Earlier I mentioned recent research suggesting that the heartland of Mesoamerican agriculture was in western Mexico, which has important implications for the place of that region in Mesoamerica as a whole and in areas, like the Southwest, subject to Mesoamerican influence in prehistory. The main research I was talking about is contained in two papers published in PNAS (which for some reason seems to be the main journal for this sort of research) about a year ago describing recent work in the Rio Balsas drainage of northern Guerrero (which is not actually very far west of Central Mexico, but apparently falls within West Mexico as a defined cultural area). One paper discusses new radiocarbon dates associated with botanical evidence for domesticated maize and squash starch grains and phytoliths that implies initial domestication of those two plants sometime around 7000 BC, which is the earliest date for agriculture yet found in Mesoamerica. The presumed wild ancestors of domesticated maize and squash occur in this area, so the very early dates associated with clearly domesticated varieties (this is apparent from the nature of the starch grains and phytoliths, which differ from wild examples) strongly implies that this is where initial domestication occurred, and that it later spread to the rest of Mesoamerica and beyond. The other paper describes the archaeological context from which the dates and archaeobotanical specimens were taken, which was a rockshelter with unusually good preservation of undisturbed deposits. This is important because one of the reasons initial domestication of plants is so hard to pinpoint in the archaeological record is that plant materials are fragile and rarely survive for long periods. Rockshelters, however, are good places for preservation, and this one was particularly good because it had a long, undisturbed stratigraphic sequence containing charcoal that could be used for radiocarbon dating along with stone tools that could give some cultural and chronological context in addition to providing botanical specimens from the residue adhering to them.
So what are the implications of this? The two PNAS papers don’t say a whole lot about the big picture, but a more recent paper by two Mexican scholars tries to tie this together with other recent research to form some hypotheses about the origin of agriculture and the rise of Mesoamerican civilization. Their basic argument is that the early inhabitants of West Mexico made extensive use of fire to control their environment, initially for hunting, had the side effect of changing the local flora and encouraging the dominance of fire-resistant grasses such as teosinte, the wild ancestor of maize. They also argue that teosinte began to grow together with wild beans and squash at this time, which is more speculative. By around 7000 BC they see local hunter-gatherer groups in West Mexico as having effectively domesticated teosinte (creating maize), squash, and beans, thus creating the milpa system of agriculture that would become one of the hallmarks of Mesoamerican culture from then on. Once the system was created it spread rapidly along river corridors to other groups in Mesoamerica and beyond, and the basis of the later complex societies that arose in various regions was in place. From then on agriculture continued to intensify in each region where it had become established, and the plants used were continually bred for more advantageous features for human use. This part of the paper relies heavily on genetic evidence. One interesting feature of this theory is that it sees initial domestication of plants as having come well before the adoption of a fully sedentary lifestyle. In this view the first maize farmers were seasonally mobile, moving to different locations within a relatively small area over the course of a year in order to use different resources, with agricultural plots initially just being one part of a complicated subsistence strategy. Permanent sedentism only came later, together with more intensive agriculture and a greater reliance on farming to the exclusion of other subsistence practices.
So how plausible is this? I find it fairly plausible overall, but I’m very skeptical about a few parts of it. The most important is the claim that beans were domesticated at the same (very early) time as maize and squash. Beans are notoriously difficult to find in the archaeological record, since they have so few parts that are likely to survive for any significant length of time under typical conditions, so it’s entirely possible that they were present as early as the maize and squash documented in the PNAS papers (which did mention three starch grains from some sort of legume that could have been but probably wasn’t a type of bean), but the earliest solid evidence for beans specifically comes from much later, in the last few centuries BC, in Oaxaca. Genetic evidence seems to indicate that domesticated beans descend from a wild type of bean found only in West Mexico, so the later Oaxacan evidence is pretty clearly not tied to initial domestication, but apparently the people who have done that genetic research have concluded that domestication first occurred in a different part of West Mexico from the Rio Balsas area where the early dates for maize and squash were documented by the PNAS papers. Noteworthy in this context is that the earliest agricultural sites in the Southwest, which are earlier than the early Oaxacan bean dates, have maize and squash in abundance but no sign at all of beans. This seems to suggest that maize and squash were domesticated first, with beans and the full milpa system only being added at some later point (maybe during the period of increased sedentism and intensification?), but there are so many uncertainties about this subject that it’s best not to leap to conclusions.
Finally, there are some implications of this for the Southwest. Since West Mexico is generally thought to be the main source area of Mesoamerican influence in the Southwest, the idea that agriculture, and possibly other core aspects of Mesoamerican culture, originated there very early on suggests that Mesoamerican influence in the Southwest wouldn’t necessarily consist of “filtered” ideas from the “core” areas of Central Mexico and the more southerly regions. Instead, the same ideas and technologies may have diffused separately, at different times, both north and south (and east), where they were elaborated in different ways by people starting with their own cultural traditions. I think this may explain some of the more puzzling aspects of Southwestern-Mesoamerican relations, particularly the way Southwestern societies clearly seem to have adopted various Mesoamerican items and practices but never the entire Mesoamerican cultural system. In any case, this is an area worthy of much more research than it has received so far.
Piperno, D., Ranere, A., Holst, I., Iriarte, J., & Dickau, R. (2009). Starch grain and phytolith evidence for early ninth millennium B.P. maize from the Central Balsas River Valley, Mexico Proceedings of the National Academy of Sciences, 106 (13), 5019-5024 DOI: 10.1073/pnas.0812525106
Ranere, A., Piperno, D., Holst, I., Dickau, R., & Iriarte, J. (2009). The cultural and chronological context of early Holocene maize and squash domestication in the Central Balsas River Valley, Mexico Proceedings of the National Academy of Sciences, 106 (13), 5014-5018 DOI: 10.1073/pnas.0812590106
Zizumbo-Villarreal, D., & Colunga-GarcíaMarín, P. (2010). Origin of agriculture and plant domestication in West Mesoamerica Genetic Resources and Crop Evolution DOI: 10.1007/s10722-009-9521-4
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