Maize is a staple crop across much of the Americas, and new research suggests it's been that way for at least 4,700 years.

Reconstructing the diets of early human populations in tropical regions like Central America has proven quite difficult, but scientists recently discovered a burial site in present-day Belize that has been continuously used for over 10,000 years.

Given the shallow nature of most graves and burial sites, it's rare for human remains to end up layered sequentially -- let alone a burial site featuring well-preserved remains.

"Finding intact burials from any time period is rare in tropical environments as the high temperatures and humidity are favorable to bacteria and other decomposers," archaeologist Mark Robinson, a postdoctoral research fellow at Exeter University in Britain, told UPI in an email. "The rock shelters we excavated just happen to have the perfect set of factors to protect from the elements, creating a very dry, loose sediment, with no tree roots disturbing the interred bones."

To find out when humans that frequented the two rock shelters first started eating maize, scientists analyzed bones from 44 human skeletons.

"The food we consume is absorbed and builds our body's tissues," Robinson said. "The chemical signature of that food leaves its mark in our bodies, meaning we really are what we eat."

Because maize photosynthesizes differently than other kinds of plants, it leaves a unique chemical signature in bones.

"Maize, in the grass family of plants, creates a different carbon signature than the fruit, nuts, and leafy greens we consume from trees and herbs," Robinson said.

The oldest remains that researchers recovered from the burial site were dated to between 9,600 and 8,600 years ago. Humans continued to bury the dead at the rock shelters until just 1,000 years ago. The isotopic ratios measured in the ancient bones showed humans in Central America began eating maize nearly 5,000 years ago.

Prior to the introduction of maize, humans in the region relied on herbs, fruits and nuts from local trees and shrubs, as well as with meat from hunting native land animals. Isotopic analysis of the bones of two infants suggests their mothers were eating large amounts of maize by at least 4,700 years ago.

As early as 4,000 years ago, maize made up nearly 70 percent of the diets of some people in Central America.

By the time the first European explorers arrived in the Americas, corn was ubiquitous among native populations. The new study -- published Wednesday in the journal Science Advances -- can help researchers reconstruct the growing importance of maize among native populations.

"Decades of research has revealed the importance of the crop for cultural identity, cosmological beliefs, creation myths, landscape management, the economy, as well as dietary nutrition," Robinson said.

"Although the impact and importance of maize has been extensively researched, how, why and when maize was domesticated and adopted by so many diverse groups is still little understood," he said. "The data from these contexts enables us for the first time to chronologically explore these questions in a single location."

As the new analysis revealed, what began as a supplemental food source slowly became more important over several generations.

"Farmers would have to learn how to cultivate the crop and make the decision to increase the labor investment into its cultivation, becoming more reliant on the grain at the expense of other sources of nutrition," Robinson said.

"By the later periods, whole socioeconomic systems were built around maize agriculture, with landscapes transformed to maximize cultivation," Robinson said. "These data provide the first lines of evidence for how that adoption process took place and how long it took."

The latest isotopic analysis can provide context to the archaeological remains of early Central American settlements. The adoption of corn as a staple diet was closely intertwined with the development of farming tools and food processing technologies, as was the evolution of local economies and social and political structures.

By the time the Mayan civilization emerged some 2,000 years ago, maize wasn't just a source of calories, it was a crop of tremendous importance -- culturally, economically and politically.

"We are currently analyzing genomic data from the skeletal material, which will provide new insight into who these people were," Robinson said. "This will help us unravel the process of adoption of the technology. Did a new population enter the area, bringing maize agriculture, or were the seeds and plants passed between communities."

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