’t just travel in a straight line.”

According to Professor Kennett, genetic research has been challenging due to the scarcity of suitable corn cobs in an environment that is not kind to organic material. 

“Well-preserved maize is extremely rare in the Americas, but the El Gigante rock shelter has over 10,000 specimens to work with,” said Professor Kennett. “Most of these fragmentary remains date later than 2,500 years ago, and locating earlier material in the assemblage was challenging and required directly radiocarbon dating large numbers of maize cobs.”

“Using this approach, we did identify about 20 early cobs dating to between 4,300-4,000 years ago and we attempted to extract ancient DNA from all of these along with a set of cobs dating to between 2,300 and 1,900 years old. Using the best available techniques we were able to extract working genomes from only three cobs and all of these dated to between 2,300 and 1,900 years ago. This is the frustrating reality of ancient DNA work.”

Professor Kennett noted that research on maize has entered into a new era of discovery because whole genome sequencing is revolutionizing our understanding of the past.

“We can now know much more about the biochemical processes involved in the domestication process and the sequence of domestication alleles that became fixed during the domestication syndrome. It is also allowing us to track the spread of maize in much greater detail and how these changes altered the path of human history.”

While researchers now know that improved varieties of maize spread from South America into parts of Central America, they do not know precisely when it happened, explained Professor Kennett. The experts theorize that it occurred by 4,300 years ago based on cob morphology.

“Testing this hypothesis will require recovering earlier samples with well-preserved genetic material, We also don’t know how far north these new varieties spread. This will require whole genome work on cobs of equivalent age farther north (e.g., Mexico).” 

“We also don’t know if people migrated north carry these newly improved varieties or if seeds simply passed through preexisting exchange networks. This will require working collaboratively with Indigenous populations in Central America to sequence ancient genomes to determine if the appearance of new varieties parallels the appearance of new populations.”

The study is published in the Proceedings of the National Academy of Sciences.

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By Chrissy Sexton, Earth.com Staff Writer