For many of us, corn is a staple food, a versatile ingredient that appears in everything from breakfast cereals to savory side dishes. But have you ever stopped to wonder about the origins of this ubiquitous grain? The corn we know today, with its plump kernels and substantial cobs, is a far cry from its ancestor. Around 9,000 years ago, in what is now southwestern Mexico, ancient inhabitants encountered a wild grass called teosinte. This wild relative bore ears no bigger than a pinky finger, yielding only a meager handful of hard, stony kernels. Yet, these early Indigenous farmers, through ingenuity and perseverance, recognized the potential within this wild grass. They incorporated it into their diets, setting in motion a transformative journey that would turn teosinte into corn, or maize, a domesticated crop that now sustains billions across the globe.
Despite corn’s undeniable importance in modern life, the complete story of its evolution across time and continents remains shrouded in some mystery. Recent groundbreaking research, spearheaded by scientists at the Smithsonian Institution, is shedding new light on these enduring questions. By delving into ancient DNA, a team of researchers has begun to piece together a more detailed and nuanced understanding of corn’s 9,000-year history.
According to Logan Kistler, curator of archaeogenomics and archaeobotany at the Smithsonian’s National Museum of Natural History and co-lead author of the new study, this research exemplifies how fundamental investigations into ancient DNA can unlock crucial insights into human history that would otherwise remain inaccessible.
“Domestication—the process by which wild plants have been transformed over millennia into the crops we depend on today—is arguably the most profound development in human history. And maize stands out as one of the most critical crops cultivated worldwide,” Kistler explains. “Gaining a deeper understanding of the evolutionary and cultural context of domestication is vital. It provides us with invaluable knowledge about this essential food and its profound role in shaping civilization as we recognize it.”
Published in the Proceedings of the National Academy of Sciences, Kistler and his international team unveiled the fully sequenced genomes of three remarkably preserved corn cobs, approximately 2,000 years old, discovered at the El Gigante rock shelter in Honduras. The genetic analysis of these ancient specimens revealed a surprising twist: these ancient Central American corn varieties possessed South American ancestry. This discovery adds a fascinating new dimension to the unfolding narrative of corn’s domestication history, suggesting a more complex and interconnected story than previously understood.
“Our research demonstrates that humans actively transported maize from South America back towards its domestication center in Mexico,” Kistler notes. “This movement would have introduced a valuable influx of genetic diversity, potentially enhancing resilience and boosting productivity. It underscores that the journey of domestication and crop improvement is not a linear progression, but rather a dynamic and intricate process.”
Tracing Corn’s Evolutionary Path from Teosinte
The domestication of corn began around 9,000 years ago in Mexico, as early peoples started selectively breeding teosinte. However, it took another 1,500 to 2,000 years for these partially domesticated corn varieties to reach other parts of Central and South America.
For many years, the prevailing theory among scholars was that corn was fully domesticated in Mexico before spreading to other regions. This straightforward model suggested a single origin point and a linear dispersal. However, the discovery of 5,000-year-old corn cobs in Mexico that were only partially domesticated challenged this conventional wisdom, prompting researchers to re-evaluate the established narrative.
A landmark 2018 study, also led by Kistler, utilizing ancient DNA analysis, revolutionized the understanding of corn domestication. This research revealed that while the initial steps towards domestication occurred in Mexico, the process was still ongoing when people began carrying corn southward into Central and South America. Instead of a single, linear path, the study suggested a more complex scenario where domestication and crop improvement proceeded in parallel across these three regions, albeit at different paces.
Building on this refined understanding, scientists, including Kistler, previously found that 4,300-year-old corn remnants from the El Gigante rock shelter in Central America represented a fully domesticated and highly productive variety. The presence of fully domesticated corn in El Gigante, located relatively close to the region where partially domesticated corn was found in Mexico, presented a puzzle. This unexpected finding prompted Kistler and project co-lead Douglas Kennett, an anthropologist at the University of California, Santa Barbara, to investigate the genetic origins of the El Gigante corn.
El Gigante Rock Shelter: A Window into the Past
“The El Gigante rock shelter is an exceptional archaeological site, preserving plant remains spanning an incredible 11,000 years,” Kennett explains. “We have identified over 10,000 maize remains, ranging from complete cobs to fragments of stalks and leaves. While many of these are from more recent periods, our extensive radiocarbon dating efforts have pinpointed some remains dating back as far as 4,300 years ago.”
The researchers meticulously examined the archaeological layers surrounding the El Gigante rock shelter, searching for any corn remnants suitable for genetic analysis. They focused on the site’s oldest corn samples, dating back 4,300 years. Over two years, the team attempted to sequence 30 samples, but only three yielded DNA of sufficient quality for full genome sequencing. Intriguingly, these three viable samples originated from a more recent layer of the rock shelter, carbon-dated between 2,300 and 1,900 years ago.
By comparing the three sequenced genomes from El Gigante with a comprehensive dataset of 121 published genomes representing diverse corn varieties, including 12 derived from ancient corn cobs and seeds, the researchers uncovered significant genetic connections. The analysis revealed subtle but consistent genetic overlap between the Honduran samples and corn varieties from South America.
“The genetic link to South America was not immediately obvious, but it emerged consistently across multiple analyses,” Kistler emphasizes. “We rigorously repeated the analysis using various methods and sample compositions, and each time, the South American connection persisted.”
South American Influence on Corn Development
Kistler, Kennett, and their colleagues hypothesize that the reintroduction of South American corn varieties into Central America may have played a catalytic role in accelerating the development of more productive hybrid varieties in the region. Although the current findings are based on El Gigante corn samples dating back approximately 2,000 years, the physical characteristics of the cobs from the older, 4,000-year-old layer suggest they were nearly as productive as the sequenced samples. This observation leads Kistler to believe that the significant crop improvement likely occurred prior to, rather than during, the 2,000-year interval separating these archaeological layers at El Gigante.
The team further proposes that the introduction of South American corn varieties and their genetic material, possibly as early as 4,300 years ago, may have been instrumental in enhancing corn productivity in Central America and increasing its prevalence in the diets of the people inhabiting the broader region, as indicated by recent dietary studies led by Kennett.
“We are observing a convergence of evidence from multiple studies in Central America, suggesting that maize was becoming an increasingly productive staple crop of growing dietary importance between 4,700 and 4,000 years ago,” Kennett points out.
Collectively, these latest discoveries, combined with Kennett’s recent research, strongly suggest a pivotal moment in corn domestication around 4,000 years ago in Central America. The introduction of genetic diversity from South America may have been a crucial factor in this transformation. Intriguingly, this timeframe aligns with the emergence of the first settled agricultural communities in Mesoamerica, which ultimately paved the way for the rise of great civilizations such as the Olmec, Maya, Teotihuacan, and Aztec. However, Kistler cautions that this connection remains speculative and requires further investigation.
“We are eager to delve deeper into the specifics of what transpired around the 4,000-year mark,” Kistler concludes. “Vast numbers of archaeological maize samples remain genetically unanalyzed. By expanding our analysis of these samples, we can begin to address the lingering questions about the precise impact of this reintroduction of South American varieties and further illuminate the complex origins of corn.”
This research was made possible by funding and support from the Smithsonian, National Science Foundation, Pennsylvania State University, and the Francis Crick Institute.
