Soil isotopes place likely maize farming at Belize wetland edges 6,000 years ago

The evidence is chemical. Maize is a C4 plant. That photosynthetic pathway leaves a carbon isotope signature that can still be detected in tropical soils thousands of years later ^[6]. The team collected soil from wetlands and uplands near the ancient city of Uxbenka, then ran chemical and microscopic analyses in the United States to map how food production changed over time ^[8].

The isotope signal tracks the pathway, so the account calls the later upland plantings "C4 crops" ^[6][10]. Prufer also hedges his own claim. "The most significant discovery is that we were able to document that people were farming what was almost certainly maize by 6,000 years ago along the edges of wetlands where soil moisture is abundant, and that over time they changed practices to mitigate risks to their crops as populations grew and the environment changed," said Keith Prufer, a University of New Mexico anthropologist who co-led the study with Megan Walsh of Central Washington University ^[2][3]. On the evidence summarised, I'd treat the 6,000-year figure as a record of C4 cultivation at the wetland margins. The maize identification is a strong inference ^[2][6].

At least 2,000 years separate the earliest wetland-edge farming in the record from the first upland fields ^[15]. The wetland margins stayed in cultivation for 4,000 years or more before farmers largely left them ^[16].

The team set its soil sequence against early climate records to assess how changes in past rainfall might have shaped farmers' choices ^[9]. That comparison fixes the order of events: the upland move followed the end of a dry interval ^[10]. It cannot on its own separate climate from the other pressure Prufer names. In his account, population growth and environmental change run across the same millennia ^[2].

Part of the interpretation comes from farmers working today. Mes shared farming strategies, talked through risks and explained how growers judge each climate condition to get the best harvest ^[14]. Around his village, most people grow maize, beans and squash in the same fields and leave land fallow for several years before planting it again ^[4][5]. "This research study began in the early 2000s," Prufer said. "The team has been working in southern Belize in the Maya Golden Landscape, which is a region made up of 35 traditional Maya communities." ^[7]

The thing this account doesn't tell you is the denominator: how many soil samples built the sequence, how tightly the 6,000- and 4,000-year marks are dated, and how the no-loss-in-productivity claim was measured ^[12]. The paper is "An early shift from wetland-adjacent to upland farming in the Maya lowlands," in Communications Earth & Environment ^[13].