Researchers have missed the bigger picture of ancient agriculture in the Americas, a team of archaeologists argues, and part of the issue can be cleared up by corn.

Using both population growth rates calculated from settlement patterns and productivity levels for ancient maize, the authors of an American Antiquity paper assert that the shift to agriculture in Mesoamerica and the Southwest did not unfold uniformly, but rather occurred in central areas and then radiated out over regions. As they wrote in the paper, published May 4, analyzing larger areas of historically connected regions resolves "apparent chaos at the regional scale" that obfuscates the two-phase reality of how humans moved from hunting and gathering to agriculture: First, one central area in a larger region would have a low-productivity transition; and later, satellite areas had much faster, high-productivity transitions, building on earlier advances.

And when the researchers charted the productivity of maize over time, they found that changes in corn lined up with their other evidence.

"Inter-regional processes are definitely seen as super important, but what has not gotten a lot of play are what we're calling macroregional phenomena," said Richard G. Lesure, the lead author of the paper and an archaeology professor at the University of California, Los Angeles. In this paper — partially inspired by research into language distribution by the archaeologist Peter Bellwood — Lesure and his co-authors argue for the importance of examining "macroregions," which are contiguous and historically connected regions. 

"People have focused on individual regions that they can go and study with their own project," said Lesure, who is UCLA's Marilyn Beaudry-Corbett Endowed Chair in Mesoamerican Archaeology. "There's a lot of regional research. There's less macroregional research, in terms of trying to trace and explain historical connections between adjacent regions."

With this lens, Lesure and his co-authors suggest that the long Mesoamerican agricultural demographic transition began around 1900 B.C. — when more permanent residences and ceramics appear in archaeological sites, and when the proportion of skeletal remains of young people grows, meaning there were more young people living and implying that fertility increased, as is expected with the transition to agriculture. The researchers suggest that in the Sonoran Desert, which is in present-day Arizona, California and Mexico, the long transition began around 1200 B.C., while on the Colorado Plateau it began around 400 B.C. In both the Sonoran Desert and the Colorado Plateau, they saw similar evidence: larger and more permanent settlements, abundant remains of ancient maize and markers in human bones that indicate they were eating corn.

In Mesoamerica, the second high-productivity phase happened between 1000 and 200 B.C., the researchers say; in the Southwest, it was between 500 and 1300 A.D. In Mesoamerica, the population growth rate reached 0.4% around 1000 B.C., and then it went as high as 1.4% around 500 B.C.

The initial low-productivity phases happened in "heartlands": the Valley of Oaxaca, Morelos, the southern Basin of Mexico and the Tucson Basin.

The Valley of Oaxaca exemplifies the dynamic. In the valley, located in current-day southern Mexico, there was rapid growth between 2000 and 1000 B.C., but the surrounding areas were barely inhabited by people. Then, in the middle of the first millennium B.C., "Farming communities appeared everywhere," the researchers wrote, and population growth rates went up.

The researchers drew population data from previously published research and mapped out changing population growth rates over time. Earlier work on population estimates in Mexico was based on regional surveys that determine hectares occupied. In the Southwest, which has a somewhat harder-to-survey architectural tradition, these researchers calculated changes in total site area occupied and previously published estimates based partly on the number of structures.

"[In] Mesoamerica, compared to other areas of the globe, the archaeologists got into this [survey-based] research sooner than many areas, and it's really quite developed," Lesure said. "This kind of data in Mesoamerica is some of the best quality in the world, and also the number of these studies is very good."

R.J. Sinensky, a graduate student at UCLA, led the maize portion of the research. Sinensky, Lesure said, "did a lot of work to dive into that literature on maize, and tried to see if he could connect actual, concrete evidence of changes in maize to the changing rates of population growth that we saw reflected in the survey evidence, and it actually worked so much better than we thought it would." The history of maize and maize farming lined up with the timeline they laid out based on population data — changes in the productivity in maize seemed to coincide with changes in the agricultural demographic transition.

Previously, "What hasn't been around is this idea of these two distinct phases of growth," Lesure said, "and the idea that they might be related to the gradual changes in maize." 

He hopes that the paper will inspire researchers to think bigger.

"The macroregion is a crucial scale for understanding this transition to agriculture," Lesure said, adding, "The macroregion of multiple contiguous regions [is] way more than any single researcher could actually do in terms of going out and surveying. It would have to be synthesizing the work of multiple researchers."

The paper, "Large-scale patterns in the agricultural demographic transition of Mesoamerica and Southwestern North America," published May 4 in American Antiquity, was authored by Richard G. Lesure, R. J. Sinensky, Gregson Schachner and Thomas A. Wake, University of California, Los Angeles; and Katelyn J. Bishop, University of Illinois at Urbana-Champaign.